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When an area of local low pressure develops, the air converges ... Local Weather. Local weather systems depend on the movement of the polar front (jet streams) ... – PowerPoint PPT presentation

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Title: Weather

  • Dynamics in Earths Atmosphere

What Drives Air Motion?
  • Warm air rises and cools as it expands. Cool air
    falls to fill the space left by warm air. This
    sets up a vertical convection current.
  • Particles exert a force as they move and this
    force per unit area is called pressure.

What Drives Air Motion?
  • If two columns of air have the same temperature,
    they will have the same distribution of air
    particles and therefore the same pressure.
    However, a warmer column of air will have more
    height. The pressure at the same altitude in a
    cold and warm air column will be different.

Diagram from R.S Naylor
The columns in the first set have the
same temperature and pressure at each altitude.
The second set shows a cold and warm column.
You have to go higher in the warm column to get
the same pressure as in the cold. Therefore,
the pressure is higher at the same height in the
warmer air column compared to the cold.
Diagram from R.S Naylor
Particles move from areas of high pressure to
areas of lower pressure because of a pressure
gradient. The moving particles exert a
pressure-gradient force. In the case of cold
and warm air columns, air will move from the
warm to the cold column at the same height.
This movement generates wind.
Pressure in the atmosphere
This is a contour display of pressures at the
surface. Blues represent lower pressures
while yellows and reds indicate higher
Global Heating and Air Circulation
  • Unequal heating of the Earth causes general air
    circulation. As we have discovered, global
    regions receive more direct radiation and
    therefore have higher temperatures. The warmer
    equatorial air rises and moves toward the polar
    regions (cooling as it goes). The colder polar
    air sinks and is drawn toward the equator.

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Global Heating and Air Circulation
  • The fact that the Earth rotates complicates the
    circulation. Free-moving objects appear to
    deviate from straight-line paths as the Earth
    spins. This deflection due to Earths rotation
    is called the Coriolis effect. The effect is
    greatest near the poles and non-existent at the

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Global Heating and Air Circulation
  • Because the Earth rotates, the air circulation
    develops three cells in each hemisphere. These
    cells redistribute heat across Earths surface
    and generate weather systems.

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Global Winds
  • The three cell circulation generates global wind
    patterns. The general winds include polar
    easterlies, westerlies, northeast or southeast
    trade winds, and doldrums (at the equator).
  • Wind direction is always designated as the
    direction from which the wind blows.

Also see Hewitt Figure 26.18
High and Low Pressure Systems
  • When an local area of high pressure develops in
    Earths atmosphere the air moves outward from the
    center (high to low). The air is diverging and
    moves downward.
  • When an area of local low pressure develops, the
    air converges toward the lower pressure center
    and up.

Air circulation around a high pressure system
Air circulation around a low pressure system
Wind Directions
  • In the Northern Hemisphere, upper level winds
    move clockwise around a high pressure system and
    counter-clockwise around a low pressure system.
  • Wind directions around high and low pressure
    systems are reversed in the Southern Hemisphere.

Pressure contour plot (blue is lower pressure,
red is higher)
Plot of wind direction- Note that the winds
are moving counter-clockwise around the low and
clockwise around the high region.
Upper Atmospheric Circulation
  • In the upper troposphere rivers of rapidly
    moving air circle the Earth. These high speed
    winds are referred to a jet streams.
  • The polar jet stream is a result of a temperature
    gradient at the polar front where warm tropical
    air meets cold polar air.

Upper Atmospheric Circulation
  • The subtropical jet stream forms when warm
    tropical air is moved from the equator to the
    poles, generating a sharp temperature gradient
    along the subtropical front.

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Jet Streams
  • The polar front is strongest during the winter
    and weakest during the summer.
  • During major cold outbreaks, the polar front
    dives south over the country. Consequently, the
    polar jet stream also dives south in response.
    The polar jet stream often aids in the
    development of storms and it also tends to steer
    the storms.

Jet Streams
  • The subtropical jet stream tends to develop
    during the winter season. The subtropical jet
    stream can also help develop and steer storms and

Local Weather
  • Local weather systems depend on the movement of
    the polar front (jet streams), seasonal
    conditions (temperature variations), and local
    topography (mountains, bodies of water).

  • A front represents a boundary between two air
    masses that contain different temperature, wind,
    and moisture properties.

When Fronts Meet
  • When a warm front advances on a cold front, the
    warm air flows up and over the cold air mass.
    Rain occurs at the boundary as the warmer air is
    cooled and water vapor condenses.

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When Fronts Meet
  • When a cold front advances on a warm front, it
    wedges under the warmer air causing the warmer
    air to rise and cool. Rain occurs where the
    fronts meet.

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When Fronts Meet
  • When a faster moving cold front advances and
    begins to merge with a warm front an occluded
    front forms. This is associated with a low
    pressure system. Rain occurs where the fronts

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Weather map for Monday, April 21, 2003
Surface temperature contour map for April 21, 9 AM
Visible satellite image for April 24, 2003 at 6 PM
Enhanced IR (infrared) image for April 24, 2003,
6 PM
  • The preceding visible and IR images came from the
    following site
  • http//

  • The climate of a location may be defined as a
    "composite" of the long-term prevailing weather
    that occurs at that location. In a sense, climate
    is "average weather". Climate can be measured
    quantitatively by calculating the long term
    averages of different climate elements such as
    temperature and rainfall.