Air pressure is the weight of the atmosphere pressing down on the earth.
You may have seen the weatherman talking about air pressure on TV during a weather forecast. It is an important thing to know, as it can actually determine the type of weather we get where we live.
Air Pressure is measured by a barometer. You can see a barometer on the left. Barometers may also use mercury in a glass column, like a thermometer, to measure the change in air pressure. When the weather is calm the mercury in the barometer stays fairly low against the measure.
If a high pressure system is on its way, often you can expect cooler temperatures and clear skies. If a low pressure system is coming, then look for warmer weather, storms and rain.
Air pressure also changes with altitude. When you move to a higher place, say a tall mountain in Scotland, air pressure actually decreases because there are fewer air molecules as you move higher up.