Gravity pushes the layers of air down to the Earth's surface. This push is called air pressure. Consequently, 99% of the total mass of the atmosphere is below 32 kilometers.
Like all fluids (gases and liquids), the air exerts a pressure on everything within and around it, although we are not aware of it. Pressure is a force, or weight, exerted on a surface per unit area, and is measured in Pascals (Pa). The pressure exerted by a kilogram mass on the Earth's surface is approximately 10 Pa. The pressure exerted by the whole atmosphere on the Earth’s surface is approximately 100,000 Pa.
Usually, atmospheric pressure is quoted in millibars (mb). 1 mb is equal to 100 Pa, so standard atmospheric pressure is about 1000 mb. In fact, actual values of atmospheric pressure vary from place to place and from day to day. At sea level, commonly observed values range between 970 mb and 1040 mb. Because pressure decreases with altitude, pressure observed at various stations must be adjusted to the same level, usually sea level.
Sometimes, atmospheric pressure is quoted in millimetres, centimetres or inches of mercury. This older form of measurement is related to the traditional method of measuring atmospheric pressure using a mercury barometer. Typical sea level atmospheric pressure is 76 cm mercury (Hg) or 30 inches.
Variations in atmospheric pressure lead to the development of winds that play a significant role in shaping our daily weather.