8 - Cal State LA - Instructional Web Server

Unit 8: Air Pressure and Winds
Author’s photo
Fast-moving air in persistent directional flow shapes features such as this dividivi tree on Aruba in the Caribbean.
• Explain atmospheric pressure and its
altitudinal variation
• Relate atmospheric pressure to windflow
at the surface and aloft
• Understand the large scale winds forces
affected by pressure systems and air
• Apply these relationships to the
operation of small-scale local wind
Atmospheric Pressure
• Pressure is a force/unit area
vertical pressure is influenced by gravity
Surface pressure, the standard sea-level air pressure
is 1013.25 mb (millibars =101.325 hectopascals),
about 14.7 lb/in2.
• Pressure lines on surface weather
chart are called isobars, and are
drawn at 4 mb intervals.
Measurement of Pressure: The barometer
• Invented by Torricelli, a liquid-in-glass barometer. Standard barometer uses
• Standard sea-level pressure is 760 mm (29.92”)
• Aneroid barometer, like altimeter in airplanes, uses partial
vacuum instrument to measure pressure.
The greater the atmospheric
pressure, the higher the column
of mercury. The values give
standard sea-level pressure in
several commonly used pressure
Atmospheric Pressure and Altitude
Mass of the atmosphere as a function of altitude. A greater
proportion of the atmospheric mass is concentrated near the
Earth’s surface. Atmospheric pressure depends on the overlying
mass of air, so it also decreases with altitude.
Wind-the Movement of Air in the Atmosphere
• Ultimate source of energy for wind is the Sun
• Uneven heating by latitudes results in movement of heat
towards cooler regions
• Air movement is also influenced by the Earth’s rotation-Coriolis
We need to verify we have permission for these two images.
Latitudinal radiation balance averaged over the
Northern Hemisphere and resulting poleward
transfer of heat.
Latitudinal profiles of solar radiation absorbed and terrestrial radiation
emitted per unit area of Earth’s surface, averaged over all Northern
Hemisphere longitudes. The consequent poleward transfer of total
heat and the separate atmosphere and ocean transfer components are
Wind is the movement of air caused by forces:
Pressure gradient force
• Coriolis force
• Friction
Surface Winds
Upper Air Winds
Source: http://www.physicalgeography.net/fundamentals/7n.html
Pressure gradient force
The pressure gradient force is caused by differences in density
(pressure) over distance. The greater this gradient, the stronger
is the force.
The direction of the pressure gradient force is always directed
from higher to lower pressure.
Air movement is always from areas of higher pressure (H) toward areas of lower
pressure (L). The larger the pressure difference between H and L, the greater the
pressure gradient force and the faster the wind.
Source: http://usatoday30.usatoday.com/weather/wpress.htm
Coriolis Force
• The Coriolis force causes deflection of moving objects, like the
wind, to the right of flow in the N. Hemisphere and to the left
of flow in the S. Hemisphere.
• The magnitude of the Coriolis force increases with velocity and
latitude (zero at the equator, maximum at the poles).
For the two children on the rotating merry-go-round the ball appears to curve to the right after it is tossed. The child on the
adjacent ground observes the ball to move in a straight path. The apparent deflection of an object moving over a rotating surface is
created by the different frames of reference, and is accounted for by the Coriolis force.
500-mb Pressure Map
Figure 4.10
Frictional Forces
• Friction acts opposite the direction of wind, causing
• Its magnitude is determined by the roughness of the
Earth’s surface.
• Friction decreases with distance away from the Earth’s
surface and is insignificant usually above 1 km
• Since friction slows winds, it also decreases the Coriolis
force that depends on velocity
Surface and Upper Air Geostrophic Winds
• Above the friction layer, only
pressure gradient and Coriolis
forces act on winds.
• These two forces act in opposite
directions, causing a balance called
a geostrophic wind.
• This wind flows parallel to isobars
in the upper air.
• Surface winds, with friction, deflect
towards lower pressure across
Surface Pressure
• Cyclones, or low pressure systems,
have counterclockwise (in the N.
Hemisphere, opposite in the S.
Hemisphere) flow towards the low
center, converging.
• Anticyclones, or high pressure
systems, have clockwise (in the N.
Hemisphere) flow outwards towards
lower pressures, diverging.
Air circulation patterns associated with a cyclonic low-pressure cell (A) and an
anticyclonic high-pressure cell (B) in the Northern Hemisphere. In the
Southern Hemisphere air circulation around low- and high-pressure cells is in
the opposite direction (clockwise toward cyclones and counterclockwise away
from anticyclones).
Local Wind Systems
Sea breeze/land breeze
Mountain/valley breeze
Katabatic winds-cold drainage winds
Chinook winds, includes Santa Anas, Foehn
Sea breeze/land breeze air circulation systems. These reversing, cell-like airflows develop in response
to minor pressure differentials associated with day/night temperature variations along the land/sea
coastal zone.
Mountain/Valley Breeze
Formation of valley (daytime) and mountain (nighttime) breezes. Red
arrows represent upslope and upvalley winds (warmer); blue arrows
indicate downslope and downvalley winds (cooler).
Local Winds
• Katabatic wind-cold drainage winds, such as off the
Antarctic plateau down to the coastal plain.
• Chinook (“snow biter”) wind is a warm, dry wind
caused by compression as air flow off mountains to
lower elevation and higher pressures.
Called Foehn wind in the European Alps.
Called Santa Ana wind in southern California.
• Other local winds have names given due to
unusually warm or cold nature.
• Warm: sirocco (N. Africa), khamsin (N. Africa),
harmatten (W. Africa), sharav (Israel)
• Cold: mistral (S. France), bora (Balkans), taku
• For other local wind names see:

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