Plague of the Plains

Rain splattered last week over the northern plains, but it was too late and too little to save them from the ravages of an all-too-familiar plague: the summer drought. After a summer so far almost without rain, the all-important spring wheat crop in Montana, the Dakotas and the Canadian prairie provinces will be a near failure. Some of the fields are hardly worth harvesting; others have been mowed for forage. East of the Rocky Mountains most of the ranges are bare, and cattle are being fed with trucked-in hay or grain. If heavy rain falls in late July, it will turn the ranges green--but it will not rescue the wheat.

Although it cannot prevent the frequent droughts that parch the high plains, the Weather Bureau has learned their meteorological causes and, to a degree, how to predict them. Like most major weather events in mid-latitudes, they can be blamed on the planetary wind that circulates around the temperate zone at high altitude. Its general motion is west to east, but it often veers to the north or south in great horizontal waves. In the bends of the waves are "ridges" of high pressure and "troughs" of low pressure, which affect the movement of the winds on the earth's surface.

Early in May, a high-pressure ridge formed over the Rocky Mountains in the U.S. and Canada. Since air circulates clockwise around a high-pressure area, the ridge brought dry air streaming down from northern Canada. In a normal year the ridge would have shifted gradually eastward, allowing warm, moist air to flow northwest from the Gulf of Mexico and bring normal rain to the high plains. But this year the high-pressure area stuck stubbornly over the Rockies during June and the first half of July. The dry, sunny weather that it brought dried out and heated the earth's surface, and hot air rising upward intensified the high-pressure area and its drought-producing effects. The moist winds from the Gulf were deflected to the Eastern seaboard.

Last week the pesky ridge finally moved westward toward the Pacific--at least temporarily--permitting moist air to reach the high plains and letting a little rain fall. The Weather Bureau's 30-day forecast, issued late last week, predicts that the ridge will move farther out into the Pacific, allowing more than normal rain to moisten the droughty area.

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