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 #2 Orographic
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 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 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. For clouds to form, air must be lifted
QUIZ: T or F
For clouds to form, air must be lifted
There are FOUR different types of lifting
Orographic lifting refers to air moving up 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
                                                                               

51. Lightning on L.Michigan

52. Lightning strike

53. Quiz Yourself!! TRUE or FALSE

54. Cumulonimbus clouds can become thunderstorms
TRUE or FALSE?????
Cumulonimbus clouds can become thunderstorms
Stratus clouds produce sudden, severe thunderstorms and lightning
Lightning forms because of unequal charges

55. Tornadoes
A funnel shaped cloud that extends downward from a cumulonimbus cloud.
Called a tornado only once it hits the ground.
Winds inside travel at up to 800km per hour in counterclockwise direction.

56. Hurricanes
Caused by a warm, vertical wind interacting with an existing storm and causing the storm to turn inward and spiral.
Wind speeds of up to 300km per hour.
Hurricanes grow as more moist air rises into it.
Severe winds rotate around a calm, low pressure zone in the middle, which is the eye of the storm.