Raindrop Formation

Were you ever told that raindrops are God’s tears? Of course they are not, but for us to understand what raindrops really are, we must understand what a cloud is. If there is not first a cloud, there can be no rain.

Steve Horstmeyer, a meteorologist based out of Cincinnati, Ohio, has given one of the best definitions of a cloud: “A cloud is a mass of air, liquid drops of many sizes, solids like dust, minute sea-salt particles, pollutants and gasseous (sic) water, aka water vapor.” The puffy, streaked, or piled-up mounds of white and gray you see in the sky are the visible manifestations of non-water particles and water in various states massed together.

How raindrops are formed in clouds is still a matter that scientists are researching. Researchers hope by knowing how raindrops are formed they can be more precise about what a climate for any given area is supposed to be and how global warming is affecting the climate of that area.

What do scientists know? They know that raindrops are more likely to come from stratoform clouds lower in the atmosphere. The higher a cloud in the atmosphere the more the water present will be in the form of ice crystals. Scientists know that aerosols released into the atmosphere tend to keep clouds stable and less likely to allow rain to fall over an area. They also understand that the amount of pollution in the atmosphere can change the size of the raindrops that fall.

A droplet within a cloud can be about .00008 inches in size, about the width of a single human hair. These droplets stay aloft in the cloud. In time the droplets bump against each other, bonding together to make larger drops. A large raindrop can take over sixty minutes to gain its size.

A raindrop about to fall is about one millimeter wide. Once falling from the cloud, a raindrop is pulled by gravity and begins to descend at .02 miles per hour. It soon achieves terminal velocity, or fall speed, the highest speed at which a drop of its size can fall. As the raindrop plummets through the atmosphere to the earth gravity, air pressure, and air turbulence deform the shape of the raindrop. A droplet less than a millimeter in size is ball-shaped. Drops a little larger than that flatten out like a disk. If a drop is over 4.5 millimeters in size, a little under the size of a common housefly, it assumes the shape of a mushroom with a tubular “stem” and a cap-like top. The different shapes are due to the surface tension (the bond formed by the water molecules that holds the drop together) of the droplet and the opposing force of air pressure. The larger mushroom-like drops are eventually broken up into smaller droplets. Larger drops tend to fall to the earth faster than smaller drops, the largest at about 20 miles per hour. Never are raindrops tear-shaped.

The mechanism scientists are studying in raindrop formation are what forces make smaller droplets of water vapor churn enough so that they form large rain drops big enough to escape the cloud and fall to earth. As research proceeds to find the answer, scientists are fairly certain that two processes work together to cause rain to fall. One of these processes is called entrainment, in which dry air at the edge and upper portions of a cloud will mix with the moist air in the cloud. The second process working with entrainment to produce rainfall is air turbulence. Water vapor and small droplets are tossed to and fro in the cloud, during which time they collide with other droplets. Water molecules bond to water molecules and soon form larger drops.

Research into ways to stimulate larger or smaller sized raindrops to fall may, in the future, relieve the suffering and economic hardship in drought-stricken regions.