Air Masses, Fronts, and Middle-latitude Cyclones

1
Air Masses, Fronts, and Middle-latitude Cyclones

• Chapter 8

2
Air Masses Fronts

• Very warm this weekend huh?
• 80s
• dewpoints in the 60s (compared to the teens a
  couple of weeks ago)
• southerly winds
• So where did this warm/humid air come from? How
  did it form?

3
Air Masses Fronts

• Defn. - Air Mass - a large body of air which has
  similar moisture temperature in any horizontal
  direction (at all altitudes)
• Exs.-
• 1) This warm air over us right now
• 2) Much of the eastern U.S. is hot humid during
  summer
• 3) Cold air outbreaks in the winter

4
Air Masses Fronts

• Two reasons that we have different air masses
• 1) Earth is heated unevenly
• 2) Much of the earth is covered by water
• So, the properties of any air mass will be
  dependent upon where the air mass originates
• Moist air masses over water for example
• Where an air mass originates is called a source
  region

5
Source Regions

• Not all areas are good source regions
• Generally a couple of things needed to have a
  good source region
• 1) Flat w/ uniform composition
• ocean, desert, etc. (allows for uniform
  characteristics)
• 2) Surface winds should be light
• 3) Influence of high pressure
• this means air will have been over the same area
  for quite a while before beginning to move

6
Source Regions

• Ideal locations
• Oceans
• Deserts
• Snow/Ice covered plains
• Middle-latitudes (like where we live) are not
  good. Why?
• Too much variability
• water, forests, plains, mountains, etc.

7
Air Masses

• Grouped into 4 categories
• Really only two things we need to consider in
  order to classify them
• 1) Land or water?
• 2) Cold or hot?

8
Air Masses

• 4 categories
• 1) c (continental) - source region over land
• 2) m (maritime) - source region over water
• 3) T (Tropical) - source region in the tropics
• E (Equatorial) - extremely warm version of
  Tropical
• 4) P (Polar) - source region in the polar regions
• A (Arctic) - extremely cold version of Polar
• Just combine 1 or 2 with 3 or 4 and thats it

9
Air Masses
10
Air Masses (cP)

• Continental Polar/Arctic (cP or cA)
• Cold/dry air masses
• Originate over the ice/snow covered regions of
  northern Canada and Alaska
• Long, clear nights allow for maximum cooling
• Very dry
• Usually stay in place but occasionally break away
  and are carried southeast by the upper level
  winds
• Drops in toward the plains so there is not much
  terrain to impede it

11
Air Masses (cP)

• Moderate (warm up) as they move southward
• Particulary if they move over warm water
• Produce lake-effect snow up north

12
Lake-effect Snow

• Focus section on pg. 200
• Occur November-January downwind from lakes
• Great Lakes in particular
• Why not past January? Colder in February
• Lakes get too cold or frozen over

13
Lake-effect Snow

• Huge impact on annual snowfall near lakes
• 50 here
• 200 here!

14
Lake-effect Snow
15
Lake-effect Snow
16
Lake-effect Snow

• Cold air moves over (relatively) warm water
• Air is warmed, picks up moisture, and rises
• Lifting of air by hills over land plus
  convergence (air slows down when it hits land)
  causes heavy snow

17
Lake-effect Snow

• Intensity is controlled by many factors with the
  3 most important being
• 1) Temp difference between air and lake
• Greater difference - better likelihood of snow
• 2) Direction of wind
• Determines how far over water air must travel -
  longer - more snow
• 3) Upwind bodies of water
• Air spends more time over water

18
Lake-effect Snow

• Who gets the most?
• Middle city
• Air must travel a long distance over several
  lakes before it reaches land in Ohio

19
Air Masses (mP)

• Maritime Polar (mP)
• Cold/moist air masses
• Affect the Pacific northwest and northeast U.S.
  coastal areas
• More rare in NE though - weather systems
  generally move west to east
• Often start out as cP over Asia and travel toward
  U.S. over the Pacific
• Water warms and adds moisture to air changing it
  to mP by the time it reaches the coast

20
Air Masses (mP)

• mP air masses are responsible for rain and snow
  along the West Coast
• Rain at the coast
• Snow in the mountains

21
Air Masses (mP)

• mP air is drier (modified mP) once it reaches the
  plains
• Moisture is forced out by terrain

22
Air Masses (mT)

• Maritime Tropical (mT)
• Warm/moist air masses
• These are the ones that affect us in Tallahassee
  the most (every day during summer)
• Originate
• Gulf of Mexico
• Sub-tropical eastern Pacific
• Western Atlantic

23
Air Masses (cT)

• Continental Tropical (cT)
• Hot/dry air masses
• In North America, only source regions are
• Desert southwest
• Northern Mexico
• Generally clear skies
• High pressure
• Responsible for summer drought conditions

24
Air Masses

• Weather associated with the previous 4 air masses
  is called Air Mass Weather
• Particularly common in the Southeast (us) during
  the summer (hot/humid) and the Pacific Northwest
  in winter (cool/rainy)
• But.the good stuff as far as weather goes
  actually occurs at the edges of air masses rather
  than within them.Fronts

25
Fronts

• Areas of transition between air masses
• Technically Front - transition zone between two
  air masses of different densities
• Also different temperatures since temp is related
  to density (ideal gas lawremember)
• Often different humidities also
• Recall air masses have both horizontal and
  vertical extent.so the upward extension of the
  front is called a frontal surface or frontal
  zone

warm
cold
26
Fronts

• The various fronts (4 total) are zones of
  transition between air masses

27
Cold Fronts

• Cold fronts are transition zones where cold/dry
  air masses are replacing warmer/more moist air
  masses
• mP replacing cT or mT
• cP replacing mP, cT, or mT

28
Cold Fronts

• Marked w/ blue triangles pointing in direction of
  cold air movement
• Look like icicles (cold!)
• Cold air this side

29
Cold Fronts

• Finding fronts
• Sharp change in temp over a short distance
• Change in moisture content
• Shift in wind direction
• Pressure change
• pressure drops as front approaches, rises as it
  passes
• Clouds and rain ahead of and at the front

30
Cold Fronts

• Ahead of the cold front
• Warm, humid
• Winds generally have a southerly component to
  them
• Rain and maybe thunderstorms

31
Cold Fronts

• Behind of the front
• Cold, dry
• Winds switch to north or northwest
• Clearing skies
• Pressure begins to rise
• As high pressure moves in

32
Cold Fronts

• Approach can be detected by observing cloud
  patterns
• Usually see cirrus clouds first as the tops of
  thunderstorms 100s of miles away are carried
  rapidly by the upper level winds
• T-storms can form due to the steep slope of a
  cold frontal surface - acts like a plow causing
  rapid ascent of air

33
Cold Fronts

• So far weve describe a typical cold front
• In reality, cold fronts can be much more
  difficult to see or feel
• Sometimes the temperature doesnt change much
• Maybe only the moisture or wind direction changes
• Can be very subtle

34
Warm Fronts

• Transition zones where warm/moist air masses are
  replacing colder air masses
• mT or cT replacing mP or cP

35
Warm Fronts

• Red semi-circles
• Again, symbols point in the direction of movement
• Big differences from cold fronts
• 1) Frontal surface is much less steep
• 2) Warm fronts move at about 1/2 the speed of
  cold fronts

36
Warm Fronts

• Because the frontal surface is not as steep as
  with a cold front, thunderstorms are not likely
• Usually light to moderate precipitation as the
  air gradually ascends along the frontal surface -
  overrunning
• This is the way sleet an freezing rain is usually
  produced
• Precipitation is more uniform than w/cold fronts
  and covers a larger area

37
Stationary Fronts

• Often, air masses stop moving
• Hit a boundary maybe (mountains)
• Or just move up against another air mass that is
  not willing to move
• These types of fronts which do not move are
  called stationary fronts

38
Stationary Fronts

• Alternating cold and warm front symbols
• symbols point in the direction of movement
• cold to north, warm to south
• Winds are generally parallel to the front

39
Occluded Fronts

• Recall that warm fronts move at 1/2 the speed of
  cold fronts
• This means that cold fronts will eventually catch
  up to and overtake warm fronts
• When this happens the front is called an
  occluded front indicating cold air on both
  sides of the front
• Up until now, weve been talking about fronts
  which have cold air on one side and warm air on
  the other

40
Occluded Fronts

• Represented by alternating warm and cold front
  symbols
• purple
• both pointing in the same direction
• 2 types of occluded front...

41
Occluded Fronts

• Cold Occlusion
• Air behind the cold front is the coldest
• Most common in the eastern US where cP air drops
  south out of Canada

mP
cP
mT
42
Occluded Fronts

• Coldest air is the heaviest so it wedges itself
  under the less cold air (ahead of the warm front)

43
Occluded Fronts
44
Occluded Fronts

• Warm Occlusion
• Occurs when the coldest air is ahead of the warm
  front
• Very common on the West Coast as storms move
  onshore from the Pacific

cP
mP
mT
45
Occluded Fronts

• Unlike with the cold occlusion, the air behind
  the cold front is not the coldest
• So when the cold front overtakes the warm front,
  the air is forced to rise over top of the coldest
  air

46
Occluded Fronts
47
Occluded Fronts

• Summary
• Occluded fronts occur where cold fronts overtake
  warm fronts
• Can be either cold or warm occlusions
• Usually a great deal of cloudiness and rain near
  occluded fronts due to rising air

48
Mid-latitude Systems

• The main mechanisms for the movement of air
  masses and fronts are called mid-latitude
  systems
• These are the low pressure centers in the middle
  latitudes (30º-60º)

49
Mid-Latitude Cyclone
L
50
Mid-latitude Systems

• Remember from last chapter.
• Low pressure at 60º latitude
• Called the Polar Front
• Back in the early 1900s, a group of Norwegian
  meteorologists developed a model of the lifetime
  of mid-latitude storms (low pressure systems)
  which develop along the polar front
• Called Polar Front Theory
• Did it all with surface based observations - just
  like the extra credit project

51
Polar Front Theory

• 6 stages
• Starts off with air flowing in opposite
  directions on either side of the front
• Causes cyclonic shear along the front
• gets things spinning

52
Polar Front Theory

• Under the right conditions, the shear will cause
  the front to kink into the beginning of a
  frontal wave
• This begins to send cold air southward and warm
  air northward (fronts)

53
Polar Front Theory

• The fronts join at the lowest pressure
• Start to get clouds and precipitation as air
  converges and rises
• System start to move to the east

54
Polar Front Theory

• Fully developed system now - open wave
• Pressure is dropping at the center (more isobars
  now)
• More precipitation
• Warm sector appears ahead of the cold front

55
Polar Front Theory

• Usually nice weather within the warm sector
• Sometimes thunderstorms if unstable though
• We almost always experience warm sector weather
  before a cold front

56
Polar Front Theory

• Warm sector shrinking as the cold front catches
  up to the warm front

57
Polar Front Theory

• Occluded front forms as the cold front finally
  overtakes the warm front
• Most intense stage of the storm
• More precipitation and wind (isobars)

58
Polar Front Theory

• Notice that the warm sector is now detached from
  the low center
• The warm, moist inflow of air to the center is
  cut off
• beginning of the end

59
Polar Front Theory

• Dissipating stage
• Cold air all around the center
• Just a weak low pressure center now with dying
  winds and little if any precipitation

60
Polar Front Theory

• These storms are also called wave cyclones
• looks like a breaking wave
• Entire process lasts several days to 1 1/2 weeks
• Can occur as families of cyclones

61
Families of Cyclones

• Just starting
• Mature stage
• Occluded stage

62
Cyclones

• The development of a wave cyclone is termed
  cyclogenesis
• Preferred regions in the U.S.

63
Cyclones

• Gulf and Hatteras lows sometimes become
  Northeasters
• Heavy snow in New England
• Like to see these if youre a skier