chinook, warm, dry air mass that descends the eastern slopes of the U.S. and Canadian Rocky Mts. after having lost moisture by condensation over the western slopes. Chinooks occur mainly in winter. They sometimes replace the cold continental air mass over the western plains, causing rapid melting of snow and temperature increases as great as 40°F (22°C) within a few hours. Similar winds occurring in the Alps and elsewhere are known as foehn winds. The term chinook was originally applied by Oregon settlers to a moist Pacific wind blowing from the direction of a Chinook camp.