Weather
Dynamics in Earth’s 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 reds
indicate higher
pressures.
Where would the winds be
highest on this date?
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.
Global Heating and Air
Circulation

The fact that the Earth rotates
complicates the circulation. Freemoving objects appear to deviate
from straight-line paths as the Earth
spins. This deflection due to Earth’s
rotation is called the Coriolis effect.
The effect is greatest near the poles
and non-existent at the equator.
Global Heating and Air
Circulation

Because the Earth rotates, the air
circulation develops three cells in
each hemisphere. These cells
redistribute heat across Earth’s
surface and generate weather
systems.
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 Earth’s 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 counterclockwise 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 directionNote 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.
The polar front and the two jet streams have an
influence on the movement of weather systems
across the globe.
Air masses are steered by the
jet stream.
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
disturbances.
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…).
Fronts

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.
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.
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.
Warm air
Cold air
Cool air
Weather map for Monday, November 26, 2001
Surface temperature contour map for November 27, 2001 9 AM
Visible satellite image for November 26, 2001 6 PM
Enhanced IR (infrared) image for November 26, 2001 6 PM

The preceding visible and IR images
came from the following site:
http://www.weather.unisys.com
Global circulation
as viewed from
the South Pole
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.