Atmospheric pressure is the force exerted at any given point on the Earth’s surface by the weight of the air above that point. The average value of surface pressure on Earth is 985 hPa. However, the standard atmospheric pressure at sea level is 1013.25 millibars, or about 14.7 pounds per square inch. This value is used to define another unit of pressure in common use, the atmosphere (atm) . In relation to the English system of measurement, one atmosphere (atm) equals 29.9213 inches of mercury (inHg) . Other conversions include:

• 1 atm = 101,325 pascals (Pa)
• 1 atm = 1,013.25 hectopascals (hPa)
• 1 atm = 760 millimeters of mercury (mmHg)
• 1 atm = 1,013.25 millibars (mbar)

The pressure of the air at a given place varies slightly according to the weather. Atmospheric pressure is measured using a barometer.