Air Masses and Severe Weather

1. Air Masses and Severe Weather Integrated Science

2. Air Mass • Huge volumes of air that can cover entire continents or oceans • Several types that each have specific characteristics • see text p. 689

4. Atmospheric Lifting • Clouds are indicators of weather • For clouds to form, air must be lifted • Three types of lifting cause clouds.

5. Lifting Type #1 • Convectional Lifting • Earth’s surface is heated by the sun, which heats the air mass above it. The heated air will then rise.

6. Convectional Lifting

7. Convectional Lifting • If cooling occurs near the air mass’ saturation temperature, often cumulus clouds may form

9. Convectional Lifting • Often happens in the mid-west during summer

10. Atmospheric Lifting Type #2 • Orographic Lifting • An air mass is pushed upward over an obstacle, such as a mountain range.

11. Atmospheric Lifting Type #2Orographic • As the air mass is pushed over the obstacle, the air mass will lose moisture in the form of clouds, rain, or snow

12. Atmospheric Lifting Type #2

13. Atmospheric Lifting Type #2

14. Atmospheric Lifting Type #2 • The clouds dump their load of rain on the windward slope • The down slope (leeward) air will be dry = rain shadow

15. Rain shadow- orographic lifting

16. Rain shadow • Western Kansas is so dry only certain short grasses can grow; not enough rain for trees

17. Rain Shadow; Big Hatchet Mt, New Mexico. Notice how dry the land is

18. Orographic Lifting – Chinook Winds • At the high elevations of the top of a mountain, the air pressure is low. • As the air descends the mountain is under more pressure. • The increased air pressure heats the air causing warm winds called Chinook Winds • Temperatures can rise 20-300 as air moves down the mountain

19. Chinook Winds

20. Atmospheric Lifting Type #3 • Frontal Lifting • This occurs at the mesh point between two different temperature air masses. • One air mass (warm) is pushed upward by the other (cold). • This can be associated with thunderstorms.

21. Frontal Lifting • Three types of frontal lifting • A) warm front • B) cold front • C) Stationary front

22. Frontal Lifting • Cold Front: When a cold air mass moves into a warm air mass. • Cold, denser air forces the warm air up where it cools and condenses, forming clouds

23. Cold Front: • Warm air is abruptly pushed upward, cooling, condensing moisture into cumulus or cumulonimbus clouds

24. Cold Front • Notice how steep the angle is between the two air masses • Typically brings sudden, heavy rains and storms

26. Cumulonimbus: Lake Superior

27. Warm Front • WARM FRONT: when a warm air mass moves into a colder,denser air mass. • Warm air rides up and over the colder air

28. Warm front

29. Warm Front • Notice the angle of slope between the two air masses.

30. Warm Front • From the ground, one would observe the following cloud sequence • A) cirrus • B) cirrostratus • C) altostratus • D) stratus • E) nimbostratus

31. Warm Front • The weather during a WARM FRONT starts with cirrus clouds about 24-48 hours before the rain begins • Cirrus clouds are “at the front of the front”

32. Warm Front • As more warm air is pushed upward, more moisture condenses forming cirrostratus clouds

33. Cirrostratus on Lake Superior

34. Warm Front • As warm more warm air is pushed up, heavier clouds form mid-way up over the cold air • Altostratus and stratus

35. Altostratus clouds: Lake Superior

36. Warm Front • The final cloud type in a warm front is the nimbostratus • “nimbo” = “rain”

37. Warm Front • Warm front: rain or snow is steady over several hours or days

38. Quiz Yourself!!! T or F

39. QUIZ: T or F • For clouds to form, air must be lifted • There are FOUR different types of lifting • Orographic lifting refers to air movingup and over a mountain

40. QUIZ: T or F • Convectional lifting is the circulation of warm air rising, cold air sinking • A rain shadow is caused by WET air descending down the leeward side of a mountain • A WARM FRONT means warm air is riding up over cold air • A WARM FRONT produces sudden, severe rain and storms

41. Severe Weather 

42. Thunderstorms • Cold Fronts • Begins with humid air rising, cooling, and condensing into a single cumulus cloud. • Cloud builds as they are “fueled” by warm, moist air from below.

43. Thunderstorms • The droplets of water grow larger until they are so big that they fall as rain.

44. Cumulonimbus clouds: L Superior

45. Thunderstorms (con’t) • Lightning may be associated with T.storms • Hits Earth 100 times per second • Over 200 people in the US die each year from lightning.

46. Lightning • Lightning is the cause of thunder • returns nitrogen to the soil • may cause fires • May have helped start pre-biotic formation of amino acids(?)

47. Lightning Formation • During thunderstorms strong updrafts cause molecules to bump together and their charges to separate • Negative charges concentrate at the base of the clouds

48. Lightning Formation • The ground has lost electrons, giving it a positive (+) charge • Opposites attract: The (-) from the cloud base is “pulled” to the (+) • First downward movement of (-) is called a leader

49. Lightning Formation • When the (-) and (+) touch, they create a conductive path to the ground. • The other (-) rush down this path = lightning

50. Lightning on L.Superior