A Mixed Blessing


OBSERVING the rapid disappearance of the snows of Saturday, I was reminded of the "snow eater" - a warm dry wind from the west that has a similar but more dramatic effect on the Great Plains of the US.

More properly known as the Chinook, the snow eater sweeps down from the Rocky Mountains in the spring and by causing the thick blanket of winter snow to disappear entirely in the space of a few hours, it quickly changes the whole appearance of the landscape.

When a moist westerly flow of air approaches the US from the Pacific Ocean, is forced to ascend by the mountains ink its path. As it moves upwards, the air expands and cools, and in due course its temperature falls sufficiently for condensation to take place; much of its moisture then falls to the ground as rain or snow.

Now the secret of the snow eating capability of the Chinook lies in the fact that the temperature of dry air changes, much more rapidly by expansion or compression than that of moist air. As the wind passes over the crest of the Rockies it has already lost much of its moisture on the ascent being relatively dry, its temperatures rises very quickly with compression as it descends, and by the time the air reaches the Great Plains it may be 10 or 15 degrees warmer than it was before it began to cross the mountain barrier. A Chinook that suddenly displaces a pool of cold winter air that has been stagnant for some time over Wyoming, Colorado, or Montana, can result in a local temperature increase of as much as 20C in less than half an hour.

The Chinook is named alter the Chinook Indians: it blew from over the Chinook camp, they used to say. It is welcomed for its superb efficiency at clearing away the troublesome blanket of snow left after the long harsh winter, but like similar warm dry winds in other parts of the world, it has unpleasant side effects. It is widely associated with a variety of symptoms ranging from lethargy to nausea, headaches and insomnia.

No one knows what causes "foehn disease", as it is called after the generically similar foehn wind common near the Alps. For many years it was believed to be associated with a characteristic oscillation in atmospheric pressure which often precedes the warm wind - a variation of several hectopascals over a few minutes.

But such pressure changes are now regularly experienced in lifts and aeroplanes with no ill effects - even in the case of persons with a history of foehn disease. The ailment remains one of the great unsolved mysteries of biometeorology.