Air pressure corrections

When you read a barometer the reading directly from it is the "station pressure."

Two things affect the barometer's reading, the high or low air pressure caused by weather, and the air pressure caused by the station's elevation, or how high it is above sea level.

No matter what weather systems are doing, the air's pressure decreases with height. If you're trying to draw a weather map of air pressure patterns, you need a way to remove the effects of the station's elevation. That is, you want to see what the pressure would be at the station if it were at sea level. Otherwise, all high-elevation locations would be mapped as having low pressure.

You need to calculate, sea-level pressure, which is defined as: "A pressure value obtained by the theoretical reduction of barometric pressure to sea level. Where the Earth's surface is above sea level, it is assumed that the atmosphere extends to sea level below the station and that the properties of that hypothetical atmosphere are related to conditions observed at the station."

To do this, you have to take into account the barometric reading at the station, the elevation above sea level, and the temperature.

Another kind of barometric reading is the altimeter setting, which aircraft use. It's defined as: "The pressure value to which an aircraft altimeter scale is set so that it will indicate the altitude above mean sea level of an aircraft on the ground at the location for which the value was determined." For it, all you need is the station pressure and the elevation, you can ignore the temperature. (Related: Calculate altimeter setting).

How pressure decreases with altitude

As you go higher in the air, the atmospheric pressure decreases.

The exact pressure at a particular altitude depends of weather conditions, but a couple of approximations and a formula can give you a general idea of how pressure decreases with altitude.

A rule of thumb for the altimeter correction is that the pressure drops about 1 inch of mercury for each 1,000 foot altitude gain. If you're using millibars, the correction is 1 millibar for each 8 meters of altitude gain. These rules work quite well for elevations or altitudes of less than two or three thousand feet.

The standard atmosphere is a table giving values of air pressure, temperature and air density for various altitudes from the ground up. You can think of these values as averages for the entire Earth over the course of a year. (Related: Standard atmosphere tables).