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.

• Hail

• Downbursts(Microbursts)

• Lightning and thunder

• Tornadoes

Violent Associations

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 Upward leader Return stroke

Electrons down Protons up 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 (30,000C, 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