With the change of time, the atmospheric pressure of a certain place also changes. The variation of water vapor in the air is the cause of this change. The density of air also changes with the change of vapor in the air. We can understand the change of atmospheric pressure by observing the change in the height of the mercury column of a barometer.
- If the height of the mercury decreases gradually then it would be understood that the pressure of water vapor is gradually increasing because water vapor is lighter than air. It indicates the possibility of rain.
- All of a sudden if the mercury height falls down then it is to be realized that the atmospheric pressure of the surrounding area has fallen down and low pressure has been created. The air at a high pressure from the neighboring area will rush with tremendous speed to the depression area. So, there is a possibility of storm.
- When the height of the mercury column of the barometer slowly increases, then it indicates that water vapor from the air is being disappeared and thy air is occupying that space. So, the weather will remain dry and clear. By determining the air pressure by a barometer in this way weather can be forecast.