How Does a Cloud Rain?
Why do some clouds produce rain or snow while others do not? The answer turns out to involve some rather surprising properties of water and of things which are very small.
Clouds are made up of very small water droplets. In continental clouds, most droplets are smaller than one seven hundredths of an inch in diameter, and have fall speeds of less than 2 feet per second. It would take such a drop 45 minutes to fall from a height of a mile, so a large number of cloud droplets must join together to make a raindrop large enough to fall fast enough to reach the ground before it evaporates. But strangely enough, the tiny cloud droplets cannot collide with each other. Rather, they are carried around each other, protected from colliding by a cushion of air. How, then, can they grow?
To answer this question, we must look at another interesting property of water: contrary to general opinion, it does not automatically freeze when the water temperature is 32 degrees F or colder. In order to freeze, water must contain a special kind of particle, called a freezing nucleus. The lower the temperature, the more different kinds of particles can act as freezing nuclei, until at forty below even clean water will freeze. Silver iodide has become famous for its ability to freeze supercooled droplets at temperatures in the plus twenties, but certain bacteria may also act as freezing nuclei in the atmosphere, as may some clay minerals. A large body of water will normally freeze at temperatures very close to the freezing point, because it will usually have a number of effective freezing nuclei. A very small cloud droplet, however, is unlikely to have a freezing nucleus that will be active until rather low temperatures are reached. These subfreezing liquid droplets are referred to as being supercooled. At temperatures around zero, a number of cloud droplets have generally frozen, and the process of snowflake growth can start.
Once a droplet is frozen, it has a greater affinity for the water in the air than does a liquid droplet. Consequently, the tiny ice crystal grows at the expense of the surrounding water droplets, which evaporate as the ice crystal sucks the water from the air around them. Soon the crystal becomes a snowflake, which is able to collect even more droplets from the surrounding air as it collides with them while falling. (Collisions are perfectly possible as long as one of the colliding bodies is more than about one five hundredth of an inch across.) In air with many supercooled droplets, the snowflake may collide with so many that it becomes a white pellet of rime ice, called graupel.
If the snowflake or graupel pellet continues to fall into air with temperatures above freezing, it will melt and become a raindrop. In Alaska, this often happens below the level of the cloud base. Snow is more reflective than rain, so if you look away from the sun at precipitation falling from a distant cloud, the fall streaks may look brighter just below the base of the cloud, where it is still snow, and darker lower down, where it has melted into rain.
