The weather is no longer quite as unpredictable as it used to be. Yet we are hardly ever sure of it even a few hours in advance. One week is about the limit of the period of any prediction. Where the best men lack knowledge untrammeled fantasy has a field day. Weather has so far remained a safe topic of conversation and of speculation.
Nuclear explosions have, of course, been made responsible for the weather—for any kind of unusual weather. Be it rain or drought or a hard season of hurricanes—the nuclear tests are dragged in. The weather bureau says: no. But then—the weather bureau has not always been correct. Indeed it would be a miracle if the popular talk and the popular press would not have seen some connection between atomic explosions and the wayward behavior of the seasons.
In one case—and to our knowledge only in one case—there has occurred a chain of events starting with a nuclear test and ending in a copious and unusual downpour. In the spring of 1955 a test shot of moderate size was fired in Nevada. At the same time the last storm of the season was blowing itself out in California. According to the usual rules of meteorology the radioactive cloud should have been carried east by the steady westerly winds which blow over the temperate zone. But this time the cloud was caught up by the swirl of the dying California storm and some of the radioactivity was carried to the west coast.
Hours after the explosion radioactive rain began to fall in California. The activity was weak enough and did not give rise to any worry. But a remarkable thing happened. As the active cloud arrived over California the storm revived. It developed into an abundant rain which is not usual at that place and time. Did we—quite unintentionally—do something about the weather?
The weather bureau said: no. One must certainly admit that this single case proves nothing. Only greatly improved methods of weather observation and weather prediction would make it possible to decide if such a chain of events consists of the strong links of cause and effect or else of a simple sequence of haphazard occurrences.
Even though our knowledge is incomplete there is at least one simple fact which should be borne in mind. All the energy in that Nevada explosion was not quite sufficient to evaporate the water droplets in a cloud one mile broad, one mile wide, and one mile deep. This is not a very big rain cloud. Such a cloud would give about one third of an inch of rain water over one square mile—not an impressive amount. Even the biggest hydrogen bomb would give only energy enough to evaporate a cloud ten miles by ten miles and towering to the top of the “boiling” portion of our air, which we call the troposphere. This would give roughly three inches of rain over a hundred square miles—a more impressive amount but vanishing in the vastness of the Pacific Ocean.
Nuclear explosions are violent enough. But compared to the forces of nature—compared even with the daily release of energy from not particularly stormy weather—all our bombs are puny. Offhand one might guess that our nuclear fireworks could not swing the scales in the massive energy changes that we see around us in the common occurrences of wind and rain.
But the interplay of clouds and sunshine, of water evaporating, freezing, dropping and thawing—in short the vagaries of weather—are both involved and tricky. Small causes can give rise to big effects. Some processes of air masses sweeping over oceans and continents are irresistible and predictable. Others, like the first upsurge of hot air from the overheated ground, may be a question of close competition and trigger action. This is what makes it so difficult to predict the weather.
One of the most delicate processes we must think about is the formation of water droplets. When some water molecules are mixed with air molecules, we have moist air. If such air rises, expands and cools, the water molecules lose some of their agitated motion and have a greater tendency to stick together to form droplets. But it is not easy to get them started on this joint enterprise.
If two or three molecules stick together, they soon are shaken apart. If, however, two or three dozen are collected, this is enough to start a growth which ends in a droplet of water. If moist air is cooled, droplets will form, provided there is a meeting place from which the growth can start. If there is no such meeting place, there are no droplets and we get no cloud. If there are few meeting places, each will collect a rather great amount of water, we will get big drops, and we may get rain. If there is an abundance of meeting places many tiny droplets are formed which will remain suspended as a cloud. The present attempts at rain-making are connected with a birth-control of droplets.