PRESSURE

FACTORS EFFECTING ATMOSPHERIC PRESSURE :-

Atmospheric pressure depends on three main factors :-

1. Altitude: As you go higher up the pressure of the air decreases. Generally speaking, this decrease in pressure is at a rate. of 1 cm for every 110 meters of ascent. At 5500 meters it is about half that figure and at a height of 48 km it is one-thousand the pressure at sea-level. This is because at great heights the air is thinner or less dense than the air at sea-level. Pressure, therefore, decreases as altitude increases.

2. Temperature: When the temperature rises, air expands, that is, it becomes less dense and exerts less pressure. When temperature falls air becomes more dense and its pressure increases.

3. Water Vapour: Air containing water vapour is lighter than dry air. The more water vapour there is the lighter the air. This is because water vapour in humid air displaces an equal volume of dry air. A certain volume of dry air contains more nitrogen and oxygen, but the same volume of humid air with more water vapour contains comparatively less nitrogen and oxygen. This makes the humid air lighter than dry air.

ISOBARS :-

The pressure of air is shown on weather maps by means of lines called "isobars" meaning "equal weight". An isobar is an imaginary line drawn on a map (or a weather chart) joining all places having equal atmospheric pressure, supposing these places to be at sea-level.

When the isobars are far apart form the one another, there is little difference of atmospheric pressure and the weather is warm. When they are close to one another there is a great difference of atmospheric pressure over a small area of the earth's surface and the weather is stormy. When isobars are close together they indicate a rapid change of pressure.

The daily weather map (or synoptic chart) shows isobars at a particular moment on a particular day. It is the basis of weather forecasting. The pressure map depicts the average air pressure over a long period of time. It is not useful in weather forecasting but it is invaluable for climate study.