Describe the stages in the development of a thunderstorm.
List and contrast two types of thunderstorms.
Summarize the events that lead to lightning and thunder.
Describe the atmospheric conditions that are most conducive to the formation of
tornadoes.
Distinguish between severe weather (thunderstorms, tornadoes) watches and warnings.
Contrast Doppler radar and conventional radar.
What Are Thunderstorms?
storm containing lightning and thunder
thunderstorms occur when warm, humid air rises in an unstable environment
instability enhanced by high surface temps (afternoon, early evening)
strong up and down movement
commonly forms along cold front
cumulonimbus cloud
Air-Mass (Ordinary) Thunderstorms
scattered thunderstorms that form in summer
develop in warm, maritime tropical air masses away from weather fronts
usually short-lived and rarely produce strong winds and large hail
Severe Thunderstorms
capable of producing large hail, strong winds, flash floods and tornadoes
(The National Weather Service defines a severe thunderstorm as having 3/4-inch hail
and/or surface wind gusts of 50 knots)
form as warm moist air ahead of a cold front is forced to rise into
unstable air
strong vertical winds may cause the storm to rotate - possible tornado
formation
Distribution of Thunderstorms
Lightning and Thunder
rapid discharge (spark) occuring in mature thunderstorms
within cloud, cloud to cloud, cloud to ground
lightning stroke heats air to extremely high temperature - air expands explosively
producing a shock wave, thunder
speed of light = 300,000,000 m/s or 186,000 miles/s;
speed of sound (thunder) = 340 m/s
Tornadoes
local windstorm of short duration
rotating column of air (vortex) extending down from a cumulonimbus cloud
low pressure inside
air near ground rushes inward, spirals upward to cloud
max winds 300 mph
spawned along cold fronts
Development and Occurance of Tornadoes
770 tornadoes per year in U.S.
April thru June peak tornado season
direction of travel - SW to NE
diameter - between 150 and 600 meters
speed - 30 mph
path length - 16 miles
Doppler Radar
radar transmits electro-magnetic wave that is then reflected and returns to receiver -
distance determined from time interval
weather radar detects precipitation
Doppler radar - reflections from precipitation moving toward or away from radar station
will be frequency shifted (Doppler Effect)
two or more radar units can give a three-dimensional picture of motion in a thunderstorm
able to detect and provide advanced warning of approaching tornadoes
NEXRAD (NEXt generation RADar) - network of 135 new Doppler radar units
in U.S. WSR-88D Doppler radar
