Chapters 10, 11, and 12

1. Chapters 10, 11, and 12 Weather of Middle Latitudes Severe Weather Tropical Systems

3. Air Masses • Types • cT – continental tropical • mT – maritime tropical • mP – maritime polar • cP – continental polar • A – arctic • Air masses are modified three ways • Exchange of heat or moisture • Radiational heating or cooling • Adiabatic heating or cooling with vertical motion

5. Fronts • Stationary • No lateral movement • Wind blows approximately parallel to isobars • If precipitation occurs it is light and it occurs on the cold side • Warm • Cold air retreats and warm air advances • Widespread steady precipitation ahead of front • Light drizzle and fog along the front

8. Fronts • Cold • Cold more dense air displaces warm less dense air • Slope of the front is much steeper, so in warm unstable air there is significant lift and storms • Squall Line • A band of intense thunderstorms that develop along or ahead of a cold front • If warm air is stable precipitation is brief and showery in a narrow band close to the front

10. Back Door Cold Front

11. Fronts • Occluded • Cold Occlusion • Air behind the advancing cold front (cP) is colder than the cool air ahead of the warm front (mP) • Warm Occlusion • Air behind the advancing cold front (mP) is relatively mild compared to cold air ahead of the warm front (cP) • Neutral Occlusion • No temperature change, but showers present and a shift in winds

13. Extratropical Cyclone • A Low pressure system that is a major weather maker for mid-latitudes • Cyclogenesis – the birth of a cyclone • An extratropical cyclone has 4 stages • Incipient • Wave • Occlusion • Bent back occlusion

14. Extratropical Cyclone • Triple Point • Point where occluded, cold, and warm front meet • Sometimes a secondary cyclone can form here • Bomb • A rapidly developing extratropical cyclone • Central pressure drops at least 24mb in 24 hours • Cyclolysis (Filling) • When the central pressure in the low begins to rise • Death of a cyclone

15. Idealized Mature Cyclone

16. Principal Cyclone Tracks

17. Circulation Systems • Land/Sea Breezes • Chinook Winds • Foehn, Zonda, Santa Ana • Desert Winds • Dust devil • Haboob – caused by strong thunderstorm downburst • Mountain/Valley Breezes

18. Sea Breeze • Land Breeze

19. Santa Ana Winds

20. Dust Devil

21. Haboob

22. Valley Breeze • Mountain Breeze

23. Thunderstorm Life Cycle

24. Thunderstorm Classification • Single Cell Thunderstorm • “pop up” storms in warm humid air masses that are shortlived • Multicell Thunderstorms • Lightning, thunder, and rain that persist for many hours • Each cell may be at a different stage • Two types • Squall line • Mesoscale Convective Complex (MCC) • An area of many interacting thunderstorm cells (very large area) • Supercell Thunderstorm • Strong updraft with rotation that may spawn a tornado

26. Conditions for Thunderstorms • Humid air in the middle to lower troposphere • Atmospheric instability • A source of uplift

28. Severe Thunderstorms • Must have at least one of the following • Hailstone greater than ¾” in diameter • Tornadoes or a funnel cloud • Surface winds greater than 58 miles per hour • For Development • Vertical wind shear • Mature synoptic scale cyclones

30. Thunderstorm Hazards • Lightning (thunder) • Downbursts • Macro (>2.5mi, winds ~ 130mph, 30 min) • Micro (<2.5mi, winds ~ 170mph, 10 min) • Flooding • Hail • Tornado

31. Tornadoes • Violently rotating column of air that is in contact with the ground made visible by condensation, dust, and/or debris • Most violent, short lived, lots of damage • 10% of severe tstorms produce tornadoes • April 3-4 1974 – 148 in 13 states, 315 deaths, $600 million in damage • May 3 1999 – 70 in 3 states, 55 deaths, $1.1 billion in damage, (max wind = 318mph) • March 18, 1925 – 1 in 3 states, 695 deaths, 3.5 hours, path 219 miles

34. Average Annual Number of Tornadoes Per 10,000 square miles

35. Why Tornadoes Are Dangerous • Extremely high winds • Strong updraft • Subsidiary vortices • Abrupt drop in air pressure

38. Hurricane • A violent tropical cyclone that originates over tropical ocean waters with maximum sustained wind speed greater than 74mph • Different from extratropical cyclone • Smaller, but more intense (lower central pressure) • No fronts • Upper air flow is anticyclonic • Presence of an eye and and eye wall

39. Hurricane Season in the western Atlantic is June 1 to November 30, with the peak being September 10

40. Hurricane Hazards • Heavy rains and floods • Some rain can be extremely beneficial, especially if suffering from a drought • Strong winds • Tornadoes • Storm Surge • 60% of all hurricane deaths are due to flooding (1970-1999) – before 1970 storm surge was major cause

41. Storm Surge

42. Saffir-Simpson Hurricane Intensity Scale

43. Life Cycle of a Hurricane • Tropical Disturbance – organized cluster of cumulonimbus clouds over tropical seas with a detectable low pressure center • Easterly Wave – a ripple in the trade winds featuring a weak trough of low pressure • Tropical Depression – winds > 23mph • Tropical Storm (name) – winds > 39mph • Hurricane – winds > 74mph

44. Hurricane Tracks

45. Hurricane Gilbert - 1988

46. Hurricane Mitch - 1998