Understanding air pressure
The weight of the air pressing down on the Earth, the ocean and on the air below causes air pressure. Earth's gravity, of course, causes the downward force that we know as "weight." Since the pressure depends upon the amount of air above the point where you're measuring the pressure, the pressure decreases as you go higher.
Air pressure is related to its density, which is related to the air's temperature and height above the Earth's surface.
Air pressure changes with the weather. In fact, it's one of the most important factors that determines what the weather is like. You can do some basic weather forecasting by using the wind and barometric pressure.
Air pressure is also called barometric pressure because barometers are used to measure it.
The National Weather Service reports surface air pressure in inches of mercury, while air pressure aloft is reported in millibars, also known as hectopascals (hPa). Scientists generally use hectopascals to measure air pressure.
In the rest of the world, measurements are usually given in hectopascals, although you'll sometimes see them in centimeters of mercury, especially on older barometers.
The term hectopascals is replacing the term millibars. The hectopascal is a direct measure of pressure, like pounds per square inch, but in the metric system. Since the measurement is in the metric system, 1,000 millibars equal one bar. A bar is a force of 100,000 Newtons acting on a square meter, which is too large of a unit to be a convenient measure of Earth's air pressure.
Inches of mercury and centimeters of mercury measure how high the pressure pushes the mercury in a barometer.
The use of direct pressure measurements goes back to the late 19th century. This was when the Norwegian meteorologist Vilhelm Bjerknes, a leader in making meteorology a mathematical science, urged weather services to use direct pressure measurements because they can be used in the formulas that describe the weather, unlike measures of the height of the mercury in a barometer.
Air pressure and your body
Changes in air pressure, especially rather quick changes, can affect your body. The most obvious of these are the discomfort or even pain you feel in your ears when your gain or lose altitude rather quickly, such as in an aircraft, or even a fast elevator that goes up or down several floors.
Air pressure changes can also be felt in other ways. The links below have more on what happens:
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.
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.