Weather from Aloft

People who want to plan picnics may never have a good word to say for the weatherman, but nonetheless he is making progress. In the current Scientific Monthly, Meteorologist Jerome Namias of the U.S. Weather Bureau explains an improved method for general weather forecasting over longer & longer periods.

Namias believes that in the north temperate zone the chief cause of weather changes is a high-altitude wind that roars from west to east around the earth. It is strongest and most constant above 10,000 ft., sometimes reaching 200 m.p.h., but it drags low-level air along, whipping up the disturbances that affect the weather on the surface.

Waves Around the World. If the earth were a smooth sphere, the circumpolar wind might flow in a neat circle following the parallels of latitude. But the earth is ridged with mountains and mottled with alternate patches of land and ocean. These blemishes scallop the great wind into snakelike, horizontal waves whose southern lobes sometimes reach the tropics. On the western sides of the waves the wind aloft blows toward the southeast, carrying with it masses of cold northern air. On the eastern sides the wind blows toward the northeast, carrying tropical air into the temperate zone. When two such dissimilar air masses clash, they produce the familiar "cold front" or "warm front" that marches across oceans and continents.

The wave-forms usually move from west to east in a stately procession. They drift fastest when the wind aloft is blowing hard and when the distance between the waves is comparatively short. When the waves are far apart and the wind is feeble, the waves may actually "retrogress," moving feebly toward the west.

This relationship gives the meteorologists a toehold. By measuring the speed of the wind aloft and the distances between the waves, they can predict with some accuracy how fast the waves will drift. They gather this information by means of sounding balloons that carry small radios. When such reports from all over North America are evaluated and combined with local data, the Weather Bureau predicts cautiously what sort of weather the waves aloft will bring to each part of the U.S. during 1) the next five days, and 2) the next 30 days.

A hostess planning a garden party finds little use for these forecasts. They do not say, for instance, whether it will rain in Little Rock two weeks from Tuesday. They merely predict whether the temperature during the period covered can be classed as "much above normal," "above normal," "normal," "below normal" or "much below normal," and whether the precipitation will be "light," "moderate" or "heavy."

Better Than Guessing. On this basis their accuracy is fairly good. Namias says that the five-day forecasts are "within one class" of being right about the temperature 80% of the time; the monthly forecasts are right 75% of the time. Rain and snow are much harder to predict than temperature is; the five-day forecasts are right about 40% to 50% of the time, and the monthly forecasts less than 40%. Even this low accuracy, however, is better than mere guessing based on "most likely" conditions for a given place and season.

The bureau now has more than 3,000 subscribers ($4.80 a year) for its monthly forecasts. They are much appreciated by fuel distributors, ice-cream manufacturers, sportswear makers et al., whose business is affected by general trends in the weather. Namias hopes that as more observations flow in from remote parts of the earth, the forecasts will become both longer-range and more accurate.

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