In some parts of the world local winds with certain wellknown and reliable characteristics are such frequent visitors that they have been given names by the inhabitants of the district. Example are the bitter mistral, which sweeps down the Rhone valley in France, the gentle foehn, which is common in Swiss Alpine regions, and the notoriously squally bora, which blows on to the Adriatic from the mountains of the former Yugoslavia. The best-known American local wind is the chinook.
When a moist westerly flow of air approaches North America from the Pacific, much of its moisture falls on to the Rocky Mountains in the form of rain or snow. By the time this air has passed over the crest, it is relatively dry, and it is a well-known phenomenon in meteorology that the temperature of dry air changes much more rapidly with expansion or compression than that of moist air.
As the eastward-moving air descends on to the Great Plains, therefore, its temperature rises very quickly with compression, and the resulting warm, dry wind is the chinook.
A chinook which suddenly displaces a pool of cold winter air that has been stagnant for some time over the Great Plains can result in a local temperature increase of as much as 20C in less than half an hour. The wind is named after the Chinook Indians - it blew from "over the Chinook camp" - and its sudden onset is eagerly awaited in spring by the inhabitants of Wyoming, Colorado and Montana.
It is superbly efficient at clearing away the blanket of snow left after the long, harsh American winter, so much so that locally it is often called the "snow-eater".
The story goes that in the early spring of 1886 a young American artist, Charles M. Russell, was eking out a living as an unknown range rider in Montana. His employers "back east", it seems, were worried about the effects of the harsh winter on their livestock and asked for a report.
Instead of using pen and ink, Russell took up his brush and sent them a painting, a work which has become a powerful symbol of the hardships endured in the heyday of the old-time western ranching.
The painting shows the vast inhospitable Great Plains thick with snow and a few starving coyotes, too weak to kill, circling hungrily around the last surviving steer, waiting for it to die from cold. The steer's only hope of survival is a timely onset of the warm, benevolent wind of the Montana spring. Russell's painting, now a very famous work, is titled Waiting for the Chinook.