Wind blows due to air-pressure gradients that are the result of the uneven heating of the Earth's atmosphere. Prevailing winds blow in a consistent direction over large areas of the Earth's surface, while local winds are dependent upon regional geography.
The Earth's curvature and tilt upon its axis cause the sun to heat the atmosphere unevenly. This results in some regions of the world having warmer air and others having cooler air. Warm air expands and increases the space between air molecules, creating a high-pressure system. In cool air, molecules are more tightly packed, and the air pressure is lower. Air molecules naturally flow from areas of low pressure to high pressure, so there is a natural movement of air between different areas in the Earth's atmosphere. This movement is responsible for wind.
Because large regions of the Earth experience seasonal changes in temperature, they also experience predictable wind patterns. These patterns are the prevailing winds. The trade winds are easterly prevailing winds in the tropics. The westerlies prevail in the middle latitudes.
Some regions of the world also experience local winds. These occur due to regional geographic features, such as water bodies and mountain ranges. Large bodies of water heat up to a higher temperature than the land, and lowlands become warmer more quickly than mountaintops. These temperature patterns create localized air flows from cooler to warmer areas.