2. Severe Weather 1.Thunderstorms 2.Tornadoes 3.Hurricanes

3. Thunderstorm Formation Three Conditions: 1.Abundant source of moisture. As moisture condenses heat is released. So cloud is warmer than air around it. How is it moving? 2.Mechanism to lift the air. 3.Atmosphere must be unstable. Air must continue to cool with altitude so that the cloud stays warmer than the surrounding air.

4. Limits to Growth 1.Rising air meets stable air that it cannot overcome. 2.Rate of condensation is not producing enough heat.

5. Air-Mass Thunderstorms Air rises because of unequal heating of Earth’s surface within one air mass – Max >> during mid- afternoon TWO TYPES: 1.Mountain: air mass moving up side of mountain 2.Sea-breeze: temperature differences between air over land and air over water.

6. Frontal Thunderstorms Produced by advancing cold fronts and sometimes warm fronts Cold Fronts: -Air rises rapidly -Lines of thunderstorms -Get lift from push of cold air -Can persist long into night Warm Fronts: -Air rises gradually -Relatively mild thunderstorms

7. Classified by direction of air movement Cumulus: air rises vertically; clouds Mature: downdrafts & updrafts; rain cools air Dissipation: smaller downdrafts; no more energy Stages of Thunderstorms

8. Stages of Development 1.Cumulus Stage: – Air rises vertically upwards – Creates updrafts = moisture goes to the upper parts of the cloud 2. Mature Stage -- Strong updrafts and downdrafts What do you think causes downdrafts? --As precipitation falls, it cools the air around it. This newly cooled air sinks rapidly = downdraft

9. Stages of Development 3. Dissipation Stage: -- Storm loses energy -- lingering downdrafts The convection cell can only exist if there is a steady supply of warm, moist air. – Knowing this, What do you think is the ultimate cause of a thunderstorm losing energy? DOWNDRAFTS! – They spread in all directions when they reach the surface and cool the surface air.

10. Severe Thunderstorms Continuous supply of surface moisture – Cold fronts moving into warm areas Instability of air – Increases strength of updrafts and downdrafts Supercells: intense, rotating updrafts Only about 10% severe in US

11. Thunderstorm Hazards 1.Lightning:  Electricity caused by rapid rush of air in a cumulonimbus cloud. (friction)  Heats air to 30,000 °C  What is thunder?!  Sound from heated air expanding and contracting

12. Lightning

13. Thunderstorm Hazards 2. Floods:  Rain falls faster than it can be transported or absorbed 3.Hail:  Water droplets encounter ice pellets and freeze  Ice pellets can get caught in the up and downdrafts

14. Hail Storm Video http://video.nationalgeographic.com/video/n ews/samaras-hail-storm?source=relatedvideo http://video.nationalgeographic.com/video/n ews/samaras-hail-storm?source=relatedvideo

15. Thunderstorm Hazards 4. Wind:  Downbursts: concentrated in one area  Macrobursts and microbursts

16. TORNADOES http://www.discovery.com/tv-shows/storm- chasers/videos/tornado-near-miss/

17. Tornadoes Form when wind speed and direction change suddenly with height – Wind Shear Funnel cloud: a tornado before it reaches the ground Why can we see tornadoes? – Dust and debris that is drawn into the swirling air

18. Tornado Formation Wind Shear can cause horizontal rotations near Earth’s surface. Updrafts can tilt this column of wind vertical and speed up their rotations. As rotation speed increases, wind is removed from the center = low air pressure Pressure gradient produces violent winds https://www.youtube.com/watch?v=0L- XExpb3pY

19. Tornadoes

20. F-scale: path of destruction, wind speed, and duration – F0-F5 Only 1% reach F4-F5; Most are F1 When do most tornadoes occur? – Spring. Why? – Large temp. contrasts between polar and tropical air Where do most tornadoes occur? – Central United States. Why? Think air masses! – Cold cP air collides with warm mT air from the gulf

21. April 27, 2011 Tornadoes https://www.youtube.com/watch?v=5ohIVzIZL uQ https://www.youtube.com/watch?v=5ohIVzIZL uQ https://www.youtube.com/watch?v=vhKjv9Gu ARQ https://www.youtube.com/watch?v=vhKjv9Gu ARQ

22. Tornado Alley

23. Fujita Scale

24. SAFETY! Warning signs: – Dark, greenish skies – Hail – Towering wall of clouds – Roaring noise SEEK SHELTER! http://ngm.nationalgeographic.com/2013/11/ biggest-storm/draper-text

25. TROPICAL CYCLONES  Large, rotating, low-pressure storms  Hurricanes = strongest  Coriolis effect causes rotation

26. TROPICAL CYCLONE FORMATION CONDITIONS:  Abundant supply of warm ocean water  Disturbance to lift warm air and keep it rising  What oceans would cyclones NOT form in?  South Atlantic and South Pacific oceans. Why?  Cooler waters

27. Stages of Development 1.DISTURBANCE:  Tropical disturbance causes air to rise 2.DEPRESSION:  Cyclonic circulation at a low pressure center 3.TROPICAL STORM:  Winds exceed 65 km/hr (40 mph) 4.HURRICANE:  Air pressure falls; winds can reach 120 km/hr (75 mph)

28. Classifying Hurricanes

29. Hurricane Map

30. Hurricane Paths

31. HURRICANE FORMATION  Water evaporates, latent heat is stored  Heat is released as air rises and clouds form  Disturbances produce more precipitation; more energy is released  Rising air creates low pressure at ocean’s surface  Coriolis effect causes moving air to turn counterclockwise in the northern hemisphere  Cyclonic rotation of a tropical cyclone

32. HURRICANE HAZARDS Storm surge: mound of ocean water towards coast Flooding: caused by vast amounts of rain http://www.youtube.com/watch?v=s76Qn7bp CsQ&safety_mode=true&persist_safety_mode =1 http://www.youtube.com/watch?v=s76Qn7bp CsQ&safety_mode=true&persist_safety_mode =1

33. Droughts are extended periods of well-below- normal rainfall Floods can occur when storms and hurricanes unleash enough rain and remain over an area for an extended time Both are considered recurring weather. Floods and Droughts

34. Heat waves usually come with drought and involve extended periods of above-normal temperatures Cold Waves are extended periods of below- normal temperatures Both are brought on by high-pressure systems except cold waves are brought on by continental polar or arctic air masses. Heat and Cold Waves