Thunderstorms
Thunderstorms
• Definition: a storm containing lightning and
thunder.
• Associated with midlatitude cyclones, localized
convection, orographic lifting and tropical
cyclones.
Thunderstorm Formation
• Ingredients
– warm, moist air (often mT)
– unstable (or conditionally unstable if lifting mech.)
– encouraged by diverging air aloft
Thunderstorm Life Cycle
The Geography of Thunderstorms
Frequency decreases with distance from equator. None above 60o
Most occur during summer’s warm temperatures.
The Geography of Thunderstorms
Thunderstorm Frequency
• Probably 1500 to 2000 thunderstorms active
around the world at any given time.
Violent Associations
• Hail
• Downbursts
(Microbursts)
• Lightning and thunder
• Tornadoes
Hail
• Formation
• Largest?
Coffeyville, KS, 1970
(1.75 lb, 14 cm diameter)
Downbursts
• Macrobursts - extreme downdrafts associated
with thunderstorms.
• Microburst
– smaller
– < 5 minute duration
– winds up to 146 mph
Lightning
• discharge of electricity that
occurs in mature thunderstorms
• Cause: charge separation in
cloud sets up electrical potential
• Role of lightning is to equalize
these differences in electrical
potential.
• Important fixer of Nitrogen.
Stepped leader
Electrons down
Upward leader
Protons up
Return stroke
Circuit complete
Repeats every few microseconds with new leader.
Frequency of Occurrence
Globally, lightning flashes  100 times per second.
Geography of Lightning
USA: Real-time Lightning
http://www.weather.com/
Thunder
• Air poor conductor of
electricity
• Huge electrical potential
develops
• Lightning bolt
‘superheats’ the air
(30,000C, 54,000 F)
• Surrounding air expands
violently  sound wave
Distance from the Thunderstorm
• Speed of light: 300,000,000 km/s
• Speed of sound: about 343 m/s
• Distance calculation
– for each 3 seconds, 1 km away
– for each 5 seconds, 1 mile away
Key Points
• Thunderstorm formation and life cycle
• Thunderstorm geography and frequency
• Violent associations
– hail, downbursts, lightning/thunder
– next lecture….tornadoes