What is Adiabatic cooling; dry adiabatic rate?

Adiabatic cooling is the cooling of a parcel of air that expands without exchanging heat with its surroundings. The rate at which rising unsaturated air cools in this way is the dry adiabatic rate, close to $9.8{}^{\circ}\mathrm{C}$ per kilometre of ascent; sinking air warms at the same rate.