9.8 Pa. The pressure exerted by the whole atmosphere on the Earth s surface is approximately 100,000 Pa. 1 mb is equal to 100 Pa, so standard atmospheric pressure is about 1000mb. In fact, actual values of atmospheric pressure vary from place to place and from hour to hour. 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.
Atmospheric pressure is measured by a barometer.
A mercury barometer measures the pressure by noting the length of mercury which is supported by the weight of the atmosphere. An aneroid barometer is a more compact instrument for measuring pressure. It consists of a box of partially exhausted air which expands and contracts as the pressure falls and rises. The box is connected through a system of levers to a pointer which, in conjunction with a dial, indicates the pressure. In a high-pressure system, air pressure is greater than the surrounding areas.