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. We have seen earlier that in each radioactive decay charged particles are emitted. As these move along their paths, they tear up more atoms and leave in their wake an assembly of charged particles. These charged particles strongly attract the molecules of water. They attract the molecules of air much less. The reason is that in a water molecule positive and negative charges are separated to a considerable extent whereas in the nitrogen and oxygen molecules of air the charges are distributed more evenly. As a result the track of each particle emitted in a radioactive decay provides many meeting places for the formation of water droplets.