Gas

Gases are one of the states of matter. Specifically, very low density, compressible (which means changes volume when pressure increases, as opposed to liquids which are generally incompressible). Gases are fluids, which means that they flow easily. Most materials will turn into a gas if they get to a high enough temperature.

The study of gases includes a fair amount of fluid mechanics, which is relevant for both the atmosphere and wind power. To learn more about gases please see UC Davis's Chem wiki.