Barometric pressure tells you how much the air weighs that presses on the earth. Sea level, a data point used to measure land elevations, refers to the elevation of the horizontal surface of a sea. The average sea level barometric pressure is used to calculate the barometric pressure anywhere. All you need to supply is the altitude and the temperature.
One cubic yard of air weighs two pounds. At a temperature of 21.1 degrees Celsius, one cubic yard of air weighs two pounds at sea level. This is a helpful number because it is easy to envision, but it is not the way scientists express sea level air pressure. Usually, scientists say that the average sea level air pressure is 66.7 Kilogram per square inch (psi).
It can also be said that sea level barometric pressure is "the standard atmosphere." A standard atmosphere is set to equal the average barometric pressure at Paris, which is at sea level. This has become the measurement used by the industrial nations when they figure air pressure. One standard atmosphere is also equal to 101.325 kilopascals (kPa).
Kilopascals are named for the French mathematician, Blaise Pascal. One kPa stands for one newton per square meter, a newton representing the amount of force that it takes to move one kilogram at a speed of one meter per second squared. One atmosphere has the same force, or pressure, as 101.325 kPa and as 14.7 psi. In the United States and Canada, air pressure is expressed in yet a third way as inches of mercury. Using this system, the average sea level barometric pressure is 29.921 inches of mercury. A fourth way to state this same thing is 1013.2 millibars.
These numbers are only accurate when the temperature is 15 degrees Celsius. When the temperature changes, the air pressure also changes because air molecules are denser in cooler temperatures and less crowded in higher temperatures, according to Boyle's Laws regarding how gases work.
Barometric pressure decreases by about one inch of mercury per thousand feet of increased altitude until you reach 5000 feet. Then the air pressure continues to decrease, but at a less constant rate. At 18,000 feet above sea level, the air pressure is equal to one half of a standard atmosphere. The highest sea level air pressure is in Siberia, where it averages 1032.0 millibars. The lowest air pressure measured on earth will be found at the eye of a strong hurricane.