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Air Pressure and Winds
Air Pressure and Winds
Air pressure is the weight of the air above you.
Air pressure decreases with altitude as you go
up, because less air remains above you.
Air Pressure and Winds
• Horizontal (east/west, north/south)
differences in air pressure cause the air to
move  Wind!
• The force that causes the air to move is
called the pressure gradient force.
• The pressure gradient force always moves
air from higher pressure towards lower
pressure, in an attempt to equalize pressure
everywhere.
• Air pressure and wind:
The greater the
pressure difference
between two
locations, the
faster the wind will
blow from high
pressure to low
pressure.
Factors Affecting Wind
•pressure gradient force
•Coriolis effect
Pressure Gradient Force
•friction force
•air flows from high pressure to low pressure
•wind speed depends on “steepness” of pressure gradient
isobar spacing shows steepness of pressure gradient
Air Pressure and Winds
• Winds begin with differences in air pressures.
Pressure that's higher at one place than another
sets up a force pushing from the high toward the
low pressure.
• The greater the difference in pressures, the
stronger the force. The distance between the area
of high pressure and the area of low pressure also
determines how fast the moving air is accelerated.
Wind
• Measuring Winds:
An anemometer
measures wind
speed; A wind vane
measures wind
direction.
Coriolis Effect
• wind is deflected to the right (Northern
hemisphere)
• due to earth’s rotation
•effect greater with
greater wind speed
•effect greatest at
poles, decrease to
zero at equator
Coriolis Effect
• Ball follows a
straight line as
wind does, but
the earth turns
under the
wind giving it
the illusion of
curving.
The Weather Highway Created by the
Coriolis Effect
• The rotation of the
earth creates the
Coriolis effect.
• The Coriolis effect
causes the air and
water to be deflected
to the right north of
the equator.
• This creates global
weather highways
Geostrophic Wind
• Curved Flow and the
Gradient Wind
• wind flow (Northern
hemisphere):
• clockwise around a High
- anticyclone
• counterclockwise around
a Low - cyclone
In N. Hemisphere:
Air flows
counterclockwise
and into a low
pressure center.
Air flows clockwise
and out of a high
pressure center.
• A high
pressure center
is where the
pressure has
been measured
to be the
highest relative
to its
surroundings.
High Pressure
•That means, moving any direction away from the
High will result in a decrease in pressure. High
pressure centers often represent the centers of
anticyclones.
• When cooler air
sinks and is
warmed, the air
can hold more
moisture
•This usually means sunny skies
•Winds tend to move clockwise
around a high
Low Pressure
• A surface low
pressure center is
where the pressure
has been measured to
be the lowest relative
to its surroundings.
That means, moving any horizontal direction away
from the Low will result in increasing pressure. Low
pressure centers often represent the centers of
midlatitude cyclones.
• When warm air
rises and is
cooled, the air
can not hold as
much moisture
•Often, these areas are associated with
precipitation and stormy weather
•Winds tend to move counter clockwise
around the low
Mid-latitude
Cyclone
Locations under the influence of a mid-latitude cyclone (low
pressure system) experience more clouds and precipitation
than locations under the influence of a high pressure center.
At the intersection of air masses (fronts) warm air is forced
to rise, which allows it to cool and form clouds.
Cyclonic and anticyclonic winds in
northern hemisphere
Surface Winds - Friction
• Surface winds
experience friction
force friction force
depends on terrain
inflow
(convergence)
around a Low
outflow
(divergence) around
a High