Understanding Rain Drops

Understanding Raindrops

The classical raindrop shape we see in illustrations everywhere is wrong! This may be the shape of a droplet the instant it lets go from a faucet, or some other surface, but it does not stay that shape for long (microseconds maybe). The reason is a phenomenon called surface-tension.

Surface-tension exists when two substances (particularly liquids or gases) are in contact with one another. The more different the two substances are from an electronic properties perspective, the larger will be the surface-tension between them. When two substances are very similar in their electronic properties, they will mix in each other or become miscible. In that case, the surface-tension is nearly zero. A good example of low surface-tension would be that between water and alcohol. Carefully pour a bit of alcohol into water, and no mater how carefully you pour, the two will combine into a single liquid, a solution of alcohol and water.

On the other hand, pour a little olive oil into water, and no matter how hard you shake it, it will eventually separate into two different liquids. When you do that, watch the shape that the oil droplets take as they rise through the water. If they are rising slowly, they will be almost perfect spheres. The smaller the droplets the closer to perfect spheres they will be. There is a large surface-tension between oil and water. If the droplets are large, or rising very rapidly, they will start to flatten out. This is very similar to raindrops.

Just like oil-in-water, very tiny raindrops will take on a spherical shape, and the smaller the droplet the closer to a perfect sphere it will remain. The reason is that a sphere is the shape with the smallest surface area for a given volume of material. Because air and water have very different properties, the molecules in the raindrop will prefer to be in contact with other water molecules rather than air molecules. Therefore, their attraction to each other will pull them into a shape that minimizes contact with air molecules. And the best shape for that is a sphere.

Now that is the ideal, and if you were aboard the International Space Station and carefully formed and released a drop of water into the air, it would eventually settle into a sphere (in the absence of air currents or other disturbances). However, on earth and in the atmosphere there are two forces that tend to distort the spheres.

The first of these forces is gravity. Gravity pulls the water through the air, because water is denser than air. A water droplet sitting on a non-wettable surface, like Teflon will tend to flatten out. The reason is that the gravitational pull on water molecules in the droplet, partially overcome the surface-tension that tries to keep it in a sphere. The droplet adopts a compromise shape in which these two forces equal each other. If the droplet is too big, it breaks into smaller droplets, until all of the droplets can reach a compromise shape. The smaller the droplet the larger is the ratio of surface area to volume. Surface-tension is a function of surface area; mass (or gravitational pull) is a function of volume. So the smaller the droplet, the greater the ratio between surface-tension and gravitational pull, so the closer the droplet will remain to a sphere.
The second of these forces is friction. A raindrop falling through the air must push air molecules out of the way as it falls. This takes energy, and that energy causes the raindrop to slow its descent. In other words, air resists the raindrop’s fall. In doing so, it acts a little like the Teflon surface above. In slowing the raindrop, it brings pressure to bear on the lower surface of the raindrop causing it to flatten out. The water in the droplet wants to fall at a rate determined by the acceleration of gravity, but the air resistance, or friction slows it down so that there is a net downward force inside the raindrop, just as if it were sitting on a non-wettable surface. The greater the rate of descent through the air, the stronger this force of resistance, and the flatter the raindrop becomes.

Because of the constant acceleration of gravity, the raindrop wants to accelerate, or descend faster and faster. Eventually the raindrop feels enough force that the pancake-shaped drop bows upward in the middle, like a parachute. As the speed of the descent further increases, the frictional forces pushing up on the bottom of the droplet overcome the surface-tension, and the drop breaks into smaller droplets, and the cycle of deformation begins again.

High in the rain cloud, most of the raindrops are tiny, even microscopic, hence they tend to be more spherical. But the conditions that cause them to form also cause them to grow larger. As tiny drops bump into each other, they merge, because the surface-tension between drops is close to zero. So with time they grow larger, which means they have more mass. The ratio of surface area to mass gets smaller. Since the frictional forces from the air, slowing their descent, is also a function of surface area (on the bottom of the drop) the gravitational forces begin to exceed the wind forces that keep them suspended in the air, and they begin to fall faster and faster.

Raindrop properties cause other storm-related phenomena.

Spherical droplets have optical properties that break white light into its spectral colors and send it back in the general direction from which it came, but at a specific angle. Therefore when there are a large percentage of raindrops that are spherical, viewed in opposition to the sun, a rainbow forms. The higher the proportion of spherical drops the more crisp and brilliant will be the rainbow. One can see the equivalent by spilling fine glass beads onto the ground, the kind used in reflective road-marking paint. If you stand with your back to the sun so that the shadow of your head falls on the spilled glass beads, a “rainbow” or “glory” will appear around the shadow of your head. The same effect can be seen sometimes from an airplane when you look down onto the top of the clouds at the shadow of the airplane.

Movement of raindrops (and ice crystals) through the air causes electrical charge separation. Frictional forces between the air and raindrops will break bonds between the water molecules, leaving dangling charges on the raindrop (or ice crystal) surfaces. The net result is that positive charges move one way, and negative charges move the other way. The charge built up on the surface of a single drop may be small, but when multiplied by the huge number of droplets in a roiling thundercloud, the charge can become quite large. When the charge becomes large enough, ZAP, lightning results.

When raindrops hit the ground or other surfaces, the force of impact breaks the raindrop into smaller droplets. The initial shapes may be elongated, similar to the “classical” raindrop shape. But the strong surface-tension pulls the droplet into a spherical shape so rapidly that it literally “snaps” into shape, creating a miniature shock wave as the air molecules speed to fill the void left behind the drop. This is the reason for the familiar “splat” sound made by a falling drop, and the “pitter-patter” of rain.

Surface tension, therefore, not only shapes raindrops, but determines how raindrops produce other storm related phenomena. We could delve further into the causes of surface-tension, and why it is particularly strong in water, but that is complicated enough to reserve for another (much longer) essay. So the next time you draw a picture of rain, draw the raindrops as spheres, flattened spheres, pancakes and parachutes. Virtually nobody will recognize them as raindrops, but you will be technically correct!