Explanation of Why Chinook Winds Are Warm & Dry

Prevailing Westerlies

• The prevailing westerlies, sometimes just called the westerlies, are winds in the middle latitudes. They are known as the "anti-trade winds" since ships had to sail against them when traveling west. In the Southern Hemisphere, they blow from the northwest. In the Northern Hemisphere, they blow from the southwest. The westerlies are strongest in the winter when the pressure is lower over the poles. They are weakest in the summer when pressures are highest over the poles.

Rain on the Coastal Ranges

• The westerlies blow across the Pacific Ocean in the winter, picking up heat and moisture from the underlying ocean surface. When the winds meet the coastal mountain ranges, they rise. As the flowing air mass rises toward the summit ridges, it cools until it eventually reaches its saturation temperature. Water vapor condenses into liquid water, first forming clouds and then precipitation. Much of that liquid falls as rain and snow on the coastal ranges.

Warming Winds

• The airmass cools by expansion as it rises over the mountains. It gains back heat from the process of water vapor converting to liquid water. The heat of condensation amounts to 2.5 kilojoules or 597 calories per gram of water. Additional heat can come from the heat of fusion if the water freezes in the air at high altitudes. By the time the westerlies have traversed the coastal ranges, the moisture they brought from the ocean has been dispelled and the process has heated up the winds and clouds as they continue eastward from the coast toward the Rocky Mountains.

Snow Eating

• When the winds finally cross over the Rocky Mountains and descend, they are compressed, as air is denser at lower altitudes. This compression warms the air even more as it moves over the eastern slopes of the mountains. The air warms by about 9.8 degrees Celsius for every 1,000 meters it descends, or about 5.4 degrees Fahrenheit per 1,000 feet of descent. Since ridge lines in the parts of the Rockies are 3,000 meters (10,000 feet) or more above sea level, the air will warm as much as 20 degrees C (36 degrees F) or more as it comes down off the mountains. The air remains dry since it loses moisture over the mountains but gains very little once leaving the ocean.