Air masses and pressure systems

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Air masses and pressure systems
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Air Masses

• Air masses take on the characteristics of the
  places where they originally formed.
• These characteristics are primarily restricted to
  wet/dry and hot/cold.

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Continental (land) air masses may form over..
cP
The polar regions in which case they are known as
Continental Polar - cP and are dry and cold..
cP

• or the tropics in which case they are dry
  and hot and are referred to as Continental
  Tropical - cT.

cT
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Maritime (ocean) air masses may form over...

• Polar regions in which case they are cold and wet
  (Maritime Polar - mP)
• or in the tropics in which case they are warm and
  wet (Maritime Tropical - mT).

mP
mT
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Maritime Polar
Continental Polar
Maritime Polar
Continental Polar
Continental Tropical
Maritime Tropical
Maritime Tropical
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Pressure Centers
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Air pressure..

• Is most simply thought of as the weight of the
  overlying column of air.
• This weight or pressure is affected by factors
  such as temperature and humidity.
• Cold temperatures lead to low kinetic energy and
  closely packed molecules (more dense high
  pressure)
• Warm temperatures lead to high kinetic energy and
  widely spaced molecules (less dense low
  pressure)

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Air pressure (continued)

• Since water vapor is a very light weight gas, air
  that has a lot of water vapor in it (high
  humidity) is less dense and lighter than dry air.
• Dry air higher pressure
• Wet air lower pressure

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High and Low Pressure Centers
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Wind patterns around Highs and Lows bring about
different weather.
H
L
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High pressure.

• Results as cold air descends from above, reaches
  the ground and spirals outward in a clockwise
  direction.
• High pressure generally brings cool dry
  conditions we refer to as Fair skies.

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Low Pressure

• Results from cool air from a neighboring High
  circulating along the ground and pushing warmer
  moist air spiraling upward in a counter-clockwise
  direction.
• Lows generally bring cloudy warm weather.

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Note how the winds blow from the High into the
Low.
L
H
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The instrument used to measure air pressure is
called a barometer. The instrument seen here
is a Mercury barometer. Air pushing down on a
dish of mercury forces the liquid metal up into
the hollow glass tube.
The level to which the mercury rises in the tube
(in inches) indicates how heavy the air is, or
how much air pressure there is . Normal air
pressure will support a column of mercury 29.92
inches high.
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Lines connecting points of equal barometric
pressure are often seen on weather maps and are
called Isobars
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When isobars are closely spaced, a rapid change
in air pressure and a corresponding increase in
wind speed is indicated.
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Fronts
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Fronts are the result.

• of two air masses of different characteristics
  coming into contact with one another.
• The two air masses will not readily mix and a
  boundary forms between the two.
• This boundary is called a front.
• The type of front which develops and the weather
  associated with the passage of this front depends
  on the types of air masses involved.

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Cold Front
As heavy cold air moves into warm air, it forces
the warm air to rise rapidly causing short
periods of heavy thunderstorms.
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Simplified diagram of a cold front.
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Warm Front
As warm air moves into heavier cold air, it
slowly creeps up and over the colder air. As the
warm air rises, long periods of gentle rain
develop
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Occluded Front These fronts develop when a cold
front overtakes a warm front. The associated
weather is
therefore a combination of long gentle rain and
heavier periods of thunder- storms.