1. Thunderstorms

2. Definition: a storm containing lightning and thunder. Associated with midlatitude cyclones, localized convection, orographic lifting and tropical cyclones.

3. Thunderstorm Formation Ingredients –warm, moist air (often mT) –unstable (or conditionally unstable if lifting mech.) –encouraged by diverging air aloft

4. Thunderstorm Life Cycle

5. The Geography of Thunderstorms Frequency decreases with distance from equator. None above 60 o Most occur during summer’s warm temperatures.

6. The Geography of Thunderstorms

8. Thunderstorm Frequency Probably 1500 to 2000 thunderstorms active around the world at any given time.

9. Hail Downbursts (Microbursts) Lightning and thunder Tornadoes Violent Associations

10. Hail Formation Largest? Coffeyville, KS, 1970 (1.75 lb, 14 cm diameter)

11. Downbursts Macrobursts - extreme downdrafts associated with thunderstorms. Microburst –smaller –< 5 minute duration –winds up to 146 mph

12. 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.

13. Stepped leader Upward leader Return stroke Electrons down Protons up Circuit complete Repeats every few microseconds with new leader.

14. Frequency of Occurrence Globally, lightning flashes  100 times per second.

15. Geography of Lightning

16. USA: Real-time Lightning http://www.weather.com/

17. 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

18. 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

19. Key Points Thunderstorm formation and life cycle Thunderstorm geography and frequency Violent associations –hail, downbursts, lightning/thunder –next lecture….tornadoes