# Measurement of Air Pressure

• The atmosphere is held on the earth by the gravitational pull of the earth.
• A column of air exerts weight in terms of pressure on the surface of the earth.
• The weight of the column of air at a given place and time is called air pressure or atmospheric pressure.
• Atmospheric pressure is measured by an instrument called barometer.
• Nowadays Fortin’s barometer and Aneroid barometer are commonly used for measuring air pressure.
• Atmospheric pressure is measured as force per unit area.
• The unit used for measuring pressure is called millibar. Its abbreviation is ‘mb’.
• One millibar is equal to the force of one gram per square centimetre approximately.
• A pressure of 1000 millibars is equal to the weight of 1.053 kilograms per square centimetre at sea level.
• It is equal to the weight of a column of mercury which is 76 centimetre high.
• The international standard pressure unit is the “pascal”, a force of one Newton per square meter. In practice atmospheric pressure is expressed in kilopascals, (one kpa equals 1000 Pa).
• The mean atmospheric pressure at sea level is 1013.25 millibars. However the actual pressure at a given place and at a given time fluctuates and it generally ranges between 950 and 1050 millibars.