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Chapter 6: Air Pressure and Winds
Air pressure – pressure exerted by the weight of the air above
Pressure is measured in force (weight) per unit area
Standard air pressure is commonly expressed in several different units:
14.7 lbs / in2
US units
101 325 N / m2
SI units
1013.25 millibar
US meteorology units
750 torr
from Galileo’s student, Torricelli in 1643, sometimes in mm Hg
29.92 in Hg
US weather stations
Methods used to measure air pressure
Mercury barometer
Liquid mercury trapped in a tube with the only open end placed in an open dish of mercury
If the tube is about 1 m tall, a vacuum will form at the closed end and air pressure will
force mercury up into the tube about 760 mm
Aneroid barometer – aneroid meaning without liquid
A metal canister with a thin metal top that can change shape as it is compressed by high air
pressure or expand outward in low air pressure is connected to a pointer system with a
series of levers to magnify the changes
Barograph
An aneroid barometer connected to a recording mechanism (which can be as simple as a
rotating drum) for a continuous record of air pressure
For purposes of weather prediction, meteorologists are concerned with horizontal differences
in air pressure and, therefore, must compensate for differences in altitude
In other words, air pressure reported from weather stations around the world is always
corrected to sea level pressure
As altitude increases, the weight of a column of air above the observer will decrease
because the column of air is shorter
Correction to air pressure is made by simply calculating the weight of a theoretical
column of air from the height of the observation to sea level and adding the pressure
such a column would produce to the measured air pressure
Change in air pressure with altitude is not a constant – it is an exponential change
The change is approximately ½ for every 5 km of altitude
Example: Approximate the air pressure at a height of 15 km
15 km = 3 (5 km)
Sea level pressure is about 1013 mbar
The new pressure is 1013 mbar x (½)3 = 1013 mbar x (1/8) = 127 mbar
The standard atmosphere
In meteorology, the standard atmosphere depicts the idealized vertical distribution of
atmospheric pressure corrected for both pressure and temperature (see Table 6-1 on
page 167 of Lutgens Tarbuck. The Atmosphere, 9th edition. 2004.)
Horizontal Variations in Air pressure
To compare air pressure from station to station, corrections must be made for differences in
elevation (see the standard atmosphere chart above)
Extreme difference in pressure
30 mbar above average sea level pressure (about 1 inch of mercury)
60 mbar below average sea level pressure (about 2 inches of mercury)
Influence of temperature and water vapor on air pressure
Polar regions in the winter receive very little incoming solar radiation and are snow covered
so they reflect what little light they receive
Temperatures of –34°C (and extremes of –40°C to –50°C) are common
Air molecules at these temperatures move more slowly and therefore pack more closely
and so their density is much higher (a Canadian high pressure is formed)
The opposite occurs with air moving from near the equator over the Gulf of Mexico
Warm air has fast moving molecules which reduce the density of the air and create a low
pressure area
Effect of water vapor
Most people perceive humid air as being ‘heavy’ but, in fact, water molecules (H2O = 18
amu) are significantly lighter than the nitrogen (N2 = 28 amu) and oxygen (O2 = 32
amu) molecules in the air
This results in humid air being much less dense than dry air at the same temperature
Influence of air flow on air pressure
Convergence – when air flows into a region from many sides, the air piles up into a taller
column of air and the pressure increases
Divergence – when air flows out of a region in all directions, the air pressure will decrease
Summary:
Cold, dry air is more dense and is associated with high pressure areas
Warm, humid air is less dense and is associated with low pressure areas
Winds moving in to an area result in higher pressures
Winds moving out of an area result in lower pressures
Factors affecting winds
1. the pressure-gradient force
2. the Coriolis force
3. friction
Pressure-gradient force – horizontal air pressure differences result in an unbalanced force
causing air to move (wind) from a higher pressure region to a lower pressure region
Data from weather stations all over the United States pool data and make maps showing air
pressure
Isobars – lines connecting areas of equal air pressure
Spacing of isobars indicate the amount of pressure change between areas
Widely spaced isobars represent small pressure differences
Closely spaced isobars represent large pressure differences
Temperature, horizontal pressure gradients, and wind
Sea breeze is a microcosmic example of how temperature causes horizontal pressure
gradients
1. heating the Earth’s surface causes air
above the surface to expand and rise
2. the rising air causes a low pressure
area on the shore
3. the rising air hits cooler air aloft and
creates a high pressure aloft
4. cool water creates a high pressure area
just above the water
5. density difference creates low pressure
area high above the water
6. results in offshore breeze aloft
7. also results in an onshore surface breeze
Figure 1: Sea breeze formation
From http://oceanservice.noaa.gov/education/yos/resource/JetStream/ocean/sb_circ.htm
Pressure gradient forces alone would result in weak winds that quickly equalize regions of
unequal pressure by quickly filling in the low pressure areas
Vertical pressure gradient
Fact: Air pressure decreases with increasing altitude
Fact: Air will flow from high to low pressure areas
Question: Why isn’t there a vertical wind from high pressure near the surface to the low
pressure regions at high altitudes?
Answer: The force of gravity exactly balances the force of the vertical pressure
gradient – this balance is called hydrostatic equilibrium
Other forces like the Coriolis force, friction, and gravity (the only one of these forces that
can generate wind) generally act to modify and sometimes even enhance the
development of Earth’s pressure systems
Coriolis force – because of Earth’s rotation, all freely moving objects are deflected to the right
in the Northern hemisphere and to the left in the Southern hemisphere
The Coriolis force
1. is always directed at right angles to the direction of airflow
2. affects only wind direction, not wind speed
3. is affected by wind speed; the stronger the wind, the greater the deflecting force
4. is strongest at the poles and nonexistent at the equator
Friction – a force that acts to slow a moving object
Unopposed, the pressure gradient force would cause winds to constantly accelerate
Friction is effective at slowing this acceleration only near the surface
Winds aloft – above a few kilometers, friction is negligible and so Coriolis force is
responsible for balancing the pressure gradient force
Begin with a stationary parcel of air
Coriolis force only affects moving objects, so the initial movement of a parcel of air
under the influence of a pressure gradient force will
be directly toward the center of a nearby low pressure
area (see bottom of graphic to the right)
As the wind pick up, the Coriolis force increases (note
in graphic to the right that as wind speed increases
the Coriolis force also increases)
The Coriolis force always acts perpendicular to the wind
direction, to the right in the Northern hemisphere and
to the left in the Southern hemisphere
The wind will theoretically move in a circular motion
between isobars at a constant speed – this balanced,
constant wind speed is called a geostrophic wind
Buys Ballot’s law: in the Northern hemisphere, if you stand with your back to the wind,
low pressure will be found to your left hand and high pressure to your right
Therefore, cyclonic airflow (counterclockwise) will occur around a low pressure center
and anticyclonic airflow (clockwise) will occur around a high pressure center
Surface winds – near the surface, friction is no longer a negligible force
Since friction always acts in the direction opposite the wind, the force of the wind (and
therefore the Coriolis force) will never reach the magnitude of the pressure gradient
force
The result is that near the surface winds will always spiral into the low pressure area
instead of moving in a circle at constant speed
How winds generate vertical air motion
Surface low pressure (cyclone in the Northern hemisphere)
Will cause a cyclonic convergence of air at the surface
The air must go someplace and therefore will rise
The rate is very slow – about 1 km per day
Result is unstable air (cloud formation) usually leading to precipitation
The air column tends to build air pressure which slows the rising column
The rising column must diverge aloft (anticyclonic in the Northern hemisphere)
Surface high pressure (anticyclone in the Northern hemisphere)
Will cause an anticyclonic divergence of air at the surface
The air must come from someplace and therefore will generate a falling column of air
The falling air warms adiabatically
Result is stable air usually leading to fair weather
The falling air column tends to build air pressure at the surface due to adiabatic warming
which slows the falling column
The falling column causes convergence aloft (cyclonic in the Northern hemisphere)
Note again that low pressure systems tend to generate unstable air and cloud formation
resulting in precipitation
This is called pressure tendency or barometric tendency
Falling barometer (a low pressure system) tends to generate precipitation
Rising barometer (a high pressure system) tends to generate fair weather conditions
Factors promoting vertical air flow
Air moving from over water (less friction) to land (higher friction) results in surface
convergence generating a rising air column and cloud formation
Conversely, air moving from land to water results in surface divergence generating a
falling air column
Air passing over mountains compresses vertically causing horizontal divergence aloft
On the lee side, vertical air expands causing horizontal convergence
Air flow toward the equator where the Coriolis force is weakened causes divergence and
subsidence
Air flow toward the pole favors convergence and slow uplift
Wind measurement
Wind vanes – always point into the wind
Often connected to a remote dial that shows wind direction
N, NE, E, etc.
0° (N also 360°), 90° (E), 180° (S), and 270° (W)
Prevailing winds – when the wind blows more often from one direction than from any other
Wind rose – lines in each of the eight directions (N, NE, E, SE, etc.) are drawn such the
length of the line in each cardinal direction indicates the percentage of time winds blow
from that direction
Cup anemometer – three hemispherical cups that catch wind and by the speed of the rotation of
the device can measure wind speed
Aerovane – resembles a wind vane with a propeller on the windward side that measures wind
speed
Wind socks – often seen a airports, a conical, open-ended, material sock catches wind showing
both speed by virtue of how far the sock extends before the end drops and wind direction