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Air Pressure and Winds
Chapter 6
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6.1 Wind and Air Pressure
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Wind and Air Pressure
Wind is the result of horizontal differences
in atmospheric pressure.
Atmospheric (air) pressure is the force
exerted against a surface by continuous
collision of gas molecules.
 Atmosphere is without walls but the force of
gravity prevents its escape.

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Wind and Air Pressure
The air pressure at sea level is about 1
kg/cm2.
Specifically, a column of air is 1 sq inch in
cross section, measured from sea level to
the top of the atmosphere would weigh
14.7 lbs.
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6.2 Measuring Air Pressure
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Measuring Air Pressure
 Air pressure is measured in millibars.
1 millibar (mb) = 100 newtons/m2
 1013.25 mb = sea level pressure
 The
mercury barometer is the standard pressure-
measuring instrument.
 Air pressure pushes on a pool of mercury, which forces
the mercury up a tube.
 High pressure: The mercury goes further up the
tube.
 Low pressure: The mercury stays lower.

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Measuring Air Pressure
Fair weather = high pressure
Rain = low pressure
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Measuring Air Pressure

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6.3 Pressure Changes with Altitude
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Pressure Changes with Altitude
Air pressure is defined as the force exerted
against a surface by continuous collision of
gas molecules.
 Air pressure decreases with altitude.
 Because of the decrease of pressure above you
 Air pressure increases with depth.
 Because of the increase of pressure above you

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Pressure Changes with Altitude
Air pressure decreases about 10mb per
100m increase in elevation
Air pressure is reduced by approximately
½ for each 5 km increase in altitude.
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6.4 Why does Air Pressure Vary?
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Why Does Air Pressure Vary?
Recall that variations in air pressure causes the
wind to blow, which in turn causes changes in
temperature and humidity
Water vapor affects air pressure.
 The amount of water vapor reduces the density of
air.
 Cold dry air has higher pressure.
 Warm, dry air has higher pressure than equally
warm, moist air.

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Why Does Air Pressure Vary?
The movement of air can also cause
variations in air pressure.
 Convergence occurs when a net airflow into a
region causing pressure to rise.
 Divergence results when there is a net outflow
of air from a region and surface pressure drops.

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Why Does Air
Pressure
Vary?
Temperature
influences air
pressure.

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6.5 Factors Affecting Wind
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Factors Affecting Wind
Pressure gradient force
Coriolis force
Friction

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Factors Affecting Wind- Pressure
Gradient Force
The force that generates winds results from
horizontal pressure differences. Greater
differences the greater the wind blows
Pressure gradient force
 Spacing of the isobars indicate the amount of
pressure change occurring over a given
distance.

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Factors Affecting Wind- Pressure
Gradient Force
 Isobars are lines connecting places of equal pressure.
 If isobars are close together a steep gradient and high
winds result.
 If isobars are far apart, the result is a low gradient and
lower wind speeds.
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Factors Affecting Wind- Pressure
Gradient Force
How temperature differences generate
wind
Sea Breeze figure 6-11 page 169
A- Temp and pressure don’t vary
horizontally

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Factors Affecting Wind- Pressure
Gradient Force
B- as the sun rises the earth begins to
warm. Air above the land warms and
expands creating a high pressure area (less
dense air column) and the air aloft begins
to flow away from the land
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Factors Affecting Wind- Pressure
Gradient Force
C- The mass transfer of air aloft creates a
surface high pressure area over the land.
The surface circulation that develops from
this redistribution of mass is from the sea
towards the land = sea breeze.
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Factors Affecting Wind- Pressure
Gradient Force
Isobars on a surface chart:
 Low-pressure systems (L) that occur in the
middle latitudes are called midlatitude cyclones.
 Produce stormy weather
 High-pressure systems (H) are called
anticyclones, which tend to be associated with
clearing conditions.

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Factors Affecting Wind- Coriolis
Force
TheCoriolis force cannot generate wind; it
modifies airflow.
The Coriolis force causes all free flowing
objects including wind:
 In the Northern Hemisphere, a deviation to the
right is due to the Earth’s rotation.
 In the Southern Hemisphere, the deviation is to
the left.

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Factors Affecting Wind- Coriolis
Force
The Coriolis force is a deflecting force.
 It is always directed at right angles to the
direction of airflow.
 It controls only wind direction, not speed.
 It is affected by wind speed (the stronger the
wind, the greater the force).
 The Coriolis force is strongest at the poles and
is nonexistent at the equator.

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Factors Affecting Wind- Coriolis
Force
Coriolis force

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Factors Affecting Wind- Friction
Remember pressure gradient
force is the primary driving force
for winds. Air moves from
high to low concentrations
but wind speeds don’t
increase indefinitely.
Friction significantly influences airflow
near Earth’s surface, but its effect is
negligible at higher altitudes.

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6.6 Winds Aloft
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Winds Aloft
 Geostrophic flow occurs when the pressure
gradient force and the Coriolis force are balanced.
 Thewind flows parallel to the isobars.
 They flow in relatively straight paths.
 Wind Direction is directly related to prevailing
pressure patterns. So if we know direction we can
establish a rough approximation of the pressure
distribution.

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Winds Aloft
Buys Ballot’s Law states that if you stand
with the wind at your back, low pressure
will be found on your left, high pressure on
the right.
The situation is reversed in the southern
hemisphere.
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Winds Aloft
Curved airflow
and the gradient
wind
Curved airflow:
 Winds around cells
of high pressure or
low pressure follow
curved paths.

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Winds Aloft
Gradient winds blow at a constant speed.
 They are parallel to the curved isobars.
 Centers of low pressure are called cyclonic; winds
flow counter-clockwise in the Northern Hemisphere.
 A trough is the result of isobars curving to form
elongated regions of low pressure.
 Centers of high pressure are anticyclonic; winds flow
clockwise in the Southern Hemisphere.
 A ridge is the result of isobars curving to form
elongated regions of high pressure.
Opposite is true in the Southern Hemisphere

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Winds Aloft
Curved airflow and the gradient wind

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Winds Aloft
In Low Pressure Centers- Figure A
 The inward directed pressure gradient force is
opposed by the outward directed Coriolis force. The
pressure gradient force must exceed the coriolis
force to overcome the air’s tendency to continue
moving in a straight line.
In High Pressure Centers- Figure B
 Pressure gradient force and Coriolis force are not
balanced. The imbalance provides a change in
direction and generates the curved flow
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6.7 Surface Winds
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Surface Winds
Surface winds travel at an angle across
isobars, toward low pressure.

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Surface Winds
Air flow crosses the isobars at varying angles
depending on friction for terrain, but winds
always move from high to low pressure.
In a cyclone pressure decreases inward and
friction causes a new flow towards its center
In an anticyclone pressure decreases outward
and friction causes a net flow away from its
center
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6.8 How Winds Generate Vertical Air Motion
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How Winds Generate Vertical Air Motion
 Factors that promote vertical airflow:
 Friction
 Increased friction causes a drop in wind speed
resulting in a pileup of air upstream from the
ocean to land.
 Convergence can result in cloudy weather.
 Decreased friction causes and increase in wind
speed from the land to the ocean.
 Subsidence and divergence results in clearing
weather.

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How Winds Generate Vertical Air Motion
Vertical airflow is associated with cyclones
and anticyclones.

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How Winds Generate Vertical Air Motion
Cyclones
 Horizontal convergence- net inward transport of
air causes a shrinking of the area it occupies
 This causes a pile up of air creating a dense
column which increases pressure
 TO compensate aloft, divergence (spreading
out) occurs at an equal rate to inflow below
 Remember rising air causes bad weather
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How Winds Generate Vertical Air Motion
Anticyclones
 Convergence (pulling in) happens aloft and
general subsidence of the air column happens
 This creates good, nice weather
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6.9 Wind Measurements
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Wind Measurement
Wind direction is always measured from
the direction it is coming.
 A North wind blows north to south.
 A East wind blows east to west.
A prevailing wind consistently blows
more often than not from one direction.

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Wind Measurement

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Wind Measurement
A wind vane shows wind direction.
Wind speed is often measured with a cup
anemometer, which has a dial that shows
wind speed.
An aerovane is a combination of wind
vane and anemometer.