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The machine's forecasts do not pinpoint ground-level weather for any locality. They concern the behavior of the high-altitude waves, which have broad control over local ground weather. At present, says Dr. G. R. Cressman, head of the unit, the machine makes fine forecasts of upper-air weather for high-flying aircraft. For ground-level weather, it is not yet very good.
All authorities insist that computer forecasting should not be judged by its present performance but by its capacity to improve. Old-style forecasting is partly a subjective art, but the computing machine is objective. It will always come to the same conclusion about the same set of figures, and as the figures improve, its forecasts will improve also.
The Rossby Limit. Rossby still watches numerical forecasting, but in 1950 he began to get restless in Chicago. He had been there about ten years—the Rossby limit. Gradually, he transferred his interest to Sweden, where he hoped to find fresh contacts to keep his brain turning over.
Since the war, U.S. meteorology had continued to expand explosively. All the armed services were demanding better forecasting and better knowledge of the atmosphere. Radars had proved fine weather-observing tools, showing up rain or snow 300 miles away. Rockets could photograph from above hundreds of thousands of square miles of weather, even entire hurricanes. Weather ships were stationed at sea; weather airplanes were flying into hurricanes.
Rossby felt that the vigorous, hard-shelled U.S. type of meteorology was in good hands. It might be better for him to start at a new level, studying neglected properties of the atmosphere. With the help of the Swedish government, Rossby set up in Stockholm the International Meteorological Institute, which soon became a place of pilgrimage for meteorologists, both European and American.
Aside from continued study of atmospheric circulation, Rossby's favorite program at Stockholm has been "atmospheric chemistry." The atmosphere, he and his researchers have found, is anything but uniform chemically. Parts of it, for instance, are full of sea-salt particles which are responsible for a common kind of rainfall. Not much is known about them, although they may be one of the factors controlling the world's climates. The chemicals in the airborne salt, for instance, are not in the same proportion as they are in the sea. No one knows why, and Rossby wants to find out.
CO² Menace. Another atmospheric variable is carbon dioxide. CO² is comparatively plentiful downwind from industrial areas such as the Ruhr, and there is a good possibility that man's fires and engines are adding so much of it to the atmosphere that the world's climate may be changed drastically by the solar heat that it traps. Rossby wants to find out about this little matter too.
He is not too hopeful about human efforts to change the weather. He admits that cloud seeding with dry ice or silver iodide particles can coax rain out of a susceptible cloud, but he is not convinced that it can be done often enough to be valuable. Rossby believes that better long-range forecasting would probably be more valuable than attainable extra rain. A long-range forecast of a disastrous drought (see NATIONAL AFFAIRS), such as the one that is affecting much of the U.S. at present, could prevent much suffering.