Low pressure areas form when an airmass warms, either from being over a warm land or ocean surface, or from being warmed by condensation of water vapour in large rain or snow systems. The warming causes the air layer to expand upward, becoming slightly thicker. This expansion then causes air in the upper troposphere to flow away, leaving less mass, and so less weight (pressure) at the surface. The lower pressure air at the surface then causes higher pressure air around it to flow toward lower pressure, but as it does, the rotation of the Earth turns the wind to the right, resulting in the counter-clockwise wind flow around low pressure (in the Northern Hemisphere...it flows in the opposite direction in the Southern Hemisphere).

High pressure areas form when an airmass cools over a cool land or ocean surface. The cooling causes the air layer to shrink, becoming slightly thinner. This shrinkage then causes surrounding air in the upper troposphere to fill up the extra space. The added weight of the extra air causes higher pressure at the surface. The higher pressure air at the surface then tries to flow outward toward lower pressure, but as it does, the rotation of the Earth turns the wind to the right, resulting in the clockwise wind flow around the high pressure (in the Northern Hemisphere...it flows in the opposite direction in the Southern Hemisphere).

Air pressure

Because there are miles of air above us and it is all pushing down, the air at the bottom gets squeezed creating a pressure, like the pressure you feel at the bottom of a swimming pool. The size of this pushing force over each unit of area is called the air pressure, or atmospheric pressure.

Pressure (Pascals)= force (Newtons) /area (m2)

(The unit of pressure is called the Pascal or Pa for short, 1 Pascal = 1 Newton per m2)

Atmospheric pressure

The pressure on the earth's surface due to the air above us is about 100000 Pa - 101,325 Pa on average. That's 1Kg pushing on every square cm! 101,325 Pa is also commonly referred to as 'one atmosphere'. The weight of a column of water 10 meters high would be needed to increase the air pressure at the base of the column by 1 atmosphere.

A barometer measures air pressure. If you took a barometer up in a hot air balloon you would see the pressure reading fall the higher the balloon goes. This happens because there is less air above the balloon the higher up into the atmosphere it goes. If you went too high the air pressure would become so low that you would not be able to breathe properly. This is why modern passenger jets have 'pressurised cabins' to keep the conditions similar to that at the earth's surface so the passengers are more comfortable.

There is another unit of pressure called the "milli-bar" or mbar for short. There are exactly 100 Pascals per milli-bar, so 1000 Mb is about one atmosphere.

If you watch the weather forecast on TV you may see a map showing atmospheric pressure. This is referred to as an isobar chart.

Isobars are similar to contour lines. Instead of the lines showing areas where the ground is the same height above sea level, the lines show areas where the atmospheric pressure is the same. The closer the lines are together the more rapidly the pressure changes from one place to another. This is similar to contour lines on a map, the closer they are together the more steep the slope.