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Air Pressure
Name: Douglas Graham.
Status: educator
Age: 50s
Location: N/A
Country: N/A
Date: 2000-2001
Question:
What is the relationship between air pressure and
temperature? In general,under controlled situations, I know that as the
temperature increases the air pressure decreases, is this also true for
the weather too? Is the air pressure greater in the winter months verses
the summer months? I know that the best way to find this out is to check
the data for air pressure for the year but I am having trouble accessing
this information.
Replies:
Mr. Graham,
We have quite a mix of ideas embedded in your query.
First: Pressure does NOT decrease as temperature increases. If air is confined
in a closed container, the pressure inside will increase if the container is
heated.
If the air is unconfined and heated -- as would be the circumstances in the
atmosphere -- the density of the gas decreases. How this might affect the
barometric pressure depends on a host of complicating factors.
Cold air is more dense (has a greater weight per unit volume) than warm air.
That is why hot air balloons are able to float in the denser surrounding air.
Barometric pressure, as a meteorologist might record it, varies all year long
and, so far as I know, does not change in any regular way with season. I have
a weather station in my home. I have recorded wide swings in barometric
pressure all year long.
Contact your nearest branch of the National Weather Service. They should be
able to provide you with graphics that represent the hourly barometric
pressure at or near your location for almost any year on record.
Good luck.
Regards,
ProfHoff
Dear Douglas-
There are some physical laws that govern the relationship between
temperature and pressure. If you take a given volume of air,
and compress it, the temperature will increase, and if you expand the same
volume of air, the temperature will decrease. This is
the principle that most refrigerators and air conditioners use.
In the atmosphere, as you go up in elevation, or height, the atmospheric
pressure decreases. The temperature also decreases, but
not because of the lesser atmospheric pressure. The air close to the
ground is warmer because the earth's surface is what heats
the atmosphere, for the most part. The further away from the earth's
surface you go, the cooler the atmosphere.
The air near the earth's surface may change temperature in another manner
also. As winds blow the air up or down mountains, or
areas of higher elevation, the air pressure decreases, and the air cools.
This is called "adiabatic" cooling, and is due strictly to the
expansion of the air due to the lowering of the pressure. If moisture is
present in the air, it can cool sufficiently to condense the
moisture into rain or snow. The air blowing down a mountain undergoes
compression, due to the lower elevation, and will become
warmer. Clouds tend to dissipate in air moving down a mountainside. That
is why the windward sides of mountain ranges receive
much more precipitation than the leeward sides.
Wendell Bechtold, Meteorologist
Forecaster, National Weather Service
Weather Forecast Office, St. Louis, MO
Douglas,
A wealth of meteorological data can be found at
Argonne National Laboratories web site at
www.atmos.anl.gov/ANLMET (the uppercase letters
must be typed as uppercase). See the ANL Data Archive
section for monthly summaries of data.
Contrary to what you say, an increase in temperature
(which is a measure of the activity and movement of
atoms and molecules) produces a corresponding increase
in pressure (which is a measure of the force of the
atoms and molecules), if the volume of the air is held
constant.
It does not work that way in the atmosphere, as
the atmosphere is not a constant volume. An
increase in temperature in a high pressure area as it
is warmed by the sun causes expansion of the area
both horizontally and vertically, thereby resulting in
only a minor increase in pressure. However, the pressure
increase is measurable, because the atmospheric volume
cannot respond immediately to the increase in temperature
(that's a lot of air to move).
Air pressure is normally greater during the late summer
and early Autumn in temperate latitudes and less in the
winter (opposite to what you thought). That is because
high pressure areas during the summer are warmer and build
up more air above them. More air above the surface means
more weight, which translates into greater pressure.
David R. Cook
Atmospheric Section
Environmental Research Division
Argonne National Laboratory
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Update: June 2012
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