Download Pressure Gradient Force - Geog-1920

Air Pressure
and Winds
Triple
Productions
Air Pressure
Air Pressure: The weight of the atmosphere as
measured at a point on the earth’s surface.
 How do differences in air pressure from place to place
affect weather conditions?
Air is a gaseous substance whose
weight affects air pressure.
The air is heavier and the air
pressure is higher closer to
the earth’s surface
Physical law
For equal amounts of cold
and hot air, the cold
air is denser.
A cold morning is
characterized by relatively
heavy air, but as afternoon
temperatures rise, air
becomes lighter.
Barometers
A variety of barometers are
to record changes in air
pressure.
The air pressure at a given location
changes as the surface heats or cools.
Barometers record the drop in
atmospheric pressure when the air
heats and the rise in pressure as the air
cools.
Air Density
Visualize air as two liquids with different densities,
like oil and water:
The lighter liquid will move to the top
as the denser liquid will move to the
bottom of the container becoming
the same thickness everywhere; in
order to attain equilibrium by evening out
pressure imbalances resulting from the
heating and cooling processes.
The heavier (cold) air moves
to locations with light (warm) air .
Pressure Gradient Force
Pressure differences existing between areas cause air to
blow from an area of high pressure toward an area of low
pressure
Balancing Pressure Differences
 Heavy air stays close to the Earth’s surface
 Winds are produced
 Warm air is moved in an upward motion
High Pressure Zone
Warm Air
Wind
Low Pressure Zone
Differences in Pressure Zones
 If distance between high and low are short
1. pressure gradient is steep
2. wind velocities are great
 If gentle air movements occur when zones are far
apart
1. the degree of difference is not great
Convection System
The circulatory motion of descending cool air and ascending warm air
Land and Sea Breezes
Good example of a convectional system.
Daytime: The warmer air over the
land rises vertically, and is replaced by cooler air from
over the sea.
Breeze from sea.
Nighttime: The water is warmer than
the land, moving the cooler air from
over the land to the sea.
Breeze from land.
Mountain and Valley Breezes
Gravitational forces cause the heavy cool air that accumulates over snow in
mountainous areas to descend into the lower valley areas.
Valley Breeze: the warm air from the valley moves
up the slopes into the mountainous regions. This
usually occurs during the day.
Mountain Breezes: the cool air moves down the
slopes into the valley , lowering the temperatures in
the valley (with the potential of freezing).
This usually occurs at night.
The Coriolis Effect
As winds move from high pressure to low pressure:
winds in the N. Hemisphere tend to travel to the right,
winds in the S. Hemisphere tend to travel to the left
Coriolis Effect
The skaters in the
middle of the circle
do not need to
move very quickly,
while the out most
skaters have to
move rapidly to
maintain this line
How does this relate?
 The Earth rotates on its axis
 Equatorial regions are rotating much
faster
 Polar regions are rotating much slower
Coriolis Effect with
Pressure Gradient Force
 When together they produce spirals rather then
straight lines of wind
 The spiral is the basic form of many storms
The Frictional Effect
The cause of wind to follow an intermediate path
Wind
 Movements are slowed due to friction on
Earth’s surface
 Friction becomes ineffective about 1 mile
above the Earth’s surface
 Friction causes wind speed to decrease and
change direction
Global Air Circulation
Equatorial Low Pressure
• Areas of low pressure at the equator line
Subtropical High Pressure
• Cooled heavier air from the equatorial air pressure
Northeast Trades and Westerlies (Southwesterlies)
• Belts of wind caused by the cooled air reaching the
earths surface
Polar Air Pressure
• Subpolar low is ascending air north of the
westerlies
• Polar Easterlies connect the Subpolar low to the
Polar high pressure
• Has the greatest effect on Jet Streams
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Jet Streams
• Strongest flows of upper air wind
• 30,000 – 40,0000 feet from the earths surface
• Air travels East to West, in both hemispheres, at
100 – 200 MPH
• Tends to separate cold polar air from warm
tropical air
• Movement of the Jet Stream has the greatest
effect on monsoon season in Southeast Asia
Monsoons
• Monsoon wind changes directions
seasonally
• Sothern and Eastern Asia is
effected by this phenomena the
most