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Thunderstorms, Tornadoes, and Hurricanes
October 27/28 2010
Extreme Weather
Hurricane Ivanover Gulf, Sept. 2004
Source: NOAA
Definitions
*Climate Change:
* Changes in climate of the past, present or future associated with natural or anthropogenic (human) factors
*Global Warming:
* Warming of the 20th and 21st century associated with anthropogenic activities.
Weather VS Climate
*Weather describes whatever is happening outdoors in a given place at a given time.
*Weather is what happens from minute to minute.
*can change a lot within a very short time
*Weather includes daily changes in precipitation, barometric pressure, temperature, and wind conditions in a given location.
*Climate describes the total of all weather occurring over a period of years in a given place.
*Climate tells us what it's usually like in the place where you live
Climate is what you expect, weather is what you get!
Thunderstorms*Begin in cumulus stage
*Warm, moist air rises, cools.
*Water condenses out of air, releasing heat
*Updrafts develop
*Precipitation occurs when weight of precipitation overcomes updrafts
Thunderstorms*Cloud draws in drier air which
evaporates some moisture, causes cooling.
*This cool, dense air sinks, creating downdrafts
*Downdrafts mark mature thunderstorms
*Downdrafts and updrafts make the thunderstorm cell
*Storm may extend to top of tropopause (~12 km)
*#1: Why doesn’t precipitation fall right as condensation occurs?
Thunderstorms
*A rush of cold air (downdraft) usually occurs at onset of precipitation
*Storms usually dissipate within 15-30 min.
*Updrafts weaken, downdrafts dominate
*Provide summer rainfall for much of the US
*Cooling of up to 10˚C/18˚F on hot summer days
*Strong downdrafts can force more warm air up, causing multi-cell storms.
Thunderstorm development time lapse (Florida)
*http://vimeo.com/4806845
Severe Thunderstorms
*Stronger winds aloft than at surface vertical wind shear
*Tips over top of storm, allows for prolonged updrafts, longer storm life
*Hail forms due to strong updrafts, falls when heavy enough to overcome updrafts
*Strong downdrafts called “microbursts” may occur, winds up to 146 kt
*Dangerous to aviation!
Source: Wikipedia
Microburst#2: How do you think it could be dangerous to aviation?
Distribution of Thunderstorms
# days per year thunderstorms observed. Do you think hail follows the same distribution?
Distribution of Hail #3: How are max/min different here? Why do you think more hail over Rockies, Great Plains?
Lightning
*Lightning occurs to reduce a buildup of positive and negative charges within a cloud.
*+ charged ice crystals tend to be at top of cloud, and - charged hailstones at bottom.
*Area of positive charge on ground develops below storm
*When electrical potential gradient large enough, the electrical current flows to surface as lightning!
*Can have cloud-to-cloud, cloud-to-ground, or lightning within a cloud.
http://apollo.lsc.vsc.edu/classes/met130/notes/chapter14/graphics/lightning.mpg
Lightning*Can heat the air rapidly to 30,000˚C/54,000˚F
*This rapid heating causes air to expand and makes a booming sound wave– thunder!
*Sound takes 3 sec to go 1 km, (5 sec per mile)
*#4: If you see lightning and hear thunder 15 sec later, how far away is the lightning?
*5 km/3 miles away!
*Sometimes thunder is not heard due to the atmosphere bending the sound waves upward
Lightning*100 people per year die of lightning
strikes
*If about to strike, hair stands on end, skin tingles, hear clicking sounds
Sept 2003, Carquinez Straits, CAAfter rare influx of moisture from SW
Cloud-to-ground lightning over Las Vegas during SW monsoon season
Source: www.goldengatephotos.com
Tornadoes*Rapidly rotating winds around intense low
pressure center
*Start as funnel-shaped cloud
*Diameter of 100-600 meters (300-2000 ft)
*Move at 20-40 kt (23-46 mph)
*Last only a few minutes
*Travel up to 7 km (4 mi)
Source: http://www.cs.uwaterloo.ca
Tornadoes*Most violent event: April 3 and 4, 1974
*148 tornadoes over a 16 hour period
*Covered 13 states
*307 people killed, 6000 injured
*$600 million in damageTrailer park destroyed in Huntsville, ALF4 tornado over Parker City, IN
Source: http://www.april31974.com
Tornadoes
*Tri-state tornado event March 18, 1925
*7 tornadoes across MO, IL, IN
*695 people killed!
*US has most tornadoes of all countries
*Occur in all states, greatest number in “Tornado Alley”
*This is area where cool, dry air from Canada meets warm, moist air from Gulf
Source: http://wximpact40-88.pbworks.com
Tornadoes*Tornado alley susceptible since warm, humid air
overlain by cool air aloft
*If strong vertical wind shear occurs and thunderstorms form, tornadoes likely
*Usually occur March-July in late afternoon
*Can pick up people, animals, appliances, railroad cars
*220 kt winds max, most < 125 kt
*Can pop roofs off/collapse houses! (Bernoulli’s principle)
*#5: How can tornadoes “pop” a roof off a structure?
Tornadoes around the US
http://www.youtube.com/watch?v=43VoMesUd2Q
Fujita Scale
Hurricanes*An intense storm, winds over 64 kt/74 mph
*Generally form in tropics (23 1/2˚N/S of equator)
*Warm, humid areas
*A “tropical” aka “Easterly” wave in atmosphere disrupts usual wind flow
*In western N Pacific (Asia/Japan), typhoon
*In Indian Ocean/Australia, cyclone
*Today we will use “hurricane” to refer to all
Hurricane and SST Change
(Webster et al, 2005, Science)
Webster et al. report that the number of category 4 and 5 hurricanes has almost doubled globally over the past three decades
Be careful
*Hurricanes are natural events, and are not linearly related to climate change
*Climate change, by increase SST, indeed makes it more possible to have strong hurricane occur
Anatomy of a hurricane
Hurricanes*Convergence at surface brings warm, moist air
up
*Divergence aloft and sinking air outside the hurricane, clear skies immediately surrounding
Hurricanes*Ingredients for Hurricane:
*Winds light
*Deep layer of high humidity, warm air
*Water >80˚F
*Season lasts June-November
*Need converging winds, so form in ITCZ
*Take in heat at ocean surface, convert it to kinetic energy as wind
*Form between 5˚and 10˚N and S
*#6: Why don’t they form over equator?
*No Coriolis effect at equator to start “spin”!
Hurricanes
#7: Why do hurricanes move from East to West?
Hurricanes
*Begins as tropical disturbance/wave
*Tropical depression: winds 20-34 kts, closed isobars
*Tropical storm: winds 35-60 kts
*Hurricane: winds >64 ktsHurricane Katrina Source: NOAA
Atlantic and Pacific Storm tracks, 2010
Source: Washington Post
Source: Wikipedia
Hurricanes: Why so destructive?*High winds, can collapse structures
*Storm surge!
*High wind-driven waves
*Low pressure in storm center causes sea level to rise up to 0.5 m (1.5 ft)!
*Like water up a straw
*Strong downbursts
*Heavy, sudden rains can cause flooding
Katrina damage in Mississippi
Source: www.katrinadestruction.com
Hurricane Andrew
*August 21, 1992
*Hit Louisiana with 120 kt winds
*200,000 homes destroyed
*$30 billion in damage
*53 deaths
*Most deaths in US from hurricane:
*1900 Galveston, TX: more than 6,000 died!
*Confused when calm eye hit, went out to “check things out”, other side of storm hit suddenly!
Source: NOAA
Hurricanes and El Niño*#8: What is El Nino? What are signs it is
happening?
*Trade winds over Pacific slacken and/or reverse, warm water “sloshes” eastward towards coast of Peru.
*Causes global climate shifts!
*Atlantic: Fewer hurricanes because stronger upper level winds do not allow formation
*Pacific: More hurricanes possible because more warm water over a larger area
Naming Hurricanes
Gets name when becomes TSIf major event (like Katrina) name retired for several years
Source: Farmer’s Almanac
Heat Waves
*Definition: A period of several days with temperatures 5’C (9’F) above average for a given location at a given time of year
*Definition varies by region.
*Cause 175 deaths a year in US!
*During 1980 heat wave, 1250 people died!
*Many more die due to secondary effects of heat
*Most summers have heat waves
*Heat Index- issued by NWS
*Gives the “apparent temperature” combining effects of temperature, humidity, and wind
*Ex, “feels like” 104’, actual temp may be 98
Heat Effects on Body*Sweating dehydrates, causes dizziness and
fainting
*In heat, heart pumps more blood
*Blood vessels dilate
*Body tries to cool blood by getting it closer to surface- can’t cool if ambient temp too high!
*In high humidity, sweat does not evaporate
*No cooling of skin
*Also sunburn, stroke, heat exhaustionSource: NOAA/wikipedia
2010 Russian Heat Wave*Temperatures up to 104’F/40’C for several weeks
*Wildfires, cause poor air quality
*Heat like this not seen in 1000 year Russian climate archives
*30% crop decrease expected, ban on crop exports until 12/31/2010
*2000 people drowned trying to escape heat by bathing in rivers or lakes
*Most of them were drunk…
Source: The Economist, Rianovosti
Global Warming to blame?*Specific weather events cannot be linked to
global warming
*Warming and changing of Earth’s surfaces can increase probability of extreme events
*Many extreme events are attributed to normal oscillations
*Ex: Pakistan floods generally associated with La Nina (ENSO) summer
Global Warming to blame?*More areas with high Urban Heat Index
*More heat waves
*Warm temperatures cause more evaporation
*More intense rainfall (in some areas)
*More evaporation
*More drought/water shortages (in some areas)
*Warmer oceans
*Provide more kinetic energy for stronger hurricanes
*Higher and denser population
*Increased chance of fatalities and damage caused by events
*Especially in poorer regions without good infrastructure
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