Adiabatic expansion/compression - (n.)

Expansion (or compression) of a gas (e.g. air) without exchange of heat with the surroundings. Air cools upon expansion and heats up upon compression, and this is the main reason for the vertical temperature gradient seen in the lower atmosphere (i.e. the troposphere). The adiabatic temperature gradient in dry air is near 1°C for every 100 m change in elevation. The actual gradient on Earth is less because of the presence of water in the air.