Air masses and fronts

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Air masses and fronts

• Chapter 7 (last but not least)

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Air masses

• Air masses form when air lies over flat,
  homogenous terrain and remains in the same place
  for several days. As it sits there, the air tends
  to adopt the characteristics of the surface
  underneath.
• An air mass is an area of air that has the same
  characteristics of moisture and temperature
• Moisture continental/maritime
• Temperature polar/tropical
• cP, cT, mP, mT

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Maritime tropical

• Maritime Tropical Air Masses
• warm temperatures and rich in moisture
• Maritime tropical air masses originate over the
  warm waters of the tropics and Gulf of Mexico,
  where heat and moisture are transferred to the
  overlying air from the waters below. The
  northward movement of tropical air masses
  transports warm moist air into the United States,
  increasing the potential for precipitation.

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Continental polar

• Continental Polar Air Masses
• cold temperatures and little moisture
• Those who live in northern portions of the United
  States expect cold weather during the winter
  months. These conditions usually result from the
  invasion of cold arctic air masses that originate
  from the snow covered regions of northern Canada.
  Because of the long winter nights and strong
  radiational cooling found in these regions, the
  overlying air becomes very cold and very stable.
  The longer this process continues, the colder the
  developing air mass becomes, until changing
  weather patterns transport the arctic air mass
  southward.

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Fronts

• Fronts form the boundaries between two different
  air masses.
• 4 types cold fronts, warm fronts, stationary
  fronts, and occluded fronts.
• Fronts are the focal point of interesting
  weather.
• Ex. In the spring, cold fronts sweet across the
  midwestern states. East and south of the front is
  maritime tropical air from the gulf of mexico,
  and behind the front is continental polar air
  front canada. Often these cold fronts can be the
  location of tornadoes.

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Cold fronts

• - A cold front is the transition zone where a
  cold air mass is replacing a warmer air mass.
  Cold fronts generally move from northwest to
  southeast. The air behind a cold front is
  noticeably colder and drier than the air ahead of
  it. When a cold front passes through,
  temperatures can drop more than 15 degrees within
  the first hour.
• Symbolically, a cold front is represented by a
  solid line with triangles along the front
  pointing towards the warmer air and in the
  direction of movement. On colored weather maps, a
  cold front is drawn with a solid blue line.

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Cold fronts (cont)

• Cold fronts are characterized usually by a string
  of clouds along the frontal axis.
• Since the warm air being lifted has a near
  vertical ascent, the types of clouds are usually
  cumulus (heavy brief precipitation)

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Cold fronts on radar
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Warm front

• A warm front is defined as the transition zone
  where a warm air mass is replacing a cold air
  mass. Warm fronts generally move from southwest
  to northeast and the air behind a warm front is
  warmer and more moist than the air ahead of it.
  When a warm front passes through, the air becomes
  noticeably warmer and more humid than it was
  before.
• Symbolically, a warm front is represented by a
  solid line with semicircles pointing towards the
  colder air and in the direction of movement. On
  colored weather maps, a warm front is drawn with
  a solid red line.

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Vertical profile of a warm front

• Warm air rises over cold air, but the ascent is
  less gradual.
• Precipitation is lighter, but typically lasts
  longer.
• Look at figure 7.5 for the different cloud types
  along the warm front

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Stationary fronts

• a front that is not moving
• When a warm or cold front stops moving, it
  becomes a stationary front. Once this boundary
  resumes its forward motion, it once again becomes
  a warm front or cold front. A stationary front is
  represented by alternating blue and red lines
  with blue triangles pointing towards the warmer
  air and red semicircles pointing towards the
  colder air.
• A noticeable temperature change and/or shift in
  wind direction is commonly observed when crossing
  from one side of a stationary front to the other.

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Cyclogenesis (step 1)

• Especially during the spring and summer in
  the midlatitudes of North America, high pressure
  to the north pushes cold polar air southward from
  Canada. To the south, maritime tropical air
  streams northward toward the polar air. The polar
  front is depicted by the symbols for a stationary
  front (the alternating red semi-circle and blue
  triangles). At the location where the opposing
  streams of air meet, cyclonic shear is created
  from opposing air streams sliding by each other
  causing the air to spin

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(step 2) mature stage

• Once the air collides and cyclonic circulation
  commences, warm air from the south invades where
  cold air was once located north of the polar
  front.
• A warm front develops where warm air replaces the
  cold air. The position of a warm front on a
  weather map is depicted (in red) with a line
  showing the boundary between the air masses and
  semi-circles indicating the direction the front
  is moving.
• To the west of the center of the developing
  system, cold air is sliding south replacing warm
  air at the surface. A cold front (blue triangles)
  develops where cold air replaces the warm air.
  Soon the developing system takes on the
  characteristic wave form, hence their name "wave
  cyclone". The lowest pressure is found at the
  center or apex of the wave.

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(step 3) occlusion stage (see fig 7.8)

• Being more dense, the air behind the cold front
  can "bulldoze" the warmer and  less dense air out
  of the way.
• The advancing warm air along the warm front
  cannot push the colder air in its path out of the
  way. Instead, the warm air rises off the surface
  and glides up and over the colder more dense air
  ahead of the warm front. As a result,  there is
  less horizontal displacement and the warm front
  moves slower across the earth than does a cold
  front.
• Over time the cold front catches up with the warm
  front and the cyclone starts to occlude

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Finding fronts on a weather map (figures 7.9-7.11)

• Look for regions where the temperature and/or
  moisture content change dramatically over a short
  distance
• Fronts can be seen on a pressure map as an area
  of lower pressure extending from the low pressure
  center
• Look for an area where the wind direction changes
  rapidly (for cold front, south ahead of front,
  north behind front)

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Lab 7 Problem 3

• Sea Level Pressure

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3

• Temperature

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3

• Satellite

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3 Winds
Direction of Cold Front Movement
L
Direction of Warm Front Movement