Chapter 19
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Result of intense convection
Associated with heating Earth’s surface
◦ During spring, summer, and fall
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Three-stage life cycle:
◦ Beginning stage– formation of cumulus cloud
 Rapid heating during day, warm air rises
 Cools, water vapor condenses, clouds form
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Clouds get bigger
Small-scale convection cells form
Draw in air from surroundings
Form winds at surface
Rapid vertical development
◦ 26,000 feet in 15 minutes; base 6 miles
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Convection intensifies, creates strong winds
Rising air creates strong updrafts
 Causes water droplets/ice crystals to
form
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◦ Clump together to form precipitation
Height of cloud reaches 7 or more
miles
 Produces heavy precipitation
 Known as a cumulonimbus cloud
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Mature phase
◦ Strong updrafts and downdrafts of wind intensify
 Cause separation of lighter ice crystals at top
 Heavier rain at lower part of cloud
 Ice crystals become positively charged
 Hail becomes negatively charged
 Large electrical potential formed-released as
lightning
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Electrical discharge of more that 100
million volts
Heats air to about 45,000° F
Cool air expands and creates thunder
Lightning can travel:
◦ Cloud-> cloud cloud-> ground cloud-> air
◦ All are potentially dangerous
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Dissipating stage
◦ Strong convection that formed the
thunderstorm dissipates
◦ Decline in winds and rain
◦ Skies clear
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Spiraling column of air contacting the
ground
Form at the base of thunderstorms
◦ When steep pressure gradient develops
 Result of strong convection
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Produce visible swirling air called funnel
clouds
Composed of rapidly moving dust and
debris
Produce speeds up to 300 mph
 Usually last only a few minutes
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◦ Some up to 2 hours
Tornadoes over water form a water
spout
 Waterspouts and tornadoes move 30
to 75 mph
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Violent storm associated with large lowpressure system
◦ Also called a cyclone
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Rotate counterclockwise in the Northern
Hemisphere
Winds spiral inward
Form over the Atlantic Ocean near the
equator
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Low pressure moves over ocean to the
USA
◦ Picks up energy/moisture from warm
water
◦ Called a tropical disturbance winds less
than 20 mph
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As it moves over warm ocean
intensifies
◦ Becomes tropical depression- winds 21 to
39 mph
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Clouds thicken and rotate around the
center
◦ Called tropical storm-winds 40 to 70 mph
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When sustained winds reach 74 mph
◦ Called a hurricane in the Atlantic Ocean
◦ Winds greater than 250 mph
◦ Winds move inward and move counterclockwise
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High winds cause pile up of water ahead
◦ Called a storm surge
◦ Can be as high as 16 feet
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Intensity measured- Saffir-Simpson scale
◦ Measures pressure, wind speed, and storm surge
◦ Scale of 1 to 5- 5 highest intensity
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In Pacific Ocean called a typhoon
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Three main steps:
◦ Gather current weather information
◦ Record information on charts and maps
 Or either computer models
◦ Charts, maps, and computer models
analyzed
 Weather is predicted or forecasted
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Data gathered from
almost 1,000
weather stations
Data is recorded
each hour
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Surface air temp
Barometric pressure
Cloud cover
Wind speed
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Dew point
Precipitation
Visibility
Wind direction
Plotted every three hours- coded form
 Displayed on maps using station
model
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◦ Usually shows:
◦ Location of high- and low- pressure
systems
◦ Temperatures
◦ Fronts
◦ Precipitation
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Surface temperature maps created using
isotherms
◦ Lines that connect equal values of temperature
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Can also be created using isobars
◦ Lines connecting value of equal pressure
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Can be generated at different levels in the
atmosphere
Refer to Pg. 394 in your textbook
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Divided into two categories:
◦ Long-term
◦ Short-term
◦ Short-term are reliable up to 6 hour
◦ Long- term– a few days, weeks, or months
out
 More difficult to make
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Refer to Pg. 395 in your textbook
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Radar extremely successful over past
20 years
◦ Reflected radio waves off clouds and
precipitation
◦ Locates poor weather
◦ Up-to-date radar images on Internet
◦ Can also differentiate forms of
precipitation
◦ Predicts where snow/freezing rain may
form
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Simple observations can also predict
weather
◦ Low barometric air pressure-poor weather
◦ Rise in barometric pressure- good
weather
◦ Type of clouds:
 Cumulus- incoming thunderstorms
 Stratocumulus- approaching warm front
and precipitation
 Clearing skies in winter- drop in temp