1. Air Masses, Fronts and Global Wind Patterns
Describe air masses and how they move.
Identify types of fronts and the weather they bring

2. Air Masses

3. Weather changes Did you ever hear this riddle?
Question: Why did the woman go outdoors with her purse open?
Answer: Because she expected some change in the weather!
Weather is always changing. One day might be cold and cloudy. The next day might be warm and sunny. Even on the same day, the weather can change a lot. A beautiful morning might be followed by a stormy afternoon. Why does weather change? The main reason is moving air masses.

4. Air Masses
An air mass is a large body of air that has about the same conditions throughout.
For example, an air mass might have cold dry air.
Another air mass might have warm moist air.
The conditions in an air mass depend on where the air mass formed.
A region under the influence of an air mass will probably experience generally constant weather conditions, a situation referred to as air mass weather.

5. Air mass formation Formation of Air Masses
Most air masses form over polar or tropical regions. They may form over continents or oceans.
Air masses are moist if they form over oceans.
They are dry if they form over continents.
Air masses that form over oceans are called maritime air masses.
Those that form over continents are called continental air masses.

6. Air Mass Formation
The Sun's heat causes air masses to form and circulate in the atmosphere.
This movement creates differences in air pressure, which in turn, creates winds.
Air moves horizontally because of differences in pressure.
When air is heated, the air expands, the density decreases, and the air rises.
When air is cooled, the air, the air becomes more dense and sinks.

7. Six major air masses affect weather in the United States.
Each air mass has the same characteristics of temperature and moisture content as the area over which it formed.

8. Air Masses
An air mass takes on the conditions of the area where it forms.
For example, a continental polar air mass has cold dry air.
A maritime polar air mass has cold moist air
An air mass can cover thousands of square kilometers.
When you observe a change in the weather form one day to the next, it is due to the movement of air masses.

9. Figure below shows air masses that form over or near North America.

10. Movement of Air Masses
Which air masses have warm moist air? Where do they form?
Movement of Air Masses
When a new air mass goes over a region it brings its characteristics to the region.
This may change the area's temperature and humidity.
Humidity The ratio of the amount of water vapor in the air at a specific temperature to the maximum amount that the air could hold at that temperature Moving air masses cause the weather to change when they contact different conditions.
For example, a warm air mass moving over cold ground may cause a temperature inversion.

11. Movement of Air Masses Why do air masses move?
Winds and jet streams push them along.
Cold air masses tend to move toward the equator.
Warm air masses tend to move toward the poles.
Coriolis effect causes them to move on a diagonal.
Many air masses move toward the northeast over the U.S. This is the same direction that global winds blow.

12. Video on Air Masses

13. Continental Arctic (cA):
Frigid – record low temperatures
Dry - very low dew points
Dense - very high barometric pressure
Usually originate north of the Arctic Circle
Siberian Express
Usually once or twice a winter
very rarely form during the summer
because the sun warms the Arctic.

14. Continental polar (cP):
Cold and dry - stable
Usually originates in NW Territory of Canada
Influences mainly the northern USA
Responsible for clear and pleasant weather during the summer
Usually in winter
Creates troughs in the polar jet stream
Lake effect snow in Great Lakes areas

15. Maritime polar (mP): Cool and moist - unstable
Originate over N. Atlantic and N. Pacific
Main Influence - the Pacific Northwest and the Northeast.
can form any time of the year
Generally not as cold as cP air masses

16. Maritime tropical (mT):
Warm and very moist – unstable
Originate in the Gulf of Mexico and the Southern Atlantic Ocean
Influences the eastern USA
Most prevalent during summer
Responsible for hot, humid summer days across the South and the East.

17. Continental Tropical (cT):
Very Hot and very dry – stable aloft
Originates in Desert Southwest and northern Mexico
Occurs in the summer, rarely in winter
Usually keeps the Desert Southwest scorching above 100oF during summer
Generally clear skies, hot, low humidity

18. Source Regions

20. Jet Stream
Jet streams are relatively narrow bands of strong wind in the upper levels of the atmosphere.
The winds blows from west to east in jet streams but the flow often shifts to the north and south.
Jet streams follow the boundaries between hot and cold air.
Since these hot and cold air boundaries are most pronounced in winter, jet streams are the strongest for both the northern and southern hemisphere winters

21. Jet Stream Why does the jet stream winds blow from west to east?
If the earth was not rotating, the warm air rising at the equator would move toward both poles
The earth's rotation divides this circulation into three cells.
The earth's rotation is responsible for the jet stream as well.

22. Jet Stream
The motion of the air is not directly north and south but is affected by the momentum the air has as it moves away from the equator.
The reason has to do with momentum and how fast a location on or above the Earth moves relative to the Earth's axis.
Your speed relative to the Earth's axis depends on your location.

23. The momentum the air has as it travels around the earth is conserved, which means as the air that's over the equator starts moving toward one of the poles, it keeps its eastward motion constant.
The Earth below the air, however, moves slower as that air travels toward the poles.
The result is that the air moves faster and faster in an easterly direction (relative to the Earth's surface below) the farther it moves from

24. Jet Stream
the three-cell circulations below, the regions are areas where temperature changes are the greatest.
As the difference in temperature between the two locations increase, the strength of the wind increases so they are also where the wind, in the upper atmosphere, is the strongest.

25. Jet Stream
Someone standing on the equator is moving much faster than someone standing on a 45° latitude line.
In the graphic (right) the person at the position on the equator arrives at the yellow line sooner than the other two.
Someone standing on a pole is not moving at all (except that he or she would be slowly spinning).
The speed of the rotation is great enough to cause you to weigh one pound less at the equator than you would at the north or south pole.

26. Fronts and their symbols

27. Fronts: Cold Warm Stationary Occluded Boundary between two air masses
Characterized by shift in weather
Cold
Warm
Stationary
Occluded

28. 5 Characteristics of a Front
Sharp temperature changes over a relatively short distance.
Changes in air moisture content
Shifts in wind direction
Pressure changes
Clouds and precipitation

29. Cold Front
A cold front occurs when a cold air mass runs into a warm air mass.
The cold air mass moves faster than the warm air mass and lifts the warm air mass out of its way.
As the warm air rises, its water vapor condenses.

30. Cold Front Clouds form, and precipitation falls.
If the warm air is very humid, precipitation can be heavy.
Temperature and pressure differences between the two air masses cause winds.
Winds may be very strong along a cold front.

31. Cold Front
As the fast-moving cold air mass keeps advancing, so does the cold front.
Cold fronts often bring sudden changes in the weather.
There may be a thin line of storms right at the front that moves as it moves.
In the spring and summer, these storms may be thunderstorms and tornadoes.
In the late fall and winter, snow storms may occur. After a cold front passes, the cold air mass behind it brings cooler temperatures.

32. Reasoning for Tornadoes
Orographic Perfection
Meeting of
Moist - mT
Hot - cT
Cool – cP
Rocky Mtn.

33. Tornado Alley

34. Fujita Scale
(NationalAtlas.com)

35. Cold Fronts Temperature – drops rapidly Pressure – rises steadily
Clouds – Vertical building
Precipitation – Heavy along front
Winds – Strong and shifting
Typically move faster than warm front

36. Cold Front
A cold front is drawn on a weather map as a blue line with triangles

37. Cold Front
(Fozzy)

38. In the summer, cold fronts can trigger:
thunderstorms
large hail
dangerous winds
tornadoes

39. Graphic Depiction!

40. Warm Fronts Temperature – rises slowly
Pressure – slight rise, then fall
Clouds – strato- and cirro-
Precipitation – long, steady
Winds – variable and light
Typically will have affect for days

41. Warm front
When a warm air mass runs into a cold air mass it creates a warm front. The warm air mass is moving faster than the cold air mass, so it flows up over the cold air mass. As the warm air rises, it cools, resulting in clouds and sometimes light precipitation.

42. Warm Front
Warm fronts move slowly and cover a wide area. After a warm front passes, the warm air mass behind it brings warmer temperatures. The warm air is also likely to be more humid.

43. Warm fronts generally bring cloudy weather.

44. Warm Front

45. Effects of warm fronts
Slow-moving warm front can mean days of wet weather before warm air
Sometimes water vapor in warm fronts condense to produce
rain
snow
sleet
freezing rain
A warm front is drawn on a map as a red line with red semicircles

46. Stationary Front
A stationary front occurs when a boundary between air masses stops advancing.
Such a front may bring clouds and precipitation to the same area for many days.
A stationary front is drawn as alternating red and blue line. Red semicircles point toward the cold air and blue triangles point toward the warm air

47. Stationary Fronts Temperature – stagnent
Pressure – slightly fluctuates
Clouds – altocumulus
Precipitation – none
Winds – variable and light
Can last for days weeks

48. Occluded Front

49. Occluded Front
With an occluded front, a warm air mass becomes trapped between two cold air masses.
The warm air is lifted up above the cold air.
Cloudy weather and precipitation along the front are typical.

50. Occluded Fronts Temperature – Pressure – Clouds – cumulus
Warm – gets milder
Cold – gets colder
Pressure –
Warm - slight drop
Cold – slight rise
Clouds – cumulus
Precipitation – steady and light
Winds – variable and light

51. Occluded Front

53. Different Temperatures - Different Pressures
Less
Dense
Pressure
Cool
Air
Denser
More
Pressure
Warm
Air

54. Pressure and Air Movement

55. Pressure Gradient Force
Difference in pressure over a given distance---between isobars
Close together = step pressure gradient
STRONG winds
Far apart = gentle pressure gradient
Light winds
Just like contour lines

56. Pressure Gradient Force

57. Isobaric Maps

58. Coriolis Effect
Apparent force due to the rotation of the Earth (Think Merry-go-round)
N. Hemisphere  wind turns right
S. Hemisphere  wind turns left
Strength depends on latitude and wind speed

59. Coriolis Effect

62. Coriolis Effect

63. Centripetal Force In-ward directed force
Allows an object to remain in circular motion
Winds moving around high and low pressure areas
Clockwise around Highs.
Counter-clockwise around Lows.

64. Friction (What a Drag) The resistance to movement
Surface winds are affected by friction
Why? Ground resistance:
trees, mountains, houses, buildings, etc.
This drag causes winds to blow across pressure gradient at the surface.

65. Pressures All Together

66. General Planetary Circulation

68. Winds Horizontal movements at surface
Names from WHERE it came from…not where it is going!!!

69. Around Pressure Cells

70. Pressure Cells High – In and Up Low – Down and Out Converge at surface
Ascend in center
Diverge Aloft
Low – Down and Out
Converge aloft
Descend in center
Diverge at surface

71. X-section of Planetary Circulation

72. January Global Pressure Map

73. July Global Pressure Map

74. Summer Highs

76. Quiz Questions:
Where would you expect there to be the strongest winds? Why?
Where would you expect there to be the calmest winds?
Where would you expect clear, cool skies?
Where would you expect cloudy skies with the greatest potential for precipitation?

77. Work Cited (Incomplete)
seen 1/03/06