Factors affecting Atmospheric Pressure

1. Altitude

The atmospheric pressure decreases with height or altitude. The decrease in pressure is about 1 cm of mercury for every 110m of ascent. The atmospheric pressure is highest at sea level. This is because at higher altitudes the air is thinner or less dense than the air at the sea level. The maximum air density is at the earth's surface; air density decreases with height (because the pull of the earth's gravity is less). The fewer number of gas molecules at higher altitudes means fewer molecular collisions and a decrease in air pressure. Since the atmosphere is highly compressible, the overlying layers exert pressure on low lying layers.

As the pressure decreases, the amount of oxygen available to breathe also decreases. At high altitudes, atmospheric pressure and available oxygen get so low that people feel breathless. Mountain climbers use bottled oxygen when they ascend very high peaks. They also take time to get used to the altitude because quickly moving from higher pressure to lower pressure can cause decompression sickness. Decompression sickness, also called "the bends", is also a problem for scuba divers who come to the surface too quickly.

Aircraft create artificial pressure in the cabin so passengers remain comfortable while flying.

3. Water vapour

Water vapour concentration affects atmospheric pressure because the molecular weight of water (18 g/mol) is less than the average molecular weight of air (about 29 g/mol). When water evaporates and enters the atmosphere as a gas, the water vapour molecules take the place of other gas molecules in the air. So, a volume of wet (or humid) air weighs less than an equal volume of dry air. Therefore, humid air is less dense and exerts less pressure than dry air.