Understanding Atmospheric Circulation and Severe Weather Phenomena

1. Severe Weather

2. Solar Heating and Latitude

3. Solar Heating and the Seasons

4. Relationship of sun angle and solar radiation received on Earth

5. Air at high elevations: Cooler Expands Water vapor tends to condense Air at sea level: Warmer More compressed Can hold more water vapor Solar Heating and Atmospheric Circulation

6. Air Circulation & Convection Currents

7. Coriolis Force

8. Atmospheric Circulation & Convection Cells

9. Air masses are classified on the basis of their source region

10. Fronts • Types of fronts • Warm front • Warm air replaces cooler air • Shown on a map by a line with semicircles • Small slope (1:200) • Clouds become lower as the front nears • Slow rate of advance • Light-to-moderate precipitation

11. Fronts • Types of fronts • Cold front • Cold air replaces warm air • Shown on a map by a line with triangles • Twice as steep (1:100) as warm fronts • Advances faster than a warm front • Associated weather is more violent than a warm front

12. Cold Fronts and Warm Fronts

13. Rotating Air Bodies • Bends in the polar jet create troughs and ridges • Forms cyclones and anticyclones

14. Low Pressure Zone Formation Warm air rises Creates a low pressure zone At the Earth’s surface, air “feeds” the low pressure zone, moves counterclockwise High Pressure Zone Formation Cool air sinks Creates a high pressure zone At the Earth’s surface, winds blow clockwise Rotating Air Bodies

15. Cyclones and Anticyclones

16. Types of Severe Weather • Thunderstorms • Snow / Rain storms • Mid-latitude cyclones • Blizzards • Tornadoes • Tropical cyclones • Typhoons in the western Pacific • Cyclones in the Indian Ocean • Hurricanes in the U.S.

17. Stages in the development of a thunderstorm

18. Thunderstorms • How Lightning Works

19. Thunderstorms

20. Lightning Varieties cloud-to-ground Cloud discharge Ball lightning Blue jets Red sprites Elves (NOVA: Science Now – Lightning http://www.pbs.org/wgbh/nova/sciencenow/3214/02.html)

21. Lightning Varieties Volcanic Lightning Nuclear Lightning Triggered Lightning (NOVA: Science Now – Lightning http://www.pbs.org/wgbh/nova/sciencenow/3214/02.html)

22. Thunderstorms • Lightning - Don’t Get Struck! • Boating or swimming – get away from the water. • Try to take shelter in: • substantial, permanent, enclosed structures. • a car, truck or other hard-topped vehicle. • an area protected by a low clump of trees. • No shelter available? • Find a low-lying, open place away from trees, poles/metal objects and water. • Make yourself the smallest target possible. • Do not lie flat, as this makes you a larger target.

23. Severe weather types • Tornadoes • How a Tornado Forms • Moist air from Gulf of Mexico • Fast moving cold, dry air mass from Canada • Jet stream moving east at 150 mph • Sets up shearing conditions

24. Severe weather types • Tornadoes • How a Tornado Forms • Warm moist Gulf air releases latent heat, creates strong updraft • Updraft sheared by polar air, then twisted in a different direction by jet stream

25. Severe weather types • Tornadoes • Why do some thunderstorms spawn tornadoes while others do not? • Super Cell Thunderstorms

26. Severe weather types • Tornadoes • The Fujita-Pearson Scale • The size of a tornado is not necessarily an indication of its intensity!

27. Tornadoes • “Tornado Capitol of the World” • CNN’s “10 deadliest U.S. tornadoes” Source: cnn.com

30. Tornadoes • Tri-State Tornado, 18 March 1925 • Largest tornado known • Travelled 353 km (219 mi) across Missouri, Illinois and Indiana • Widest swath recorded - 1 mi in diameter • Devastated 23 cities • Killed 695 people and injured 2,027 In this photo, engineers examine a board that the tornado's high-speed winds drove through a larger plank. cnn.com

31. Tornadoes • The Super Outbreak, 3-4 April 1974 • 5 weather systems collided • Dry air from the SW overrode moist Gulf air, creating an inversion layer • Gulf air pushed through the inversion layer • Thunderstorms developed

32. Tornadoes • The Super Outbreak, 3-4 April 1974 • 147 tornadoes • 6 F5 tornadoes • 13 states • 16 hours

33. Tornadoes • Why don’t tornadoes strike large cities? • Occur over large regions • Cities are relatively small targets • Oklahoma City Tornado (1999)

34. Tornadoes • Safe Rooms • Best ones are underground • Some are above ground

35. Mid-latitude Cyclones • Idealized weather • Middle-latitude cyclones move eastward across the United States • First signs of their approach are in the western sky • Require two to four days to pass over a region • Largest weather contrasts occur in the spring

36. Mid-latitude Cyclones

37. Mid-latitude Cyclones • Nor’easters • The Eastern U.S. “White Hurricane” of 1993 • AKA “Storm of the Century” • Three storm fronts all converged with a trough in the jet stream • Collision began in Florida, and moved up the eastern seaboard with the jet stream • 238 people died from Cuba to Canada • 48 sailors lost at sea

38. X 1. Low pressure zone from Gulf of Mexico – lots of thunderstorms 2. Trough in jet stream drew in fast-moving arctic front 3. Trough also drew in a rain/snow front from the Pacific 2 1 3

39. Mid-latitude Cyclones • Blizzards • Form when a long cyclone brings • Cold 60 km/hr winds • Freezing temperatures • Lots of snow • Can travel very slowly • Storm itself usually doesn’t kill • Shoveling snow, auto accidents, etc.

40. Mid-latitude Cyclones • Blizzards • Northeastern United States, 6-8 January 1996 • Storm centered on Ohio, Pennsylvania, West Virginia, and New Jersey • 50 mph winds, record snow falls • 154 people died • Warm, wet weather immediately followed • 187 people died

41. Mid-latitude Cyclones • Ice Storms • Formation: • Falling snow and ice melt, change to rain, then freeze again as they reach the ground • Sleet • Freezing rain

42. Mid-latitude Cyclones • Ice Storms • Canadian Ice Storm, 5-9 January 1998 • 80 hours of freezing rain • Power systems collapsed • had to be completely replaced • People without power for up to 4 weeks • 16 U.S. and 28 Canadian deaths • Damages • $1.4 billion for the U.S. • $3 billion for Canada Source: cnn.com

43. Hurricanes

44. Hurricanes • Only natural disaster that is given a human name • Actually large tropical cyclones • Convert heat in the ocean into winds • Exports excess heat from the tropics to the midlatitudes

45. Hurricanes • How a Hurricane Works • Tropical disturbance • Low pressure zone develops and draws in clusters of thunderstorms and winds

46. Hurricanes • How a Hurricane Works • Tropical disturbance • Tropical depression • Surface winds strengthen, move about the center of the storm • Central core funnels warm moist air up towards stratosphere • Air cools, vapor condenses, latent heat released • Fuels more updrafts, cycle repeats, storm grows

47. Hurricanes • How a Hurricane Works • Tropical disturbance • Tropical depression • Tropical Storm • Storm has sustained surface wind speeds of +39 mph

48. Hurricanes • How a Hurricane Works • Tropical disturbance • Tropical depression • Tropical Storm • Hurricane • Surface winds consistently over 74 mph

49. Hurricanes • How a Hurricane Works • Tropical disturbance • Tropical depression • Tropical Storm • Hurricane • The Eye • As wind speed increases, winds are spiraled upwards prior to reaching the center • A distinctive clear “eye” is formed • Strongest winds are located on the walls of the eye

50. Hurricane Wind Patterns