Winds are directional atmospheric movements. On Earth, they are the movement of air, and exist in three basic types:
- winds due to large pressure systems, which are in turn caused by sunlight heating the ground, and are affected by the rotation of the Earth. (Without solar heating, the air would rotate against the Earth's rotation, due to the drag of the solar wind.)
- cold air falling down a mountain, creating a katabatic wind or Fohn wind.
- sea and land breezes, caused by the gain or loss of solar heat at different rates by land and sea.
Due to differences in latitude, the planet's axial tilt, and the reflection and absorption of different materials and gases, some parts of the Earth get more energy than others, and they also have different capacities for holding on to that heat. Hot air expands, and that expansion causes the air to have a lower density. Colder air, with a higher density, will tend to displace warmer air by moving in under it. Since cold air holds less water vapor, there is often rain at the boundaries, or weather fronts, between air masses at different temperatures.
Over large bodies of water near the equator, monsoons and typhoons form, driven by the heat of the underlying water.
Patterns in winds around the world have long been known, both as semi-permanent phenomena, such as the jet streams, which are primarily rotational effects, and periodical occurrences, such as the shifting of the trade winds in the tropics and such seasonal winds as the scirocco.
Wind is imperative to sailing. Windmills also use the energy of wind, either directly (as to grind grain) or to generate electricity.
Wind tends to hug the ground and travel in layers so when it hits a hill the layers bunch up resulting in high wind speeds at the top of the hill. Low pressure areas are created on the windward side of the hill and the opposite side of the hill. Eddies form in these areas with the wind eddy called the bolster eddy and the other eddy is called the lee eddy.
In the narrows of a canyon the layers of wind converge at the narrowest point of the canyon creating bolster eddies on either side of the start of the narrows and lee eddies on the opposite side.
In craters the layers of wind converge at the rim of the crater creating strong winds. Winds at the bottom of the crater are light but chaotic and disappear half way to the rim. Eddies form in middle.