NATS 101 Intro to Weather and Climate Section 05: 2:00PM TTh ILC 150

Dr. E. Robert Kursinski

TAs: Mike Stovern &

April Chiriboga

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NATS 101 - 05

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NATS 101 - 05Lecture 2

Density, Pressure & Temperature

Climate and Weather

Two Important Concepts

Let’s introduce two new concepts...

Density

Pressure

What is Density?

Density () = Mass (M) per unit Volume (V)

= M/V

= Greek letter “rho”

Typical Units: kg/m3, gm/cm3

Mass =

# molecules (mole) molecular mass (gm/mole)

Avogadro number (6.023x1023 molecules/mole)

Density Change

Density () changes by altering eithera) # molecules in a constant volumeb) volume occupied by the same # molecules

ab

What is Pressure?

Pressure (p) = Force (F) per unit Area (A) Typical Units: pounds per square inch

(psi), millibars (mb), inches Hg

Average pressure at sea-level: 14.7 psi

1013 mb 29.92 in. Hg

PressureCan be thought of as weight of air above you.

(Note that pressure acts in all directions!)

So as elevation increases, pressure decreases.

Higher elevation Less air aboveLower pressure

Lower elevation More air above Higher pressureBottom

Top

Density and Pressure Variation

Key Points

1. Both decrease rapidly with height

2. Air is compressible, i.e. its density varies

Ahrens, Fig. 1.5

Why rapid change with height?

Consider a spring with 10 kg bricks on top of it

The spring compresses a little more with each addition of a brick. The spring is compressiblecompressible.

10 kg 10 kg

10 kg

10 kg

10 kg

10 kg

Why rapid change with height?

Now consider several 10 kg springs piled on top of each other.

Topmost spring compresses the least!

Bottom spring compresses the most!

The total mass above you decreases rapidly w/height.

massmass

massmass

massmass

massmass

Why rapid change with height?

Finally, consider piled-up parcels of air, each with the same # molecules.

The bottom parcel is squished the most.

Its density is the highest.

Density decreases most rapidly at bottom.

Why rapid change with height?

Each parcel has the same mass (i.e. same number of molecules), so the height of a parcel represents the same change in pressure p.

Thus, pressure must decrease most rapidly near the bottom.

pp

pp

pp

pp

A Thinning Atmosphere

Bottom

Top Lower density, Gradual drop

Higher densityRapid decrease

NASA photo gallery

Pressure Decreases Exponentially with Height

Logarithmic Decrease• For each 16 km

increase inaltitude, pressure drops

by factor of 10.

48 km - 1 mb 32 km - 10 mb 16 km - 100 mb 0 km - 1000 mb

100 mb

10 mb

1 mb

16 km

32 km

48 km

Ahrens, Fig. 1.5

Exponential VariationLogarithmic Decrease• For each 5.5 km

height increase, pressure drops

by factor of 2.

16.5 km - 125 mb 11 km - 250 mb 5.5 km - 500

mb 0 km - 1000 mb

Water versus Air

Pressure variation in water acts more like bricks, close to incompressible, instead of like springs.

Air:Lower density, Gradual drop

Higher densityRapid decrease Bottom

Top

Bottom

Top Water:Constant drop

Constant drop

Equation for Pressure Variation

We can Quantify Pressure Change with Height /(16km)

MSL

MSL

where

is elevation in kilometers (km)

is pressure in millibars (mb)

at elevation z in meters (km)

is pre

(at elevation zin km)

ssure (mb

1

) at mean sea l

0

leve

Z

z

p

p

p

p −= ×

What is Pressure at 2.8 km?(Summit of Mt. Lemmon)

Use Equation for Pressure Change/(16 km)

MSL

(2.8km) /(16 km)

0.175

MSL

(at elevation Zin km) 10

(2.8 km) 1013mb 10

(2.8 km) 1013mb

set = 2.8 km, 10

10

(2.8 km) 1013mb 0.668 677mb

13 mb

Zp p

p

p

p

Z

p

−

−

−

= ×

= ×

= ×= × =

=

What is Pressure at Tucson?

Use Equation for Pressure Change

Let’s get cocky…

How about Denver? Z=1,600 m

How about Mt. Everest? Z=8,700 m

You try these examples at home for practice

/(16km)M

MS

S

L

L(at e

set =

levation Zin

800 m

km) 10

, 1013 mb

Z

Z p

p p −= ×=

Temperature (T) Profile• More complex than

pressure or density • Layers based on

the Environmental Lapse Rate (ELR), the rate at which temperature decreases with height.

inversion

isothermal

6.5oC/km

Ahrens, Fig. 1.7

Higher AtmosphereMolecular Composition• Homosphere- gases

are well mixed. Below 80 km. Emphasis of Course.

• Heterosphere- gases separate by molecular weight, with heaviest near bottom. Lighter gases (H, He) escape.

Ahrens, Fig. 1.8

Atmospheric Layers Essentials

• Thermosphere-above 85 kmTemps warm w/height Gases settle by molecular weight (Heterosphere)

• Mesosphere-50 to 85 km Temps cool w/height

• Stratosphere-10 to 50 km Temps warm w/height, very dry

• Troposphere-0 to 10 km (to the nearest 5 km)Temps cool with height

Contains “all” H2O vapor, weather of public interest

Summary• Many gases make up air

N2 and O2 account for ~99%

Trace gases: CO2, H2O, O3, etc.Some are very important…more later

• Pressure and Density Decrease rapidly with height

• TemperatureComplex vertical structure

Climate and Weather

“Climate is what you expect.

Weather is what you get.”-Robert A. Heinlein

WeatherWeather – The state

of the atmosphere:

for a specific place

at a particular time

Weather Elements

1) Temperature

2) Pressure

3) Humidity

4) Wind

5) Visibility

6) Clouds

7) Significant Weather

Surface Station ModelTemperaturesPlotted F in U.S.Sea Level PressureLeading 10 or 9 is

not plotted Examples:1013.8 plotted as 138998.7 plotted as 9871036.0 plotted as 360Ahrens, p 431

Responsible for boxed parameters

Sky Cover and Weather Symbols

Ahrens, p 431

Ahrens, p 431

Wind Barbs

Direction

Wind is going towards

WesterlyWesterly from the West

Speed (accumulated)

Each flag is 50 knots

Each full barb is 10 knots

Each half barb is 5 knotsAhrens, p 432

65 kts from west

temperature dew point

SLP pressure

wind

cloud cover

Ohio State website

Practice Surface Station

Temperate (oF)Pressure (mb) Last

Three Digits (tens, ones, tenths)

Dew Point (later) Moisture

Wind Barb Direction and Speed

Cloud Cover Tenths total coverage

Ahrens, p 431

72

58

111

Decimal point

What are Temp, Dew Point, SLP, Cloud Cover, Wind Speed and Direction?

Practice Surface Station

Sea Level Pressure

Leading 10 or 9 is not plotted

Examples:

1013.8 plotted as 138

998.7 plotted as 987

1036.0 plotted as 360Ahrens, p 431

42

18

998

Decimal point

What are Temp, Dew Point, SLP, Cloud Cover, Wind Speed and Direction?

Surface Map Symbols

• Fronts

Mark the boundary between different air masses…later

Significant weather occurs near fronts

Current US MapAhrens, p 432

Radiosonde

Weather balloons, or radiosondes, sample atmospheric to 10 mb.

They measuretemperature moisture

pressure

They are tracked to get winds

Ahrens, Fig. 1

Radiosonde Distribution

Radiosondes released at 0000 and at 1200 GMT for a global network of stations.

Large gaps in network over oceans and in less affluent nations.

Stations ~400 km apart over North America

Radiosonde for TucsonExample of data taken

by weather balloon released over Tucson

Temperature (red)

Moisture (green)

Winds (white)

Note variations of all fields with height

UA Tucson 1200 RAOB

troposphere

stratosphere

tropopause

temperature profile

moisture profile

wind profile

ClimateClimate - Average weather and range of

weather, computed over many years.

Whole year (mean annual precipitation for Tucson, 1970-present)

Season (Winter: Dec-Jan-Feb)

Month (January rainfall in Tucson)

Date (Average, record high and low temperatures for Jan 1 in Tucson)

Climate of TucsonMonthly Averages

Individual months can show significant deviations from long-term, monthly means.

Average and Record MAX and MIN

Temperatures for Date

Climate of TucsonProbability of Last Freeze

Cool Site: Western Region Climate Center

Climate of TucsonProbability of Rain

Cool Site: Western Region Climate Center

Climate of TucsonExtreme Rainfall

Cool Site: Western Region Climate Center

Climate of TucsonSnow!

Cool Site: Western Region Climate Center

Summary

• Weather - atmospheric conditions at specific time and place

Weather Maps Instantaneous Values• Climate - average weather and the range

of extremes compiled over many years

Statistical Quantities Expected Values

Reading Assignment

• Ahrens

Pages 25-42

Problems 2.1-2.4, 2.7, 2.9-2.12

(2.1 Chapter 2, Problem 1)

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