Air pressure is the weight of air resting on a given area of the Earth's surface. Air pressure (also called atmospheric pressure) is greatest at sea level, where the air is densest. On a mountain peak the air is less dense and the pressure is lower.

Low pressure also occurs when air is warm, expands, gets lighter and rises. High pressure occurs when air gets cold, contracts, becomes heavier (denser) and sinks (falls). Air flows from regions of high pressure (highs) to low pressure (lows) as it tries to equalise the difference between the two, known as the pressure gradient. This movement of air is wind. The greater the difference between the high and the low pressure, the greater the wind speed. The closer the isobars on a weather map are together, the stronger the winds.