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Chapter 6:
Air Pressure and Winds
Chapter Notes
Air pressure – the pressure exerted by the weight of air above.
Air pressure is exerted all around an object on Earth’s surface or in the atmosphere.
That is, air pressure is exerted in all directions.
Atmospheric Pressures in Inches and Millibars
Measuring Air Pressure
At sea level, the standard atmosphere exerts a force of 101,325 newtons
per meter2. The National Weather Service uses the unit “millibar’ to
measure air pressure. At sea level, atmospheric pressure equals 1013.25
millibars.
TV meteorologists often describe air pressure in inches. This is because
they use a mercury barometer to measure air pressure. The mercury
barometer uses a column of mercury in a glass tube as an indication of air
pressure. At sea level the mercury will raise to a height of 760 millimeters,
which equals 29.92 inches.
An aneroid barometer measures air pressure without using a liquid. The
aneroid barometer uses a partially evacuated metal chamber that is very
sensitive to variation in air pressure. It changes shape, compressing as
the pressure increases and expanding as pressure decreases.
Converting Pressure to Sea-Level Values
Pressure Changes with Altitude
The pressure at any given altitude in the atmosphere is
equal to the weight of the air directly above that point. As
we ascend through the atmosphere, the air becomes less
dense because of the lesser amount of air above.
Consequently, there is a decrease in pressure with an
increase in altitude.
The rate at which pressure decreases with altitude is not
a constant. The rate of decrease is much greater near
Earth’s surface where pressure is high. The standard
atmosphere model depicts the idealized vertical
distribution of atmospheric pressure. According to the
model, atmospheric pressure is reduced by
approximately one-half for each 5-kilometer increase in
altidude.
Influence of Temperature and
Water Vapor
In general, cold air is composed of
comparatively slow-moving gas
molecules that are packed closely
together. So, the density of this cold
air increases and so will the pressure
that it exerts. This mass of cold air
would be said to have high
barometric pressure.
In contrast, in a mass of warm air the
molecules would be farther apart, the
air less dense, and the mass would
produce low barometric pressure.
The amount of water vapor present in
a volume of air influences the air’s
density. This is because a water
molecule has less mass than a gas
molecule of nitrogen or oxygen.
We can conclude that a cold, dry air
mass will produce higher surface
pressures than a warm, humid air
mass. Also, a warm, dry air mass
produces higher pressure than an
equally warm, but humid air mass.
Cold Air Exerts More
Pressure than Warm Air
Airflow and Pressure
The movement of air can cause variations in air pressure:
Convergence – where there is a net flow of air into a region, as a
result the air “piles” up. Results in a “taller” and heavier air column
that exerts more pressure.
Divergence – a region where there is a net outflow of air Results in
a drop in surface air pressure.
Isobars-Lines of Equal Pressure
Factors Affecting Wind
Wind – the horizontal movement of air which is the result of horizontal
differences in air pressure.
Air flows from areas of high pressure to areas of low pressure. Wind is
nature’s attempt to equalize differences in air pressure.
If Earth did not rotate and if there were no friction, air would flow directly
from areas of higher pressure to areas of lower pressure. Because both
factors exist, wind is controlled by a combination of force including:
1. The pressure-gradient force
2. The Coriolis force
3. friction
Pressure Gradient Affects Wind Speed
Pressure-Gradient Force
When air is subjected to greater pressure on one side than on another, the
imbalance produces a force that is directed from the region of higher pressure
toward the area of lower pressure.
- Pressure differences cause the wind to blow
- The greater the differences in the pressure, the greater the wind speed
Pressure data are shown on surface weather maps by means of isobars.
Isobars are lines connecting places of equal air pressure.
The spacing of the isobars indicates the amount of pressure change occurring
over a given distance and is expressed as the pressure gradient.
Closely spaced isobars indicate a steep pressure gradient and strong
winds; widely spaced isobars indicate a weak pressure gradient and light
winds.
The Coriolis Force
If the only force involved in the horizontal movement of air, wind would cross
the isobars at right angles due to the pressure-gradient force. That is not the
case; winds cross the isobars at non-right angles due to the Coriolis force.
All free-moving objects, including wind, are deflected to the right of their path
of motion in the Northern Hemisphere and the left in the Southern
Hemisphere.
Coriolis Force Varies with Latitude
The magnitude of the Coriolis force is dependent on latitude. It is
strongest at the poles, and weakens as you move toward the equator.
The amount of Coriolis deflection increases with wind speed. This is
because faster wind cover a greater distance than do slower winds in
the same time period.
Geostrophic Wind
Geostrophic Winds - winds created when the Coriolis force is exactly equal
and opposite to the pressure-gradient force. These winds flow in a straight path
parallel to the isobars, with velocities proportional to the pressure-gradient
force. A steep pressure gradient creates strong winds, and a weak pressure
gradient produces light winds.
Buy Ballot’s Law: In the Northern Hemisphere if you stand with your back to
the wind, low pressure will be found to your left and high pressure to your right.
Upper-Air Weather Chart
Winds around cells of high or low pressure follow curved paths in order to
parallel the isobars. Winds of this nature, which blow at constant speed
parallel to curved isobars are called gradient winds.
It is common practice to call all centers of low pressure cyclones and the
flow around cyclonic. Cyclonic flow has the same direction of rotation as
Earth: counterclockwise in the Northern Hemisphere and clockwise in the
Southern Hemisphere.
Centers of high pressure are frequently called anticyclones and exhibit
anticyclonic flow.
Whenever isobars curve to form elongated regions of low and high pressure,
these areas are called troughs and ridges. The flow about a trough is
cyclonic; the flow around a ridge is anticyclonic.
Friction as a factor affecting
wind is important only within the
first few kilometers of Earth’s
surface. Friction slows winds
speed and as a result reduces
the Coriolis force. So, the
movement of air is at an angle
across the isobars, toward the
area of low pressure.
In whatever hemisphere, friction
causes a net inflow
(convergence) around a cyclone
and a net outflow (divergence)
around an anticyclone.
Anticyclonic Winds around High Pressure Center;
Cyclonic Winds around Low Pressure Center
Convergence Aloft Causes Air to Sink and Diverging Surface Winds;
Divergence Aloft Causes Converging Surface Winds and Rising Air
Vertical Airflow Associated with Cyclones and Anticyclones
How does horizontal airflow relate to vertical airflow? Vertical movement is
usually very small when compared with horizontal airflow, but the vertical
movement of air is important as a weather maker. Rising air is associated with
cloudy conditions and precipitation, whereas subsidence produces adiabatic
heating and clearing conditions.
In a surface low pressure system, air is spiraling inward, and the net inward
transport of air causes shrinking of the area occupied by the air mass in a
process called horizontal convergence. As the air converges horizontally it “piles
up”, or increase in height. The taller and heavier column of air above the should
destroy the surface low pressure, but if the air diverges, or spreads out, at
elevation at a rate equal to the horizontal inflow, then low pressure is maintained.
The opposite occurs at an area of high pressure. At the surface airflow is away
from the center of high pressure (divergence), but at elevation, air is converging
and the net result is the area of high pressure is maintained.
In summary: Air is rising in areas of low pressure promoting the growth clouds
and chances of precipitation. The opposite is true of areas of high pressure; the
air is sinking and warming so skies should be clear.
A Wind Vane and Anemometer
An Aerovane
Wind Measurement
Two basic wind measurements—direction and speed—are important to the
weather observer. Winds are always labeled by the direction from which they
blow; example: a north wind blows from the north to the south. A wind vane is
one instrument that is commonly used to determine wind direction. A
prevailing wind is a wind that blows more often from one direction than from
any other. The prevailing westerlies in the Untied States consistently move the
“weather” from west to east.
Wind speed is often measured using a cup anemometer, although an
aerovane can be used to determine wind direction and speed.