1 How wind forms

Atmospheric pressure affects weather and climate because a difference in atmospheric pressure between two regions at the same horizontal level causes air movement from the high-pressure area to the low-pressure area, which is wind. Pressure differences are produced mainly by unequal heating (hot air rises → low pressure; cool air sinks → high pressure), which is why heat, pressure and wind are one chain. The greater the pressure difference over a given distance (the closer the isobars), the stronger the wind.

Wind systems.

• There are several global systems.
• For example, polar winds are dry, cold winds that blow from the high-pressure areas around the North and South Poles toward the low-pressure areas in the subpolar regions.
• (Other major systems: the trade winds blowing toward the equator, and the westerlies in middle latitudes; the Earth's rotation deflects them.) Local winds: sea and land breezes (Module 3), mountain and valley winds, and the dry hot Khamsin wind of Egypt in spring.