Atmospheric Pressure

We live at the bottom of an ocean of air, and that air presses on everything — including our bodies — all the time. You feel this pressure most clearly when you climb a mountain or fly: the air gets thinner, and you feel breathless. That pressing weight of air is what this section is about.

What Atmospheric Pressure Is

The weight of a column of air contained in a unit area, measured from mean sea level up to the top of the atmosphere, is called atmospheric pressure. It is expressed in units of millibar. At sea level, the average atmospheric pressure is 1,013.2 millibar.

Because of gravity, the air near the surface is denser, and denser air exerts higher pressure. Air pressure is measured using a mercury barometer or an aneroid barometer.

Pressure decreases with height. At any given elevation, however, pressure varies from place to place — and this variation is the primary cause of air motion, that is, wind, which moves from high-pressure areas to low-pressure areas.

Vertical Variation of Pressure

In the lower atmosphere, pressure decreases rapidly with height. The decrease amounts to about 1 millibar for every 10 metres of increase in elevation. But it does not always decrease at the same rate.

The standard atmosphere gives average pressure and temperature at selected elevations:

The vertical pressure gradient force is much larger than the horizontal pressure gradient force. Yet it is generally balanced by a nearly equal but opposite gravitational force. That is why we do not experience strong upward winds.

Horizontal Distribution of Pressure

Small differences in pressure are highly significant for wind direction and for purposes of comparison. Horizontal distribution of pressure is studied by drawing isobars at constant levels. Isobars are lines connecting places having equal pressure.

To eliminate the effect of altitude on pressure, the pressure measured at any station is reduced to sea level before being shown on weather maps. The sea level pressure distribution is shown on such maps.

The patterns of isobars correspond to pressure systems:

• A low-pressure system is enclosed by one or more isobars, with the lowest pressure in the centre.
• A high-pressure system is also enclosed by one or more isobars, with the highest pressure in the centre.

World Distribution of Sea Level Pressure

The world distribution of sea level pressure differs between January and July. Near the equator, the sea level pressure is low — this area is known as the equatorial low. Along 30° N and 30° S lie the high-pressure areas called the subtropical highs. Further poleward, along 60° N and 60° S, are the low-pressure belts termed the subpolar lows. Near the poles, pressure is high, known as the polar high.

These pressure belts are not permanent in nature. They oscillate with the apparent movement of the sun. In the northern hemisphere, they move southwards in winter and northwards in summer.

Forces That Affect Wind

Three forces govern the behaviour of wind: …