Air is a mixture of several gases present in the atmosphere. The weight exerted by the air on a unit area of the Earth is known as air pressure. It is expressed in millibar. Atmospheric pressure produces changes in the weather. Winds are caused by air pressure.
- On maps, isobar lines join all places with the same air pressure at a given point in time. If the isobars are far apart, then the weather is calm and gentle. Closed spaced isobars denote great differences in air pressure and warm weather.
- Air pressure is measured by a barometer. There are two types of barometers — Fortin’s barometer and aneroid barometer.
- The rate of change of atmospheric pressure between two points on the Earth’s surface is known as the pressure gradient. The pressure gradient is denoted by the spacing of isobars on weather charts.
- Close spacing of isobars indicates a strong pressure gradient, while wide-spaced isobars point out a weak gradient.
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