Science: Jump-Line Warning

Tornadoes are notoriously unpredictable. It is impossible to tell, just by looking, whether a tall black cloud will merely drop a shower, or whether it will lash out with a twister. Last week the U.S. Weather Bureau was on the trail of a promising way of telling in advance which of the tall black clouds are apt to be pregnant with tornadoes.

According to Meteorologist Morris Tepper, the thing for tornado predictors to watch for is a “pressure jump.” When conditions are right, as they all too frequently are in tornado regions, the air contains an “inversion,” a layer whose temperature is sharply different from the air above or below it. Since cold air is heavier than warm air, the boundary between the layers may have “gravity waves” in it, just as the ocean has waves in the boundary between water and air.

Tepper is not sure what starts an atmospheric wave; often an advancing mass of cold air seems to be the cause. But once the wave gets going, its front gets steeper and steeper and the air in the wave may rise more than a mile in a few minutes. This causes a sudden rise of barometric pressure that shows as a sharp jog on the chart of a specially sensitive barograph.

The wave, which meteorologists call the “jump-line.” is not dangerous, but the Weather Bureau has found that it is apt to set off the violent thunderstorms that lash the earth with twisters. In one study, 82% of the tornadoes struck within an hour of a pressure jump.

After satisfying itself on the close relationship between jump-lines and tornadoes, the Weather Bureau (though chronically starved for funds) is now trying to make use of its new knowledge. It has designed a simple instrument that ignores ordinary changes of pressure, but rings a bell when a jump-line passes over it.

The Weather Bureau thinks that its jump-line detectors can be made in quantity for less than $100 each. Spotted through tornado areas in police stations and other always-open institutions, they should enable the weathermen to keep track of each jump-line as it moves crosscountry. Since the average speed of the tornado-triggering wave is only about about 35 m.p.h., the weatherman should have time to give plenty of warning.