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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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