Colorado ClimateSpring 2000 Vol. 1, No. 2

Inside:• Growing Season Trends• Urban Heat Islands• Where Do Climate Data Come From• Climate Prediction in the 21st Century

22 Colorado Climate

Colorado Climate CenterAtmospheric Science

DepartmentColorado State UniversityFort Collins, CO 80523-1371ISSN 1529-6059

An earlier publication withthe same name, “ColoradoClimate,” was publishedmonthly from 1977 through1996 with the support of theColorado AgriculturalExperiment Station and theColorado State UniversityCollege of Engineering.

The Colorado Climate Centeris supported by the ColoradoAgricultural ExperimentStation.

Table of ContentsGovernor’s Flood and Drought Preparedness Conference ............................................................................................ 1

Growing Season Trends Across Eastern Colorado ....................................................................................................... 3

Can We Predict Colorado Climate in the 21st Century? ................................................................................................. 5

A Look at the Past: April 1900 ....................................................................................................................................... 6

Climate on the Web – National Climatic Data Center (NCDC) ..................................................................................... 7

Where Do Our Climate Data Come From? .................................................................................................................... 8

Climate Data in Use ...................................................................................................................................................... 10

For Teachers: Climatology – It’s Perfect for the Classroom ........................................................................................ 11

Folklore ........................................................................................................................................................................ 13

Quarterly Climate Review ............................................................................................................................................ 14

October 1999 ........................................................................................................................................................... 14

November 1999 ....................................................................................................................................................... 15

December 1999 ........................................................................................................................................................ 16

Water Year Precipitation, October-December 1999 ................................................................................................. 17

Colorado’s Climate, April-June – A Look Ahead .................................................................................................... 18

Are There Urban Heat Islands in Colorado? ............................................................................................................... 20

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Single issue price is$7.50.

Cover Photo: Lilacs thrive in Eastern Colorado despite the rigors of Colorado’s springtime climate. (Photo takenby Nolan Doesken)

A Time for Time Serieshere has been much talk recently of thetrend toward very mild winters here alongthe Colorado Front Range. Is it true? WillT

January averagemaximum and minimumtemperatures, 1889-2000, for Fort Collins,Colorado.

we all be playing golf in January from now on?The following graph shows a time series since

1889 of average January maximum and minimumtemperatures. The results are not as dramatic as youmight think. If you look closely, there is a bit of an

upward trend in January average minimumtemperatures, but not much change in averagemaximum temperatures. In many ways, the climateof the last 20 years resembles that of 100 yearsearlier. The wild ups and downs of the 1930s,1940s, and 1950s have not been repeated recently.

The lack of any extreme January cold since1979 is noteworthy. While 1934 and 1954 stillstand out as exceptionally warm individual years(and the January warmth those years carried on intothe spring and summer, as well), there have neverbeen three years in a row as warm as January 1998,1999, and 2000. Each year we figure the nextJanuary will be frigid – the law of averages takingover. But it just hasn’t happened yet. After threeyears in a row, I assure you there are a lot of carbatteries that have survived beyond their normallife spans. When the first arctic blast hits nextwinter, assuming it will, there will be an awful lotof cars in eastern Colorado that won’t start.

Next issue we’ll look at trends in Coloradocloudiness.

Colorado Climate 1

Colorado Climate Summary – Second Issue!

T he second issue of Colorado Climateappears after one of the warmest winters ofthe past century. Temperatures have

averaged greater than six degrees Fahrenheit above thelong-term mean at some locations in our state. Theabsence of cold, arctic high-pressure systems this pastwinter is a major reason for the warmer weather.

Has this arctic air disappeared? Well, when welook at weather maps for the entire NorthernHemisphere, we find that some locations, such asparts of Europe and eastern Russia, have had wellbelow average temperatures. The average over theentire Northern Hemisphere is actually close to thelong-term mean, and is quite a bit cooler than just afew years ago. The message for Colorado is thatnext winter may not be so warm!

In this issue we discuss whether we can predictthe future climate of Colorado. A short answer is notvery well! However, we need to still plan for anylikely future climate. There needs to be a reduction inour vulnerability to drought, for example, as summa-rized in our discussion in this issue of GovernorOwen’s Flood and Drought Preparedness Conference.

Both of us serve on the State’s Water Task ForceCommittee, which meets monthly when a water crisismay be developing. Resiliency to water shortages inColorado ranges from just a few months for drylandagriculture and wildfire applications, which are highly

affected by recent rains and snow, to as much as two tofour years for urban supplies from large reservoirsystems.

To the extent possible, which is environmentallyresponsible and fiscally possible, we need to managewater resources wisely if we are to continue to providewater for a growing population, for recreation, forenvironmental protection while still supporting aviable irrigated agriculture industry. As we reported inour first issue, we have not had significant statewidedroughts in the last 20 years or so. Earlier in the 1900sand the later 1800s, several long-term droughtsoccurred, including the Dust Bowl years in southeastColorado. Before historical weather records were kept,even more severe and longer duration droughtsoccurred. If we had a recurrence of even a historicaldrought, are we prepared since the state’s populationand our urban demand for water has grown? Thesubject of drought before European settlement ofColorado will be an article in one of our upcomingissues.

Enough said of this climate concern. We hopeyou enjoy our magazine!

Roger A. Pielke, Sr. Nolan J. DoeskenProfessor and Colorado Assistant StateState Climatologist Climatologist

The message for

Colorado is that next

winter may not be so

warm!

N

Governor’s Flood and DroughtPreparedness Conference

December 2-3, 1999, DenverNolan Doesken

ormally, it takes a major drought or adamaging flood to get large numbersof people to sit down together to

discuss problems, develop strategies, and formulatesolutions. It is even more unusual to discussopposites – floods and droughts – at the same time,especially when neither flood nor drought isoccurring at the time. That is why it was anunprecedented, or at least highly unusual, eventwhen more than two hundred water planners,elected officials, and government and businessleaders from across Colorado gathered in Denver inearly December to discuss strategies for preparingfor both floods and drought. This unique conferencewas convened by Colorado Governor Bill OwensDecember 2-3, 1999 at the Adam’s Mark Hotel nearthe State Capitol. The Colorado Department ofNatural Resources, the Colorado Department of

Agriculture, and the ColoradoDepartment of Local Affairs co-sponsored the event.

Sunshine greeted meetinggoers on the 2nd, but in theColorado tradition, this gave wayto a moderate snow on the 3rd thatsignificantly reduced second dayattendance. The meeting beganwith a welcome and introductionby Greg Walcher, ExecutiveDirector of the Colorado Depart-ment of Natural Resources.Governor Bill Owens thenaddressed attendees with a reviewof recent catastrophic floods inColorado and a discussion of

(continued on page 2)

Tom McKee, former StateClimatologist, takes abreak with Joe Garner ofthe Rocky MountainNews (photo courtesy ofCat Shrier, ColoradoWater ResourcesResearch Institute).

2 Colorado Climate

water demands and the apparent growing droughtthreat. Tom McKee, former State Climatologist,followed with a scientific overview of the causesand characteristics of floods and drought in thestate. An economic assessment of flood and droughtimpacts was then offered by Nancy McCallin,Director of the Governor’s Office of State Planningand Budgeting. She recalled for the audience thatalthough drought has not been a big problem inColorado for nearly two decades, in the winter of1976-77 Colorado’s ski industry and relatedbusinesses lost 40 percent of their normal revenuedue directly to the lack of snow.

The keynote luncheon speaker was a familiarface to many. Former U.S. Senator Hank Brown,currently the President of the University ofNorthern Colorado in Greeley, reminded listenersof the nature of Colorado’s water supplies. The vastmajority of water supplies are only available duringthe brief snowmelt runoff period from April to earlyJuly. Without major water storage projects tocapture this water for use at other times, otherplaces, and in subsequent dry years, Coloradowould not be the diversified state that it is today.

Brown was not subtle about expressing hisviews. He stated clearly what he believes is an

urgent need to build new and expand existing waterprojects and potential transbasin water diversions tosustain a successful agriculture industry in Coloradowhile meeting the water needs of growing urban areasthrough the 21st Century. He felt this was the onlyprudent way to meet human water needs while stillproviding flexibility for meeting environmentalchallenges and interstate agreements. This would needto be done without the financial assistance of theFederal Government, which up until now has heavilysubsidized most major water development projects inthe West.

During the afternoon of December 2, concurrentsessions were held addressing flood issues, droughtissues, and mitigation plans. This format allowed avariety of specialists to share their experiences.

On the second day of the conference, speakersrepresenting federal perspectives on drought and floodmitigation spoke at a special breakfast panel discus-sion hosted by State Representative Brad Young. Thiswas followed by presentations by local officials fromoutside of Colorado who have recently faced major

flood and drought disasters. Participants then broke intosmaller groups focused on drought and flood issuesspecific to particular stakeholders: Environmental,Business, Water Management, Agriculture, County andMunicipal. Each group attempted to list and prioritizethe hurdles and potential mitigation strategies involvedwith planning for floods and drought.

Want more information?A comprehensive summary of this conference and

its recommendations are available on CD-ROM fromthe Colorado Department of Natural Resources, 1313Sherman Street, Room 718, Denver, Colorado, 80203,phone: (303) 866-3311.

A brief summary was published in theFebruary 2000 issue of Colorado Water, thenewsletter of the Water Center of Colorado StateUniversity. Copies can be obtained by contactingthe Colorado Water Resources Research Institute,Colorado State University, Fort Collins, Colorado80523. Phone (970) 491-6308, fax (970) 491-2293,or e-mail CWRRI@ColoState.edu.

Governor’s Preparedness Conference (continued from page 1)

Tom McKee and NolanDoesken listen toquestions from the press(photo courtesy of CatShrier, Colorado WaterResources ResearchInstitute).

Colorado Climate 3

TGrowing Season Trends Across Eastern Colorado

Roger A. Pielke, Sr. and Nolan Doesken

here is considerable interest in trends inclimate across Colorado. Is our climatewarming or cooling, for example. In a

recent study (Pielke et al. 2000), in collaboration withTom Stohlgren, Bill Parton, Lisa Schell, and JohnMoeny of the Colorado State University NaturalResource Ecology Lab (NREL) and Kelly Redmond ofthe Western Regional Climate Center in Reno, Nevada,we have investigated this question with respect to thegrowing season in eastern Colorado.

We have reproduced several figures from thatstudy for several observation sites in easternColorado. We chose sites with the longest set ofdata in order to see if long-term trends exist. Theconcept of statistical testing is used to estimatewhether the trends are likely to be real or not.

The results show large variability in time andin space across eastern Colorado. No one stationcould possibly capture this variability. The trend ingrowing season length, for example, for the period1940-1996 lengthened by 43 days according to thisanalysis, at one of the Agricultural ResearchStation’s sites (CPER), while it decreased by twodays at Rocky Ford. After the CPER site, thegreatest increase in number of growing season dayswas Fort Collins with 24 days. For their period ofrecord Wray had an increase of 14 days, while LasAnimas increased by 11 days, based on this trendanalysis.

We think we understand the lengthening ofgrowing season at Fort Collins since the city hasgrown substantially, resulting in an urban heatisland effect. The reason for the increase at theother locations is unknown. Over the century since1917, however, the figures show considerabledifferences in trends between the sites. There is atendency, if you clump the stations together, toconclude that growing season has lengthened, butthe stations at Akron and Rocky Ford show ashortening of growing season.

What can we conclude from such studies?First, no single station can indicate what the trendsare, even in a relatively homogeneous landscapesuch as eastern Colorado. Secondly, there aresignificant variations in the trends over time, aswell as differences across the region. This variabil-ity suggests that the climate in Colorado is stronglyinfluenced by local effects, as well as from anylarger-scale climate effects. Finally, trend analysescannot be used to extrapolate expected climate inthe future. If you started in 1940, for example, andtried to predict the climate from 1940-1950 basedon the trends prior to 1940, you would be off target! If

would be better to assume the variability observed inthe previous decades will persist into the future.

Of more concern are climate anomalies that falloutside of the existing climate record. Recent tree coreobservations from northern Wyoming suggest therewas a major drought in that region in the 1700s thatlasted for 100 years! We will talk more about that

Trends in number of growing season days per year (1940-1996) forseveral weather stations in eastern Colorado (from Pielke et al, 1999). Avalue of “P” less than about 0.2 suggests the trend is statisticallysignificant and “n” is the number of years with complete data.

Station Slope P< nCentral Plains Exp. Res. Sta. (CPER) 0.7488 0.001 57Fort Collins 0.4191 0.002 57Fort Morgan * 0.0672 0.5 43Akron 4E -0.0301 0.5 57Wray 0.3735 0.05 37Cheyenne Wells ** 0.1223 0.5 40Eads 2S 0.0824 0.5 41Holly * 0.1146 0.5 41Lamar 0.0920 0.5 51Las Animas 0.2439 0.2 45Rocky Ford 2SE -0.0419 0.5 56

* data from 1949-1996** data from 1941-1995

Trends in number of growing season days per year but for the period1970-1996 (from Pielke et al, 1999). A value of “P” less than about 0.2suggests the trend is statistically significant and “n” is the number ofyears with complete data.

Station Slope P< nCentral Plains Exp. Res. Sta. (CPER) 0.8443 0.1 27Fort Collins 0.4664 0.5 27Fort Morgan 0.5062 0.2 22Akron 4E -0.0342 0.5 27Wray 0.3941 0.5 17Cheyenne Wells 0.2110 0.5 20Eads 2S -0.2002 0.5 15Holly 0.4374 0.5 24Lamar -0.2526 0.5 24Las Animas 0.4842 0.2 21Rocky Ford 2SE 0.0402 0.5 26

(continued on page 4)

4 Colorado Climate

Growing Season Trends(continued from page 3)subject, and what we can do to reduce our vulnerability,in our next issue!

Reference: Pielke, R.A. Sr., T. Stohlgren, W.Parton, N. Doesken, J. Moeny, L. Schell, and K.Redmond, 2000: Spatial representativeness oftemperature measurements from a single site. Bull.Amer. Meteor. Soc., v.81, #4, pp 826-830.

Wray, CO

Colorado Climate 5

T

Can We Predict Colorado Climatein the 21st Century?

Roger A. Pielke, Sr.

here is certainly a lot of news in the mediareporting on gloom and doom for ourweather in the coming decades. It seems

that some scientists have concluded that increases incarbon dioxide will produce a positive feedback withother components of our climate system such that theEarth’s atmosphere will warm and our climate willchange. They base their conclusions on the knowledgethat carbon dioxide is one of the “greenhouse gases.”

A greenhouse gas, however, is actually aninaccurate analog to how a real greenhouse works.An actual greenhouse works by trapping heat whenthe glass roof is closed. Sunlight enters, but theheated air cannot easily escape. A greenhouse gas inthe atmosphere, however, works by absorbingupward propagating radiative heat, and re-emittingsome of it downward.

Carbon dioxide is a greenhouse gas, but it isalso an essential gas for photosynthesis in plants.Moreover, the largest greenhouse gas is watervapor. Water vapor, of course, converts to cloudsand precipitation, so that the net effect of the threeforms of water in the atmosphere (as a gas, a liquid,or a solid) is not obvious. The latest researchsuggests that on the global scale, the effect ofclouds is to cool our atmosphere slightly.

The conclusion that we can predict the futureclimate is based on an increase of sea surfacetemperatures, as the increased carbon dioxidewarms the atmosphere. However, an enrichment ofplant growth due to the increased carbon dioxideand/or an increase in cloud cover could prevent theatmosphere from warming due to the greenhousegas effect of CO

2.

These two feedbacks (the biological effect ofincreased CO

2 and changes in cloud cover) are just

two of the feedbacks in the climate system whichmake prediction so difficult, if not impossible. Weare naïve if we think that human influences cannotalter the earth’s climate. But understanding all thecomplex interactions of water, air, plants, andanimals is quite a challenge.

While we may have great difficulty accuratelypredicting long-term climatic trends, progress will bemade in anticipating some of the shorter term(weeks to months) variations in temperature andprecipitation patterns. The last 20 years havebrought some successes in relating long-memoryocean currents and oceanic temperature patterns torelated weather patterns. The El Niño phenomenon,for example, has shown that there are times whenaccurate forecasts of temperature and precipitation

From Pielke, Sr., R.A,and L. Guenni, 1999;Vulnerability assessmentof water resources tochanging environmentalconditions. GlobalChange Newsletter, No.39, September, pp. 22-23.

anomalies can be made several months to a year inadvance for some portions of the world. Continuedslow progress in this type of climate forecast can beexpected in the 21st century, but Colorado is too farfrom the ocean to expect much accuracy in theseforecasts.

The conclusion of the Colorado ClimateCenter is that we cannot predict Colorado’s climatein the future. It is misleading to the public tosuggest we can. However, we can inform as to whatthe climate of the past has been, and how currentweather patterns fit into this climate history. Forinstance, are we having above or below averagetemperatures, or is the current weather abnormal?

As an alternate to prediction, we propose avulnerability perspective where assessments aremade as to what are the environmental risks inColorado. The figure displayed with this articleillustrates this approach for water resources. Usingthis approach, we can assess, for example, what areour risks if the 1930s dust bowl drought reoccurred,with our current population and water uses. We donot need a prediction to be concerned about thisthreat!

Predictability requires:– the adequate quantitative understanding of these interactions– that the feedbacks are not substantially nonlinear

Figure 1. Use of ecological vulnerability/susceptibility in environmentalassessment (adapted from Tenhunen and Kabat, 1999)

NaturalLandscape

Change

ClimateChange &Variability

LocalHuman

Population

Animal andInsect

Dynamics

LandManagement

Practices

Vulnerabilityof Water

Resources

6 Colorado Climate

A Look at the Past: April 1900Nolan Doesken

he older I get, the more interested I becomein our history and how we have arrivedwhere we are today. As a climatologist, I

guess I qualify as a historian in my own right – aweather historian. We have the advantage here ofmany decades (back to the 1870s and 1880s) of handwritten as well as published and digitized weatherrecords from all portions of Colorado that we can lookat any time we please. What these data show is thatthere may be some subtle changes that have occurredin our climate over the period of instrumental climateobservations back into the 19th century. However, forthe most part, the climate that we face today is largelysimilar to what our relatives faced generations ago asthey discovered and settled this fine state in search of abetter life.

Colorado was well on its way to becoming asettled state back in 1900. We had already been astate for 24 years. There were approximately540,000 people (from Population Abstract of the U.S.1900 Census) living in Colorado and perhaps 3.8million cattle and sheep. Trains could get you almostanywhere in the state. Miners were still looking forgold and silver, but agriculture was the state’s realhope for the future. Several dams had already beenbuilt, and hundreds of miles of irrigation ditcheshad already been dug to move water to fertile landsfarther and farther from the immediate river bottoms.

Leading up to April 1900, Colorado’s weatherhad been on a wild roller coaster ride. February 1899had been the coldest month in history for much of thestate. Fort Collins, for example, had a stretch of 6 daysin a 9-day period in early February with lowtemperatures at or below –30 degrees Fahrenheit.Incredible snows piled up in the mountains, cuttingoff fuel and food supplies to mountain communitieslike Aspen. In March 1899, the Ruby mining campnear Crested Butte reported 254 inches of snow inone month. That remains the state monthly snowfallrecord to this day. In comparison, the winter of1899-1900 was very mild. Fuel supplies were

T generous and the half-million residents of Coloradowere very thankful.

The thankfulness gave way to anxiety, however,as little snow fell in the mountains, and warm early-spring temperatures evaporated what had fallen. Forthe 5-month period, November 1899 through March1900, Durango received only about 30 percent oftheir average precipitation. Drought reared its uglyhead.

Then along came April 1900. Weather patternsshifted and one storm after another moved slowlyacross the state, each picking up moisture from theGulf of Mexico as they reached eastern Colorado. Inall, five major storms, each lasting about three dayswith only one to three dry days until the next storm,brought widespread and heavy rains and wet snows. Inmany ways, it was not unlike the weather pattern oflate April 1999, which brought flooding to southeast-ern Colorado.

In 1900, there were about 80 weather stationsreporting monthly precipitation. The statewideaverage precipitation for the month came to nearly sixinches, nearly double the previous wettest month sincesystematic weather records began in the 1890s. Thegreatest precipitation totals were found east of themountains. Monthly totals exceeded ten inches fromPikes Peak eastward to Hugo and Burlington. Totalsalso surpassed the ten-inch mark in the Fort Collinsarea, in extreme southeastern Colorado, and in thefoothills west of Denver. Lake Moraine, on the slopesof Pikes Peak, recorded 16.52" of precipitation for themonth of April 1900. It was one wild month there, asmost of this moisture fell as wet snow.

This truly remarkably wet month pulled Coloradoout of the grasp of widespread drought, and resulted inplentiful water on the South Platte and ArkansasRivers for the first few months of the growing season.It was only a few months, however, until dry weatherand expanding drought conditions again tookcommand.

Leading up to April

1900, Colorado’s

weather had been on

a wild roller coaster

ride.

Colorado Climate 7

Climate on the WEBNolan Doesken

Let’s visit the National Climatic DataCenter at: http://www.ncdc.noaa.gov

for over 50 years, can be obtained. You can getprocessed radar data from anywhere in the countryfrom last summer’s thunderstorms, or you can acquiretabulations of data from 150 years ago from the variousforts that were established across the western states inthe years following the Louisiana Purchase. Most likelythey can tell you what the weather was like on the dayyou were born, and possibly even the day when yourgrandmother was born. You can even find weather datacollected by sailing ships travelling from New York toSan Francisco before the days of the Panama Canal.

I encourage you to visit the NCDC website. It willgive you an idea of the magnitude of our nation’sclimate data resources. For the weather junkie whocares about what has happened in the past, this place isparadise. But be careful. For many long-term data sets,there are sizeable fees for extracting data. Don’t startfilling your e-shopping cart with fantastic climate dataunless your credit card is handy. Increasingly, agenciesneed to collect revenue to cover data storage, handling,and dissemination costs. Yes, the data are free. Our taxdollars paid to gather it. However, keeping it andmaking it available to users like you and me is not free.Expect some fees along the way.

ou may not know it, but when you type inthis address you have made contact with thecomputer system of a remarkable organiza-

tion located in a modern federal building in downtownAsheville, North Carolina. Asheville, the home of theVanderbilt Estate, is nestled between Great SmokyMountain National Park and Mount Mitchell, thehighest peak in the southern Appalachians, reaching aheight of 6,684 feet above sea level. Ashevilleexperiences a humid, hazy climate, but with mildwinters and “relatively cool” summers, at least whencompared to the Carolina lowlands. Many back Eastfind it to be a very delightful climate.

The National Climatic Data Center (NCDC) is thehome of immense volumes of temperature, precipita-tion, snow, humidity, wind, pressure, and any other sortof weather data you can imagine. Not only do data fromall 50 states reside there from last month, last year, lastdecade, and the last century, so do the weather datafrom most countries in the world. In addition tostandard surface weather observations, many otherspecial data sets exist at NCDC. Weather data, collectedby high-flying balloons that have been launched now

Y

NOAA National Data Centers (NNDC)Contact Information

http://www.nndc.noaa.govNational Oceanographic Data Center (NODC)Telephone: 301-713-3277 Fax: 301-713-3302

E-mail: services@nodc.noaa.govhttp://www.nodc.noaa.gov

National Geophysical Data Center (NGDC)Telephone: 303-497-6826 Fax: 303-497-6513

E-mail: info@ngdc.noaa.govhttp://www.ngdc.noaa.gov

National Climatic Data Center (NCDC)Telephone: 828-271-4800 Fax: 828-271-4876

E-mail: orders@ncdc.noaa.govhttp://www.ncdc.noaa.gov

We interpret the present in the context of the pastfor the prediction of the future.

8 Colorado Climate

IWhere Do Our Climate Data Come From?

Nolan Doesken

t is an interesting world we live in. If you havea trained eye for spotting weather stations, ithas gotten to the point that it is difficult to drive

Instead of tapping into the ever-growing list ofautomated weather stations scattered across Colorado,we are still relying on the data from the CooperativeProgram of the National Weather Service.

Why do we use the data from this seeminglyoutdated network? There are several good reasons.

First and foremost, this is the oldest continuoussource of weather data from both urban and rurallocations in Colorado. Data are collected in asimilar manner today as 100+ years ago. Thismakes comparisons with the past possible. Manystations have been operated at or near the samelocation and with the same instruments for severaldecades. This is very important for historic climatecomparisons. You can’t do that with the automatedweather station along the Interstate Highway.

Second, temperature readings are likely to beconsistent. Three major sources of variability intemperature readings are 1) the height aboveground, 2) how the thermometer is shielded fromthe sun, and 3) the local instrument location andexposure. For more than 100 years, the NationalWeather Service Cooperative Observer Programhas striven for consistency in each of these areas.Meanwhile, other weather stations use a variety ofradiation shields for keeping sunlight off theirtemperature sensors. Some of these shields havenever been tested and compared to official instru-mentation.

The Cooperative Observer Program, with itssimple but rugged large diameter (eight inches)standard rain gauge, is the most accurate source ofyear-round daily precipitation measurements forboth rain and water content of snow and ice.

The Cooperative Program is now the onlysource of daily snowfall observations nationwide.Snowfall is not measured by most automated weatherstations.

Finally, data collected by the CooperativeObserver Program have a personal human touch. Insome ways this can be a problem since humansmake errors and aren’t always reliable. However,the human touch adds something that no computercan, such as useful personal remarks that helpexplain and interpret what certain weather condi-tions were really like and what impacts they had inthe community.

The Cooperative Program has survived morethan 100 years and has provided valuable data forapplications that those who began this network inthe 1800s could never have imagined – informationto design the Interstate Highway System, for layingfiber optic lines, and for determining the reliability ofmicrowave communications.

10 miles without seeing a weather station somewherealong the highway or out in a field, or on garage roofs,or at schools, or along rivers. You may even be drivinga mobile weather station. Many new cars are equippedwith on-board thermometers – a great way to study theclimate of Colorado. Yes, there are some parts of thestate, like out west of Maybell, or down south of LasAnimas, where weather stations are still a rarity. But ifyou’re in any of the populated areas or transportationcorridors, you’re going to see weather stations if youlook. It’s a new game you can have your kids play asyou drive cross-country – spot that weather station.

In this age of technology, you probably assumethat all the maps and data graphs we show in thisreport are all based on fancy electronic weather data

gathered by computerized weather data collectionsystems at airports, schools, mountain observato-ries, and cell-phone agricultural weather stations.Actually, nothing can be farther from the truth. Wedo utilize data from these many electronic weatherstations practically every day. But when it comestime to monitoring our climate; to compare thisyear to last, or this decade to the climate of manyyears ago; we revert back to one of the best andlongest-lived observing networks in our country.Most of the data we show in our reports come fromvolunteer weather stations – in people’s yards, atlocal businesses, at water and sewage treatment plants,and at reservoirs, universities, and National Parks.

Harold Reinhart,cooperative weatherobserver, Crestone, CO

Colorado Climate 9

As we end the 20th century (the new millenniumactually begins January 1, 2001), the CooperativeProgram remains the best source of statewide andnationwide data for monitoring the basic elements oftemperature, precipitation, and snowfall on a nationalscale.

Oh yes, there are some problems. Stationlocations are not all appropriate. Too often there ismissing data. Observers have varied daily observa-tion times which result in data inconsistencies. Buteven with these problems, there is still no morereliable and accurate measurement of precipitationthan what you get from an enthusiastic volunteerweather watcher equipped with one of the NationalWeather Service’s traditional standard high-capacity rain gauges.

We are currently working with the NationalWeather Service locally and nationally to raisemore support for this remarkable network ofvolunteers. After operating almost 110 years, theprogram could use some upgrading. There are stillthousands of citizens nationwide willing to serve asvolunteer weather observers, but people don’t stay athome 365 days of the year like they used to. Withmodern technology, it should be easy to measureand record elements like daily high and low

temperatures even when an observer is away for theweekend or for a week. It also needs to be easierand more rewarding for volunteers to report theirdata to the National Weather Service (and subse-quently, to organizations like ours) on a daily basis.However, it is also imperative that the instrumentalreadings of the future be as accurate as possible andconsistent with the measurements taken in the past.This is not as easy as it seems. Few people appreci-ate that replacing a rain gauge or installing a newthermometer, or moving a weather station from oneside of town to another (or one side of a building toanother, for that matter), frequently results in anapparent change in the climate. There is more tocollecting accurate weather data than most peoplerealize. Much of the value of the CooperativeObserver Program has been the historical consis-tency of data. We must be careful as we attempt to“modernize.”

If you would like more information about theCooperative Program, or if you would like to helpraise support for this incredible nationwidevolunteer program, please contact our office. Also,visit the Program’s website at:http://www.ccop.nws.noaa.gov

Cooperative ObserverLocations for Colorado.T = Temperature andprecipitation stations.O = Precipitation stationsonly.

WV M/IV F/AP EPWestern Mountains/ Foothills/ EasternValleys Interior Valleys Adjacent Plains Plains

10 Colorado Climate

EClimate Data in Use

Nolan Doesken

Now let’s get into the fun stuff. Here and in futureissues of Colorado Climate we will describe in briefdetail some of the interesting climate applications thatwe’ve been involved with.

As you know, Colorado has several of thehighest elevation heavily-traveled mountain passesin North America. Truckers don’t care for thesepasses, but it’s not just the steep roads, the snowand ice, and scary downhill grades that causeproblems. Several years ago a large nationaltrucking company called our office. Some of theirshipments of bottled liquids and spray cans fromproducers in the Midwest were found to bedamaged when they reached their destinations inUtah. Quite frankly, some of the containers hadexploded in their cases in the trucks. They neededto figure out what happened. Was the truckernegligent, was the product faulty, or did somethingelse happen? With closer scrutiny, it appeared thatthe containers most likely popped somewhere inColorado – most likely on I-70 somewhere betweenDenver and Grand Junction.

By now you’ve probably already guessed thereason. Between Kansas City and Denver, theactual pressure (not the barometric pressure thatyou see on TV or hear on the radio, which iscorrected to sea level) drops gradually from anaverage of about 29 inches of mercury at Kansas City(980 millibars) to less than 25 inches of mercury atDenver. This drop is a result of the gradual rise inelevation that occurs. However, from Denver west-ward, this change is no longer gradual. From Denverto the Eisenhower Tunnel, the average station pressuredrops sharply to about 20 inches of mercury at thetunnel and even lower atop Vail Pass. On top of this,passing storm systems impose an additional butrelatively minor pressure variation. With the drop ofexternal atmospheric pressure as the trucks climbedthe Rockies, the pressurized containers simply popped.It wasn’t the trucker’s fault.

The solution that this trucking company chosewas to route later shipments via I-80 instead ofI-70. The shipping route was longer, but theelevations were lower. An alternative solution was aredesign of the containers by the manufacturer sothat they could withstand greater variations inoutside pressure. That was a more costly solution,since only a small fraction of their product nationallyever had to withstand these lowered pressures. Wehaven’t heard back from them.

veryone seems to know and appreciate howfluctuations in daily weather affect businessand transportation, but few people stop to

As you know,

Colorado has several

of the highest

elevation heavily-

traveled mountain

passes in North

America. Truckers

don’t care for these

passes, but it’s not

just the steep roads,

the snow and ice, and

scary downhill

grades that cause

problems.

think just how important climate information, deducedfrom weather data collected for many years, really is inconducting business.

In each issue of Colorado Climate, we willbriefly describe some of the ways climate data are putto use here in Colorado. You may be surprised. Evenafter more than 22 years here at the Colorado ClimateCenter, one of the most interesting parts of my job isanswering phone calls and e-mails from businessesneeding climate information to do their jobs or to dothem better. Every call or e-mail brings a newchallenge, and offers a new opportunity to utilize themany years of data we have collected to help make animportant decision. It convinces me of the great valuein carefully collecting weather data each and everyday, even when we’re not sure anyone cares. Eventu-ally, if not today, they will.

Let’s begin with some of the more common typesof questions we receive. This doesn’t mean theanswers are easy, but these are questions that are askedvery often.

• I am planning to move to Colorado fromAny Town, USA. Could you tell me howthe climate of (where I am moving to)compares to the climate of (where I ammoving from)?

• We are considering raising a new crop. Dowe get enough precipitation at the mostcritical times to get a profitable yield, andhow often will we be frozen out by shortgrowing seasons?

• I (or a member of my family) have asthma.Where could I live where the climate would bebetter?

• How often will we experience drought?

• We had a bad flood. How much rain fell toproduce that flood? Was it greater than a 100-year storm?

• We are installing a new water line. How deepdo we need to bury it to be sure it will neverfreeze during the winter?

• Our construction company is bidding on a jobin Breckenridge (or any other place). Howmany adverse weather days can we expect thatmight delay construction?

Colorado Climate 11

I

Average monthly precipitation, 1961-1990, inches.(Remember that precipitation includes both rain and the melted water content of snow)

Steamboat Springs SterlingMonth (elev. 6,760 Ft) (elev. 3,938 Ft)January 2.36 0.33February 1.99 0.22March 2.04 1.04April 2.18 1.37May 2.11 3.16June 1.52 2.91July 1.53 2.62August 1.48 1.87September 1.64 1.03October 1.87 0.79November 2.09 0.49December 2.57 0.33Annual Total 23.38 16.16

(continued on page 12)

For Teachers: Climatology –It’s Perfect for the Classroom

Nolan Doeskent’s not surprising, but most students, be theythird graders, seventh graders, high schooljuniors, or even college students (and some-

times even high ranking government officials), thinkthat the only reason we collect weather data is to predicttoday’s and tomorrow’s weather. I agree that this is avery important reason for having networks of weatherstations. But that is not the only reason. Data, whencollected over time, allows us to define and describe ourclimate. The climate, in turn, plays a large role indetermining how we live, where we grow our food,how much energy we consume to keep our homes andbusinesses comfortable, and much more. I happen tobelieve that climatology (the study of climate) shouldhave a place in the classroom. Some might call it boring(like my college advisor who tried to talk me out ofpursuing a career in climatology), but I disagree.You just have to present it with enthusiasm andreal-world applications.

Here in Colorado, every county, every city, andeven different parts of some of the larger cities havefeatures of their climate that may differ from nearbyareas. For example, south and west Denverroutinely get more snowfall than northeast parts ofthe city. Certain portions of Boulder and ColoradoSprings are much more likely to experience verystrong winds than others. Any city that occupiesboth the bottom and the sides of a valley or ridgewill likely see dramatic local differences innighttime temperatures.

Some very simple comparisons of basicclimate elements like snowfall, precipitation, ortemperature can make a great lesson that caninclude some math, some geography, and evensome history to go along with the science. You canget a little practice on the computer, too, if youwant.

Here’s a fun little project. Perhaps you can useor adapt this in your classroom.

Let’s look at two cities in Colorado thatare both at about the same latitude –Steamboat Springs and Sterling.

Plot these monthly totals (see table at right) on abar graph using either graph paper, home-made graphpaper, or your computer. Look at the differences andthen try to answer these questions.

1) Which location receives the most precipita-tion over the entire year?

2) Which location gets the most precipitationin winter?

3) Do you think that the precipitation falls as rainor snow?

4) How would you convert these numbers frominches into centimeters? Millimeters?

5) Express as a percentage, how muchprecipitation Sterling gets compared to SteamboatSprings.

6) What might be different about houses,transportation, occupations, and recreational

COLORADO

WY NE

KS

OKNMAZ

UT

12 Colorado Climate

For Teachers (continued from page 11)

activities between residents of these two Coloradocities?

7) What meteorological explanations can you givefor why one city gets much more precipitation duringwinter and the other gets much more precipitationduring summer?

8) What about water resources? What can welearn about Colorado water management from thissimple graph?

Here are some answers and somediscussion items.

1) Steamboat Springs receives the mostprecipitation.

2) Steamboat Springs receives the majority of itsprecipitation during the winter months at the sametime that Sterling receives very little precipitation.

3) You cannot tell just by looking at these graphswhether the precipitation falls as rain or snow.However, from what we know about geography andtemperature, we know that most precipitation inSteamboat Springs for about half the year (lateOctober into mid April) falls as snow. Sterling alsoreceives some of their precipitation in the form ofsnow, but very little moisture falls in Sterling duringthose months when it is cold enough to snow. Themajority of Sterling’s annual precipitation falls as rain.

4) There are 2.54 centimeters (approximately) inone inch. There are 10 millimeters in one centimeter,which means that there are 25.4 millimeters in oneinch. To convert inches to centimeters or millimeters,multiply the number of inches for any month by theunit multiplier.

For example, if May precipitation averages 3.16inches in Sterling, this means that the May precipitationin Sterling averages 3.16 inches times 25.4 millimeters/inch = 80.264 millimeters (you can also practice yourrounding). There is usually no reason to reportprecipitation to increments of less than 1 millimeter orless than 0.01 inches.

5) Sterling Precipitation/Steamboat SpringsPrecipitation times 100% = 69.1189.

You will get the same answer regardless of theunits you are using. You will get a different answer,however, for each month. Annually, Sterlingreceives 69% as much precipitation as SteamboatSprings. However, in July, Sterling receives 171% asmuch as Steamboat Springs. You could also say thatSterling averages 71% more precipitation thanSteamboat Springs in July.

6) The precipitation graphs alone do notprovide enough information to answer thesequestions, but they can serve as the beginning ofthe discussion. More geographic information isneeded to round out this discussion. Residents ofSteamboat Springs are accustomed to deep snow,while Sterling residents expect dry winters. Housesand businesses in the Steamboat Springs area arebuilt with stronger roofs to withstand the weight ofthe snow. The Steamboat Springs economy is basedon the winter recreation industry and someremnants of the cattle ranching business that still exist.Sterling remains an active agricultural economy. Sincemost of Sterling’s precipitation falls during thegrowing season, many crops can be grown. Livestockare also important to the economy.

7) We could teach a whole class on weather andclimate as we try to answer this question. To put itsimply, the reason is “mountains” and “air masses.”Here in the mid latitudes, the air up at mountain-toplevel and higher moves primarily from west to east. Itmoves faster in the winter when the temperaturedifferences are greatest between the equator and thenorth pole, and much slower in the summer whentemperature variations are small. The air moving fromthe west to the east crosses the Pacific Ocean bringingplentiful moisture into the West Coast during thewinter. These Pacific air masses drop much of theirmoisture in the mountains of Washington, Oregon, andCalifornia, but still have enough moisture to dropmore snow as the air is forced to rise and cool as itencounters the Rocky Mountains. Steamboat Springssits at the base of a mountain range that standsperpendicular to these winter winds. This is whySteamboat Springs gets so much winter precipitation.The same air masses continue eastward towardsSterling, but after rising over the continental divide,the air descends, warms, and dries rapidly. Areas east

Average Monthly Precipitation

Steamboat Springs Sterling

Colorado Climate 13

of the mountains are “downwind” of the Pacificmoisture source and are normally very dry in thewinter.

During the spring and summer, the weatherpatterns change as the westerly winds aloft weaken.Moist air from the Gulf of Mexico and the southernand central plains states occasionally driftswestward into eastern Colorado. This moisturefuels large thunderstorms that can drop heavy rainsin short amounts of time. But this moisture isblocked by the Rocky Mountains and usually doesnot reach the Steamboat Springs area in quantity.The result is that Sterling receives much moresummer rainfall than Steamboat Springs. Stormscan be so heavy as to cause major flash floodingnear Sterling. Summer storms west of the moun-tains usually do not produce such heavy down-pours.

8) Again, this could be an entire class period ormore. This would be a good opportunity to contact alocal water official – someone from a local waterconservancy district or someone with the State ofColorado or one of the federal agencies like the U.S.Bureau of Reclamation. There are many water officialsin Colorado – many more than climatologists. Try toget one to visit with your class. Water resources issuesin Colorado are absolutely fascinating, and what we

have done to provide water for agriculture andurban development may amaze your students andgive you a whole lot to talk about – includingenvironmental issues and endangered species. Thesky’s the limit on this topic.

What have you accomplished?• Learned to access data on the web.• Learned to make graphs by hand or by computer.• Learned to interpret graphs and apply the informa-

tion.• Opportunity to discuss precipitation processes

and air motion.• Chance to talk about geography, economy, and

water resources.

This very simple exercise could be adapted toalmost any age group. If you want to do more, orgive motivated students special science projects,you could dig into the Colorado Climate Centerdata archives on the Web to extract year-by-yeardata for these two stations. Ask students to see howmuch precipitation varies from year to year and ifprecipitation totals have been changing over time.You could even explore known flood or droughtperiods and determine how much precipitation fellthen.

Folklore

“A cold April the barn will fillA dry April not the farmer’s will”

“Snow in April is manure”

“April cold and wet fills barnand barrel”

Taken from “Weather Proverbs” Signal Service Notes No. IXby H.H.C. Dunwoody, U.S. Government Printing Office,Washington, DC, published in 1883.

A cool wet spring means good crops ahead. This was more an observation than a forecast –and there was a lot of truth to it and still is. Delaying fruit tree blossoming spares early freezedamage. Cool, damp weather results in better soil moisture reserves to carry through dry summerperiods. Farmers for centuries have witnessed the advantages of cool, wet springs. While it keptthem from getting into the field as early as they may have liked, it paid off at the end of the seasonmore often than not. Spring 2000 is off to a warm start. If you believe the old-timers, this does notbode well for Colorado agriculture this year.

14 Colorado Climate

Quarterly Climate ReviewOctober 1999Climate in perspective:

October weather was truly delightful. There werean abundance of warm, sunny days, deep blue skies,dry air, light breezes and cool, crisp nights. Thepredominantly dry weather made it a great month forlate-season golfing, mountain hiking, biking, or outdoorprojects at home. Huge day-night temperature variationsmade it very interesting to figure out how to dress.Forty to fifty degree (Fahrenheit) daily temperatureswings were common. On October 1, the Puebloairport started the day at 31° F but managed to reach90° F by afternoon, only to drop back to near freezingthat night. Gunnison saw the temperature drop from ahigh of 64° down to a morning low of +9° F onOctober 25.

Precipitation:Most of Colorado was much drier than average

in October with many areas well below 50 percentof average. Little or no precipitation fell acrosssouthwest Colorado at a time of year that issometimes quite wet there. Northeastern Coloradowas also very dry in October. The only relativelywet areas of the state were found along theColorado Front Range, the urban corridor, andsoutheastern counties. Extreme southeast Coloradoreceived more than double the average precipitationwith most of this coming in one storm October 7-8.

Temperature:October temperatures ended the month near to

a little below average over eastern Colorado whilewestern Colorado was a little to as much as fourdegrees Fahrenheit above the 1961-1990 averages.

Daily Highlights1 Dry and warm with increasing clouds.

Turning cooler over northeastern counties.2-3 Warm and dry over western Colorado, but

sharply cooler east of the mountains withsome low clouds and fog patches on the 2nd.

4-5 Warm and dry again, but with increasing cloudsfrom the southwest on the 5th.

ere is a review of the first three months ofthe 2000 water year: October, November,and December, 1999. In each future issue

of “Colorado Climate” we will review recent months interms of precipitation patterns and temperatureanomalies. By the time you get this publication, these

H

> 200%

150-200%

100-150%

50-100%

<50%

October 1999 precipitation as a percent of 1961-1990 average.

BelowAverage

0 to 2

2 to 4

>4

October 1999 temperature departures from 1961-1990 average, degrees F.

months will be long gone and fading from yourmemory. But hopefully these brief narratives withaccompanying graphs and maps will serve to documentinteresting climate features for posterity.

October 1999October 1999October 1999October 1999October 1999DescriptionDescriptionDescriptionDescriptionDescription StationStationStationStationStation ExtrExtrExtrExtrExtremeemeemeemeeme DateDateDateDateDatePrecipitation (day): Walsh 2.60" Oct. 8Precipitation (total): Walsh 3.70"High Temperature: Stratton & 94 F Oct. 14

Las Animas 94 F Oct. 14Low Temperature: Hohnholz -9 F Oct. 17

Ranch

Colorado Climate 15

> 150%

100-150%

50-100%

<50%

November 1999 precipitation as a percent of 1961-1990 average.

6-8 A significant storm system with a generousmoisture supply approached from the southweston the 6th. Fairly lively thunderstorms for so latein the season developed on the afternoon andevening of the 6th and moved northeastward.Thunderstorms continued into the morning ofthe 7th in northeast Colorado. Later on the 7th,moderate to heavy rains moved across southeastColorado with very heavy rains over BacaCounty. 3.50 inches of rain were reported atthe Walsh weather station.

9-14 Sunny, dry and warm weather statewide.15-19 Cool and unsettled. Western Colorado missed

out on much precipitation, but easterly“upslope” winds helped generate widespreadprecipitation along the Front Range in twoseparate surges. The first event began on the15th in northern Colorado and spreadsouthward, eventually ending over southernColorado early on the 17th. Six inches of wetsnow fell in Boulder on the 16th. Rain andwet snow developed in the same areas againon the 18th ending on the 19th with theheaviest amounts falling southwest ofPueblo.

20-27 Sunny with warm, lovely days but coldnights. Nighttime temperatures dropped intothe single digits in some high mountainvalleys.

28-29 Windy and cooler as a Pacific storm systemspread precipitation across parts of north-western Colorado.

30-31 Sunny and pleasant.

November 1999Climate in perspective:

Winter was slow to get started as unseasonablywarm temperatures that felt more like September

November 1999November 1999November 1999November 1999November 1999DescriptionDescriptionDescriptionDescriptionDescription StationStationStationStationStation ExtrExtrExtrExtrExtremeemeemeemeeme DateDateDateDateDatePrecipitation (day): Ruxton Park 1.00" Nov. 21Precipitation (total): Ruxton Park 1.25"High Temperature: Las Animas 87 F Nov. 8Low Temperature: Walden -23 F Nov. 24

(continued on page 16)

<6

6 to 8

>8

November 1999 temperature departures from 1961-1990 average, degrees F.

persisted almost to Thanksgiving. The state was free ofany precipitation for the first 16 days of the month.Finally, a taste of winter arrived on the 21st in the formof sharply colder temperatures and a significantsnowstorm.

Precipitation:Almost all of Colorado was drier than average in

November, with the majority of the state receiving lessthan 50 percent of the 1961-1990 average. Many

16 Colorado Climate

weather stations in eastern, southern, and westernColorado totaled just a few hundredths of an inch forthe month. The only areas approaching averageprecipitation for the month were found in north centralColorado and near Colorado Springs. These areas wereparticularly hard hit by the Nov. 21-22 storm. TheColorado Springs airport reported 1.01 inches ofmoisture from 10.6 inches of snowfall.

Temperature:November temperatures for the month as a whole

were much above average statewide ranging from aboutfive degrees F warmer than average on the WesternSlope to more than eight degrees above average in someareas east of the mountains (one of five warmestNovembers on record at several locations).

Daily Highlights:1-17 Dry, sunny, and unseasonably warm with light

winds – a remarkable stretch of warm, dryweather for so late in the fall. Daily maximumtemperatures passed the 70-degree mark inDenver 13 days during this period, whiletemperatures soared into the 60s high in themountains. New daily record highs were set insome cities, especially on the 8th. There was nosign of mountain snows, and it was evendifficult for ski areas to effectively use theirsnowmaking facilities.

18 Windy and cooler as a Pacific cold frontfinally reached Colorado. Some valley rainsand mountain snows in northern Colorado.

19-20 Dry but seasonally cool as a new stormsystem developed.

21-22 The only winter storm of the month. 4-12" ofsnow fell along the northern Front Rangewith the heaviest amounts near ColoradoSprings.

23-25 Dry but cold, with some morning fog,especially over snow covered areas. Lowsfinally dipped below zero in the mountainswith single digits over portions of northeast-ern Colorado.

30-30 Warm and dry again, with melting snow. Not sowarm as earlier in the month.

December 1999Climate in perspective:

The stable and persisting weather pattern ofNovember gave way to more changeable, fastermoving systems in December, more typical of theseason. Pacific moisture made its way intoColorado’s northern and central mountains onseveral occasions bringing small doses of muchwelcomed snowfall. However, very little of thismoisture extended southward into the mountains ofsouthwest Colorado. Prevailing westerly windsaloft also helped generate downslope winds east ofthe mountains that helped raise temperatures andreduce humidity. One storm system managed todrop heavy snows east of the mountain crest insouthern Colorado early in the month.

Precipitation:December precipitation totals were again

below average over most of Colorado. Areas ofsouthwestern and eastern Colorado were particu-larly dry with almost no precipitation for the monthat places like Durango and Lamar. WhileColorado’s northern and central mountains faredbetter, only a small area near Ouray and in themountains near Meeker were actually aboveaverage for the month. The storm of Dec. 3-4targeted a few areas along the Colorado FrontRange leaving above average precipitation amountsfor December over areas south and west of Denverand along the east slopes of the Sangre de Cristoand Wet Mountains southwest of Pueblo.

Temperature:No true arctic air masses reached Colorado in

December. Relatively cold Pacific air massesdominated December weather patterns bringingseasonally cold temperatures to the central Rockiesbut with no dramatic extremes. East of the moun-tains, temperatures ended up above average again, withsome stations in northeastern Colorado at least sixdegrees Fahrenheit above average. Many stations innortheast Colorado had no low temperatures below+10° F all month.

December 1999December 1999December 1999December 1999December 1999DescriptionDescriptionDescriptionDescriptionDescription StationStationStationStationStation ExtrExtrExtrExtrExtremeemeemeemeeme DateDateDateDateDatePrecipitation (day): Ruxton Park 2.60" Dec. 2Precipitation (total): Ruxton Park 4.46"High Temperature: Kim & Pueblo 74 F Oct. 14

Reservoir Dec. 1Low Temperature: Sargents -28 F Dec. 14

Colorado Climate 17

Daily Highlights2-2 Still quite warm but with increasing clouds and

moisture. Mountain snows began on the 2nd

spreading to the Front Range.4-4 A major Front Range storm developed with

strong winds and blowing snow in someareas. The heaviest snow fell south and westof Denver on the 3rd, with heavy snowscontinuing from Pikes Peak southward toRaton Pass into the day on the 4th. Snowfalltotals exceeded two feet in some areas, withlocally more than three feet near Cuchara.

6-6 Mild and dry, but with cold nights in themountains.

10-10 A cold front approached on the 7th accompa-nied by light snow producing icy roads lateon the 7th and early on the 8th. Skies thencleared and temperatures dropped. Manylocations had their coldest temperatures ofthe month early on the 9th. Walden dropped to–21° F while Antero Reservoir recorded–23° F.

11-22 West northwesterly winds aloft pushed aseries of weakening Pacific storm systemstoward Colorado. Snows, mostly light, fellintermittently accompanied by normal rangesof temperature. The coldest days were the14th and 15th when daytime highs onlyclimbed into the teens in some mountaincommunities. Most snowfall was limited tothe northern and central mountains, butflurries and sprinkles spilled out onto theplains, especially 17-21st. Strong downslopewinds blew periodically through the periodalong and east of the mountains.

23-31 Clear skies with pleasant temperatures andclear holiday driving conditions. However,cold air settled into some of the broad snow-covered mountain valleys. Kremmling, forexample, had daily high temperatures only near20° F each day with overnight lows well belowzero.

> 200%

150-200%

100-150%

50-100%

<50%

December 1999 precipitation as a percent of 1961-1990 average.

December 1999 temperature departure from 1961-1990 average, degrees F.

BelowAverage

0 to 2

2 to 4

4 to 6

>6

Water Year Precipitation,October through December 1999

For the first three months of the 2000 water year,precipitation totals are well below average overpractically all of Colorado. Some locations in south-western Colorado have received less than 0.25 inchesof precipitation in these 3 months, making this an evendrier start to the water year than what they experiencedin the severe drought winter of 1976-77. Precipitationtotals are much better over the northern and centralmountains although all stations are still below the 1961-1990 average. East of the crest of the mountains,accumulated precipitation patterns are highly variable,reflecting the patterns from four storm systems thathave contributed most of the precipitation so far thiswinter. While most areas remain dry, above average

(continued on page 18)

18 Colorado Climate

on average, than any other month. The only otherportions of the U.S. where April is the snowiest monthof the year are some of the eastern slopes of mountainsin central Montana and eastern Wyoming such as theBig Horns near Sheridan, and the higher peaks of theBlack Hills in South Dakota. A snowstorm droppingtwo to three feet of new snow in 24 hours or lessoccurs about every other year along the Colorado FrontRange. Almost every decade there is at least one stormdropping four feet or more in one day. Colorado holdsvery few national climate records, but the amazing 76inches of snowfall in 24 hours recorded at Silver Lake,Colorado (in the Boulder watershed just west ofBoulder) back in April 1921 still stands as a NorthAmerican 24-hour snowfall record. It has beenchallenged a time or two by storms in the Sierras, thePacific Northwest, the Alaska panhandle, and the snowbelts of Lake Ontario, but more than six feet of snow inone day is a tough record to beat.

By late April, the snowpack at high elevationstypically reaches its greatest depth of the season.Skiers and snow boarders are still traversing thehighest slopes while flowers and fruit trees areblossoming in the surrounding lower valleys. Aprilis a very critical month for Colorado’s WesternSlope fruit industry as growers revel in the warmsunshine but fear the dreaded late frost that can endthe entire fruit season in one cold night. Meanwhilein eastern Colorado, freezing nighttime tempera-tures are still fairly common, but daytime tempera-tures are plenty warm for fieldwork. Mile uponmile of wheat shows steady growth. Rain showerswith occasional lightning and thunder become morenumerous as the month goes on. But on occasion,the spring rains can turn to wet snow and suddenblizzards. Some of the spring storms may last forseveral days dropping widespread moderateprecipitation that replenishes soil moisture reserves.Between storms, drying winds sweep across theplains and the mountain valleys, especially onsunny afternoons.

April and early May are especially known forsudden changes. Bright warm sunshine in theafternoon can quickly turn to evening thunder andcold winds. By morning, the ground can be coveredby deep snow. Strong winds associated withpassing storm systems can pick up clouds of dust.This is a time of year to be prepared for just aboutanything.

As we move later into May, the sun climbs stillhigher in the sky, and temperatures warm gradually.Winter-like storms become increasingly rare,especially over the southern mountains. Occasionalepisodes of hot, summer-like weather occur, duringwhich mountain snows begin to melt rapidly. Aswater levels begin to rise, kayakers and raftersmigrate toward their favorite rivers, although these

> 200%

150-200%

100-150%

75-100%

50-75%

<50%

Water Year 2000 (October through December 1999) as a percent of the 1961-1990 average.

precipitation has been observed southwest of Denver,in the Colorado Springs area, and along the easternslopes of the Wet and Sangre de Cristo Mountainssouth and southwest of Pueblo. Extreme southeasternColorado also continues to show well above averageprecipitation totals as a result of a single large stormdropping more than three inches of rainfall in earlyOctober there.

Colorado’s Climate, April-June –A look ahead

Every season in Colorado has its specialfeatures and memorable aspects. But there is noseason like spring to point out the dynamic natureof the climate in the mid latitudes and in the middleof a continent. If you have lived in Colorado evenjust a few years, I don’t have to tell you any of this,because you’ve likely already learned it on yourown. Springtime is the most changeable, the mostexciting, the most hazardous, and probably the mostimportant season of the year to the health andgrowth of all forms of life in Colorado. Even theslow steps of geologic processes quicken for a timein the spring as wind erosion, soil erosion, freeze-thaw processes, rock slides, and sedimentation allspeed up, only to slow again later in the summerand fall.

April, for all practical purposes, is still a wintermonth in the mountains of Colorado. Snow continuesto fall, and sometimes in huge quantities. In particular,the eastern slopes of the Front Range can expect veryheavy snows in April. Towns like Cripple Creek,Bailey, and Estes Park receive more snowfall in April,

Colorado Climate 19

waters are icy cold, even on hot, sunny days. Thunder-heads become a frequent afternoon visitor to the skieseast of the continental Divide. Later in May, thunder-storms increase in both frequency and severity, oftentossing huge quantities of hail stones to the ground. Afew of these storms may produce a tornado or two, butmost Colorado tornadoes in May are fairly small andshort-lived. Closer to Kansas, tornadoes and severeweather are greater hazards, since these tornadoes maybe quite large. Over portions of northeastern Colorado,May is the wettest month of the year, and rainfallprobabilities are higher in late May than any other timeof year. Widespread soaking rains are quite common.Approximately once every 10-20 years, a large Maystorm will produce heavy rains that combine withmelting snows to produce flooding over portions ofeastern Colorado.

As we move into June, winds at mountain-toplevel become noticeably lighter as the jet streamweakens and drifts northward into Canada. Largestorm systems become infrequent, and insteadlocalized thunderstorms become the dominant rainmaker. Severe thunderstorms are most common inearly June over eastern Colorado. Almost everyyear, tornadoes are spotted on at least one dayduring the June 1-14 window. Meanwhile, westernColorado sees little precipitation and many hot,clear days in June. High mountain snowpack meltsrapidly, and many of Colorado’s largest rivers reachtheir peak flows for the year in early June. Reser-voirs are filled during this very important time ofyear.

An abrupt change from cool, changeablespring weather to persistent sunshine and intenseheat takes place in the middle of June. The last halfof June is almost always hot and dry in Coloradowith only widely scattered thundershowers. Thesestorms can produce large hail and heavy rains ineastern Colorado, but heavy precipitation isnormally quite localized. By the end of June, themost of the snowpack is gone, and summer accessto the Colorado high country can begin.

Thunderheads

become a frequent

afternoon visitor to

the skies east of the

continental Divide.

20 Colorado Climate

FAre There Urban Heat Islands in Colorado?

Nolan Doesken

warmer places than the surrounding countryside hadbeen shown in Europe early in the 19th century. Around1947 the phrase “urban heat island” was coined todescribe this phenomenon.

Are there urban heat islands here in Colorado?This hasn’t been studied in much detail. A 1976publication of the Rocky Mountain Chapter of theAmerican Society of Heating, Refrigerating and AirConditioning Engineers (ASHRAE) contained amap of Denver’s “heat island” which is shown atleft. Assuming that the data they found then wereaccurate and representative, it showed a profounddifference in extreme cold winter temperaturesbetween Denver Stapleton airport and the center ofthe city. A difference of 8 degrees Fahrenheit onextremely cold days, along with reduced urbanwind speeds caused by the friction from many largebuildings and urban landscaping, is significant. Theeffect is large enough that urban furnaces andheating systems can be smaller and use less fuelthan heating systems for rural and suburbanstructures.

It should be very easy to look for urban heatislands in Colorado. All you do is find weatherstations that have been in cities or have had the citygrow up around them. Then compare their tempera-tures to that of nearby weather stations that havealways remained in a rural location. That’s a goodidea, but the reality is there are hardly any weatherstations that meet these simple requirements. Oneof the best examples may be in Fort Collins. Theweather station has always been on the campus ofColorado State University, and the city has grownup around it. The bulk of the urbanization in FortCollins has occurred since the 1950s. The bad newsis there aren’t many nearby choices of rural stationsfor comparison. The best choice may be Waterdalejust west of Loveland. While it is close in distance,its foothill location means that much of thedifference in temperature between the two sitesmay be due to topography, not urbanization. Forthat reason, we also selected Akron, Colorado forcomparison. The Akron 4E (4 miles east of theAkron Post Office) is 100 miles away but is free ofdramatic topographical effects and has excellentdata quality.

Let’s look at average minimum temperaturesfor January, the time when the heat island effect isbelieved to be greatest. The graph of temperaturedifferences does seem to show a long-termwarming trend for Fort Collins with respect to thetwo rural stations. While there are large year-to-yeardifferences in the relationship, Fort Collins has beensystematically warmer than these rural sites since the

or four years during college, I was fortunateto have a great summer job working with alarge crew of scientists from several

institutions studying the St. Louis, Missouri area. Theproject, known as METROMEX (METROpolitanMeteorological EXperiment), was trying to discover ifthat large urban area was affecting the summertimeclimate of the agricultural areas of southern Illinoisdownwind (east) of the city. We collected all sorts ofdata ranging from rainfall and rain chemistry data fromhundreds of locations upwind, in, and downwind of thecity, to surface temperature readings and much more.Later, several important conclusions were reached.St. Louis was found to be hotter and drier thansurrounding forested or cultivated lands during thesummer months. Also, the air over the city, notsurprisingly, was found to carry more particles andpollutants. With the buoyancy of the hotter air overthe city, that urban air tended to rise, causing ruralair to be drawn inward towards the city (conver-gence). Finally, it appeared that these factors allworked in combination to enhance thunderstorm

development and severitysuch that areas downwindfrom the city appeared toreceive more rain andmore severe weather thanareas immediately over orupwind of the city. Thedifferences were notdramatic, but they didappear to be detectableand non-random.

This wasn’t the firsttime that scientists andother observant peoplenoticed that the climate ofcities differed from theclimate of nearby ruralareas. Beginning in the1920s, airports wereconstructed in the ruralareas just outside of mostU.S. cities. From the

1930s and continuing into the 1950s, many of thenation’s weather stations were moved from downtownlocations, where they had been since governmentweather offices were first established in the U.S.beginning in the 1870s, out to each city’s airport whereweather observations were critical for airport operationsand aviation safety. Climatologists quickly noticed thatthe airports were typically cooler than the inner citywhere the stations had previously been. Even beforethat, the idea that densely populated urban areas were

Denver’s urban heatisland in degreesFahrenheit (with respectto Stapleton airporttemperatures) forextreme low wintertemperature days. Takenfrom Rocky MountainChapter of ASHRAE,1976, “Climate Data forAir Conditioning DesignRocky Mountain ChapterRegion Colorado,Wyoming, Montana, andEnvirons.”

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early 1970s. Since the early 1950s, the Fort Collinsstation has warmed about three degrees Fahrenheitcompared to these two rural stations.

What about other cities? It turns out that FortCollins is one of the only weather stations in anurban area of Colorado that has not been modified,frequently relocated, terminated, or otherwisegreatly disrupted. The only other reasonable choicefor a long-term urban weather station is the old U.S.Weather Bureau Denver station that resided on theroof of the Post Office in downtown Denver fromJanuary 1916 until that weather station wasterminated in the early 1970s. While its rooftoplocation was not compatible with weather stationexposure guidelines, it was otherwise a consistentand high quality climate record. For comparison,temperature data from the Kassler Water TreatmentPlant southwest of Denver was used. LikeWaterdale, Kassler temperatures are not an idealmatch since the station lies in complex topographyat the mouth of the Platte Canyon where the SouthPlatte River exits the mountains. We will alsocompare Denver to the Akron 4E temperaturerecords.

As the graph clearly shows, downtown Denvertemperatures are much warmer than the ruralstations almost every year – typically four to fivedegrees warmer than Kassler and six to ten degreesF warmer than Akron 4E. Over the 1931 to 1972period there may have been a slight warming ofDenver compared to Kassler, but no obvious trendis evident with respect to Akron. Keep in mind thatthe area near the Denver Post Office has been fullyurbanized for the entire century so there may be noreason to expect an ongoing upward trend withrespect to the rural sites.

Yes, urban heat islands do appear to exist inColorado. Some analysts believe that even smalltowns may be slightly warmer than surroundingcountryside due to roads, buildings, waste heatfrom buildings and vehicles, and vegetationdifferences. However, the combination of complextopography and irrigation complicate the picture.Colorado’s largest cities reside along the easternbase of the Rocky Mountains. Even withouturbanization, this area is often warmer thansurrounding regions during the winter months as aresult of downslope winds that warm by compres-sion as air descends the Front Range of theRockies. Vegetation also plays an interesting andimportant role in defining temperature patternsaround cities. Unlike the cities of humid climates,portions of Colorado cities may be greener andlusher than surrounding rural areas during thesummer months due to the large extent of irrigatedlandscapes that have accompanied urbanization. Duringdaylight hours, surrounding unirrigated rural areas mayheat up more than the moist vegetated urban landscape.

This is the “urban oasis”phenomenon that helpsexplain why many westerncities do not have the sameurban heat island characteris-tics as midwestern, eastern,and southern cities in theU.S.

A neat way to learn aboutthe nature of Colorado urbantemperature patterns is tocompare minute by minute thedaily temperature cycle betweennearby urban and rural weatherstations. In Fort Collins, comparable electronic weatherstations have been maintained for several years on theCSU Main Campus and four miles northwest atChrisman Field on the CSU Foothills Campusapproximately 0.5 miles beyond the edge of the city.These stations record temperatures every few minutes.Clear days during the past year were selected duringJanuary, July, and October. Temperature differenceswere calculated with respect to the Main Campusstation. Sure enough, the main campus “urban” stationis cooler during the day – a daytime oasis. A rapid shiftfrom cooler to warmer takesplace within one hour andoccurs at sunset. Throughoutthe evening and into the earlymorning hours, the urban siteaverages about two degrees Fwarmer than the nearby ruralsite – a nighttime heat island.After sunrise, the rural sitewarms more quickly than theurban site and the “urban oasis”returns. The maximum daytimedifference occurs mid morningwhen the main campus stationaverages over one degree Fcooler than the rural site. The relationship is similar inall seasons, but the magnitudes vary. During the winter,nighttime temperature differences were greatestaveraging about 3 degrees warmer in the city with verylittle urban-rural daytime temperature differences.Daytime differences were greatest in late summer whenurban vegetation was still very green, but grasslandsnear the foothills had turnedbrown.

It is also well known thaturbanization affects runoff fromrainfall and snowmelt. But isurbanization also affectingprecipitation and stormseverity? Recent scientificpapers by Stohlgren et al (1998)and Chase et al (1999) suggest

Temperature differencesbetween downtownDenver and two ruralstations.

(continued on back cover)

Temperature differencesbetween Fort Collins andtwo rural stations.

DowntownDenver stationclosed in 1973

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that irrigated croplands along the Front Range maycontribute to more summer thunderstorms. Some havespeculated that severe weather has become morefrequent in Aurora and further east in Adams County inrecent years. But others have pointed out that severeweather there has always been common but untilrecently few people lived there and those that didsimply took it in stride. We simply do not have enough

long-term weather stations east and northeast of Denveror Colorado Springs know for sure. Perhaps we canexplore this in more detail in a future issue.

References: Chase, T.N., R.A. Pielke, Sr.,T.G.F. Kittel, J.S. Baron, T.J. Stohlgren, 1999:Potential impacts on Colorado Rocky Mountainweather due to land use changes on the adjacentGreat Plains. J. Geophys. Res. in press.

Stohlgren, T.J., T.N. Chase, R.A. Pielke, Sr.,T.G.F. Kittel, J.S. Baron, 1998: Evidence that localland use practices influence regional climate,vegetation, and streamflow patterns in adjacentnatural areas. Global Change Biology, 4, 495-504.

Daily cycle oftemperature differencesin degrees F between anurban weather station(Fort Collins, CSU MainCampus) and a ruralstation (Chrisman Field,CSU Foothills Campus)for selected days in 1999.Positive values areindicated when the urbansite is warmer than therural site.

Temperature Differences BetweenFort Collins and Chrisman Field

Urban Heat Islands in Colorado? (continued from inside back cover)