Air pressure is caused by the gases in the atmosphere – mainly nitrogen (N2) and oxygen (O2) – pushing in all directions (think of gas in a balloon). These gases may be relatively light, but there are a lot of them in the atmosphere stretching approximately 24 kilometres (15 miles) above our heads. Air density/pressure varies due to: temperature – hotter air is less dense so floats above cool air; altitude: the closer air is to the planet’s surface – the more gravity packs its molecules together so increases its density; and humidity.
Paradoxically water vapour molecules H2O – two hydrogen (lightest matter in the universe) plus one oxygen – are actually lighter than air molecules, which are approximately 80% N2 (two nitrogen atoms) and 20% O2 (two oxygen atoms). That’s one – though not the only one – of the reasons clouds float! Note that liquid water is much heavier than water vapour, which is why it rains.
Because of the earth’s rotation, angle, sun striking half the surface, oceans and ice caps, the air temperature – and air pressure – varies throughout the day, the seasons and the years. These variations cause colder and hotter regions in the atmosphere. Colder regions are denser than warmer regions so have higher pressure. High pressure moves towards low pressure (prick a balloon and the gas rushes out, not in) causing wind. Wind moving from high pressure to low pressure are between a rock (earth’s surface) and a hard (high-pressure) place, so there is nowhere for all that incoming air to go. Nowhere to go… except UP!
When the air pressure drops very low, massive amounts of air are forced upwards so fast that they spin to form whirlwinds, tornadoes, cyclones and hurricanes. Note that cyclones and hurricanes are similar phenomenon but spin in different directions due to the Coriolis effect: cyclones rotate anti-clockwise in the southern hemisphere, hurricanes clockwise in the north.
However, while the Coriolis effect influences light air over thousands of kilometers, despite widespread belief it is far too weak to influence the direction small amounts of relatively heavy water spin down the plughole. That’s just an urban myth!
So in areas of relatively low pressure, air generally rises up. And as it rises, the air cools and the molecules of water it contains condense into water vapour that eventually forms white fluffy clouds. However, if the air pressure is low enough, the winds will be stronger so the incoming air will rise even higher where the lower temperature of the air can no longer support the ever denser water molecules, so they precipitate into rain, snow or even freeze as hail.
But predicting the weather isn’t quite that simple because it isn’t the actual air pressure that forecasts the weather – knowing the air pressure just tells you what you can learn by looking out the window, i.e. what the weather is now, not what’s coming. It is the change in air pressure that indicates what weather is coming, rather than the actual pressure itself.