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## Weather

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### Most of Earth's weather happens in the troposphere but is ... http://www.weather.unisys.com. Global circulation. as viewed from. the South Pole. Climate ... – PowerPoint PPT presentation

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Title: Weather

1
Weather
• Dynamics in Earths Atmosphere

2
An atmosphere is a blanket of a gases
surrounding a planet. Earths atmosphere has
distinct layers defined by changes in
temperature. Most of Earths weather happens in
the troposphere but is influenced by gas movement
in the stratosphere. http//www.glacier.rice.edu/
weather/3_atmstructure.html
3
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.

4
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.

5
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.
6
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.
7
Pressure in the atmosphere
This is a contour display of pressures at the
surface. Blues represent lower pressures
while reds indicate higher pressures.
8
Where would the winds be highest on this date?
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10
Global Heating and Air Circulation
• Unequal heating of the Earth causes general air
circulation. As we have discovered, global
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.

11
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12
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
equator.

13
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14
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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16
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.

17
Also see Hewitt Figure 26.18
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.

19
Air circulation around a high pressure system
Air circulation around a low pressure system
20
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.

21
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.
22
Upper Atmospheric Circulation
• In the upper troposphere rivers of rapidly
moving air circle the Earth. These high speed
winds are referred to as 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.

23
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.

24
The polar front and the two jet streams have an
influence on the movement of weather
systems across the globe.
25
Air masses are steered by the jet stream.
26
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.

27
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
disturbances.

28
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).

29
Fronts
• A front represents a boundary between two air
masses that contain different temperature, wind,
and moisture properties.

30
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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32
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.

33
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34
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
meet.

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38
Weather map for Monday, November 26, 2001
39
Surface temperature contour map for November 27,
2001 9 AM
40
Visible satellite image for November 26, 2001 6 PM
41
Enhanced IR (infrared) image for November 26,
2001 6 PM
42
• The preceding visible and IR images came from the
following site
• http//www.weather.unisys.com

43
Global circulation as viewed from the South Pole
44
Climate
• 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.