Air pressure also varies over time, and these temporal differences are usually caused by the temperature of the air. Cool air is denser (heavier) than warm air. Warm air is less dense (lighter) than cool air and will therefore rise above it. Areas of high pressure can be caused when cool air is sinking and pressing on the ground. At this time, the weather is usually dry and clear. In contrast, when warm air rises, it causes a region of low pressure. With low pressure, the weather is often wet and cloudy.
Air pressure varies over time and from place to place. If we travel up a mountain or go up in a hot air balloon, for example, the air pressure gets less the higher we go. We can explain this difference by comparing air pressure to a pile of blankets on your bed.
