The weight of air resting on a given area of the Earth's surface is known as air pressure. Air pressure (or atmospheric pressure) is always greatest at sea level, where the air is at its most dense. Therefore at the top of a mountain the air is less dense and therefore the pressure is lower.
The air is composed of billions of tiny particles that are constantly moving in all directions, bouncing off whatever they encounter. These collisions constitute what is known as air pressure. The more collisions occurring within a certain area then the greater the air pressure will be.
We are completely unaware of this, but the air is constantly exerting pressure on us, on average this is 14 ¾ pounds per square inch. (1 kg per sq cm ) . Air molecules are naturally drawn towards the earth by gravity, and as a consequence the density of the air is greater near the surface of the earth. Therefore the number of molecules in a given area, the air pressure, decreases with altitude. These molecules are in constant motion and this prevents them from settling at ground level.
At sea level, standard air pressure is 1013, but typically the pressure varies between 980 and 1040 millibars (mb). As with any aspect of the atmosphere there are extremes and the highest and lowest recorded pressures are as follows:
The highest recorded atmospheric pressure, 1085.7 mb, occurred at Tonsontsengel, Mongolia, 19 December 2001.
The lowest sea level air pressure ever recorded was 870 mb in the eye of Typhoon Tip over the Pacific Ocean on October 12th 1979
Air pressure is measured using a barometer. Although the changes are usually too slow to observe directly, air pressure is almost always changing.
Weather maps showing the pressure at the surface are drawn using millibars. Air pressure can tell us about what kind of weather to expect as well. Winds blow in an attempt to combat the differences in air pressure. Wind is the movement of air over the surface of the Earth, from areas of high pressure to low pressure. A large change in pressure over a relatively small distance, a large pressure gradient, can result in far stronger winds. When the isobars are tightly packed, locations within that large pressure gradient can expect windy conditions. As air rises and creates an area of low pressure, water vapour in the atmosphere will condense and form clouds. However sinking air, in an area of high pressure, means that no condensation will take place. This is why low pressure is associated with cloudy skies and unsettled conditions, and high pressure is associated with clearer skies and drier conditions.
Winds near the Earth's surface rotate anti clockwise toward the centre of areas of low pressure and clockwise outward from the centre of areas of high pressure in the Northern Hemisphere, with an opposite flow (clockwise around areas of low pressure and counter clockwise around areas of high pressure) occurring in the Southern Hemisphere. The main reason for this pattern is the Coriolis force, which results from the Earth's rotation on its axis and deflects wind to the right in the Northern Hemisphere and to the left in the Southern Hemisphere.
So keep tapping your barometers and feel the weight of the air pressing in around you!
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