Atmospheric Hazards Part II

1. Atmospheric HazardsPart II Heat Waves, Winter Storms, and Hurricanes Session 8

2. Air Mass • Large body of air (usually > 1000 miles +/- in diameter) that exhibits relatively homogeneous properties of temperature and moisture

3. Source Regions • Locations where air masses develop • Classification from temperature and the characteristics of the ground surface • Temperature can be Arctic, Polar, or Tropical • Surface type can be Continental or Maritime

4. Source Region Types • Maritime (ocean) surface (m); provides very moist air • Continental (land) surface (c) ; provides dry air • High (polar) latitudes; source of cold air • Low (tropical) latitudes; warm air source

5. Air Mass Regions

6. Letter Designation • Lower case = nature of the surface • Upper case = temperature

7. Basic Air Mass Types • Maritime • Continental

8. Maritime (m) • Maritime Polar (mP) • High latitudes • Cool and Moist • Humid all year, mild or cold, mild temperatures may have low clouds in winter, cold storms like a Nor'easter, found in North Pacific, Northwest Atlantic

9. More Maritime • Maritime Tropical (mT) • Low latitudes • Warm and Moist • Warm and humid year round • Gulf of Mexico

10. Continental (c) • Continental Polar (cP) • High latitudes • Cold and dry • Found in Interior Canada and Alberta, dry entire year, cold in winter, cool in summer, cold waves in winter • Associated with “Lake Effect” snows around Great Lakes

11. Continental Arctic (cA) • Polar and Arctic Latitudes • Extremely cold and extremely dry • Arctic Basin and Greenland Ice Cap, very stable air mass all year, produces cold waves in winter

12. Continental Tropical (cT) • Low latitudes • Extremely hot and extremely dry • Hot, dry, and unstable; cloudless skies, occasional drought as in Great Plains

13. What are Fronts? • Fronts are a boundary between dissimilar air masses • Different densities, temperature and moisture (humidities)

14. Why are Fronts Important? • Mark the beginning of a major change in weather • Produce areas of convergence and divergence aloft thus forcing vertical motion fields in their vicinity

15. Types • Frontal types are determined by the type of thermal change in air mass character • 1. Cold front • 2. Warm front • 3. Occluded front • 4. Stationary front

16. Symbols for Fronts

17. Cold Front • Marks the leading edge of a colder air mass moving into a region • Cold front usually found in a low pressure trough • Drawn as a solid blue line with the triangles along the front showing its direction of movement

18. Cold Front Features • In the Northern Hemisphere, winds ahead of the cold front at the surface (warm side) will be generally from a southerly direction (can also range from SE to SW) and shift to a more westerly direction (could range from W to N) behind the front (in the cold air) • One of the best ways to determine the time of frontal passage is in recognizing the abrupt wind shift

19. Warm Front • Marks the leading edge of a warmer air mass moving into a region • Solid red line with semicircles pointing in direction of movement • Usually found in a trough

20. Warm Front Features • Winds are generally from an easterly direction ahead of the front at the surface (on the cold side) while shifting to a more southerly direction behind the front in the warm air

21. Occluded Front • Occurs when a cold front (that typically moves faster than a warm front) catches up and Merges with a warm front • Purple line with alternating cold-front triangles and warm-front semicircles pointing in the direction toward which the front is moving

22. Occlusion Characteristics • Cold occlusion: cold front takes over a warm or stationary front (cold air in back) • Warm occlusion: warm front overtakes a cold or stationary front (cold air in front)

23. Stationary Front • Boundary between dissimilar air masses that moves very slowly back and forth so that over a relatively long period of time, it appears not to be moving at all • Winds generally from the south ahead of the front at the surface (on the warm side) while northerly behind the front in the cold air • Drawn as alternating red and blue line with semicircles facing the cold air on the red line and the triangles pointing towards the warmer air on the blue line

24. Heat Waves • “A period of abnormally and uncomfortably hot and unusually humid weather. Typically a heat wave lasts two or more days.” • Definition from National Weather Service • Location Dependent

25. Heat Advisory • Issued within 12 hours of the onset of the following conditions: heat index of at least 105̊F but less than 115̊F for less than 3 hours per day, or nighttime lows above 80̊F for 2 consecutive days • Threshold value is expected or has been present for two or more consecutive days

26. Excessive Heat Warning • Prolonged periods are expected or extremely high indices are expected for a single day

27. Heat Index • "Apparent Temperature" is an accurate measure of how hot it really feels when the Relative Humidity (RH) is added to the actual air temperature • How it really feels like to an individual

28. Heat Exhaustion • Mild form of heat stroke, characterized by faintness, dizziness, and heavy sweating

29. Heat Stroke • Condition resulting from excessive exposure to intense heat, characterized by high fever, collapse, and sometimes convulsions or coma

30. Heat Index Categories

31. Heat Wave Causes • Absence of polar air masses (jet stream further north of area; strong ridge) • Strong heating of the surface (sinking of atmosphere, high pressure, leads to a warm air dome)

32. Additional Causes of Heat Waves • Dry ground (lack of soil moisture) • Amount of vertical mixing of air (vertical mixing near surface is weak or confined to shallow depth, little mixing of drier air aloft to reach surface and mix with warm, moister air near ground)

33. Urban Heat Island • Heat is often worse in cities compared to rural areas • It can be very noticeable at night and difficult sleeping without air conditioning

34. Heat Wave Case Studies • Chicago 1995 • United States 2006

35. Midlatitude Cyclones • Source of most stormy weather in the United States • Does not include “garden variety” thunderstorms or tropical cyclones

36. Phenomena • Severe thunderstorms • Tornadoes • Severe winter storms • Blizzards • Heavy Snow • High winds • Floods

37. Cyclogenesis • Development or strengthening of a midlatitude cyclone

38. Source Regions • Major areas in the United States for cyclogenesis • Eastern slopes of Rocky Mountains • Great Basin • Gulf of Mexico • Atlantic Ocean off of North Carolina

39. Snow • Precipitation composed of white or translucent ice crystals, chiefly in complex branch hexagonal form and often agglomerated into snowflakes • Intensity characterized as light, moderate, and heavy

40. Snow Intensities • Light: Visibility is 1 km (5/8 statute mile) or more • Moderate: Visibility less than 1 km but not more than ½ km (5/16 statute mile) • Heavy: Visibility less than ½ km

41. Snow Grains • Precipitation in the form of very small, white opaque particles of ice • Solid equivalent of drizzle • Also called granular snow • Similar to snow pellets in appearance, but more flattened and elongated with diameters < 1 mm • Neither shatter or bounce when they hit hard surface

42. Snow Pellets • Precipitation consisting of white, opaque, approximately round (sometimes conical) ice particles having a snowlike structure about 2-5 mm in diameter • Crisp and easily crushed • Rebound when they fall on a hard surface and often break up • Often fall in a shower before or with snow and when the surface temperature is at or slightly below 0° C

43. Freezing Rain • Falls as liquid but freezes after hitting the surface • Surface is below freezing while atmosphere is warmer

44. Winter Storm Outlook • Major storm expected in next 3-5 days

45. Winter Storm Advisory • Mixed precipitation (snow, sleet, freezing rain) expected but not warning in 12-24 hours

46. Winter Storm Watch • Heavy snow, sleet, blizzard conditions, significant accumulations of freezing rain or drizzle, or combination of types within 24-48 hours

47. Winter Storm Warning • Winter precipitation expected in 12-24 hours, may include “near-blizzard” conditions

48. Ice Storms • Usually freezing rain • Can cause significant hazards such as ice on roads, power lines, and fallen trees

49. Blizzards • Sustained winds ≥ 35 mph, falling or blowing snow, visibilities at or below ¼ mile, and lasting a minimum of three hours

50. Blizzard Formation • Occur within circulation of extra-tropical cyclones as the cyclones provide wind and snow • North American geography has favorable environment for blizzard formation due to north-south orientation of Rocky Mountains • Three ingredients (cold air, strong winds, and moisture) are necessary to form a blizzard