Drops that grow larger than 4.5 millimeters (about three-sixteenths of an inch) become distorted into a parachute-shape as they fall, and then eventually they break up into smaller drops. The bigger the raindrop, the faster it falls, and the more it is affected by air pushing against its bottom. The top remains spherical, even on bigger falling raindrops, because surface tension-those water molecules clinging to each other-is greater than the pressure of airflow above. Drops that are 2 to 3 millimeters (just under one-eighth of an inch) in size are big enough to be affected by air pushing against them as they fall.īecause the airflow on the bottom of the raindrop is greater than the airflow on the top of the raindrop, this creates pressure on the raindrop's bottom, and its shape becomes flattened, like a sandwich bun, or punched in, so it looks like a kidney bean. Small raindrops, less than 1 millimeter in size (less than one-sixteenth of an inch), retain a roughly rounded shape because of surface tension, but drops can collide into each other as they are falling and form bigger raindrops. So, the water molecules in raindrops cling together, in their round little community, until… Farewell, Cloud Country The water molecules stick together because they are more attracted to bonding with each other than they are to bonding with air. Raindrops form into this shape because of the surface tension of water, which is sometimes described as a "skin" that makes the water molecules stick together. The drops sitting up here are like little globes of water, nearly round and spherical. Way up high in the atmosphere, dust and smoke particles suspended in clouds create places where moisture can settle and form into drops. doi: 10.1175/1520-0469(1971)0282.0.This short video explains how a raindrop falls through the atmosphere and why a more accurate look at raindrops can improve estimates of global precipitation.ĭownload this video in HD formats from NASA Goddard's Scientific Visualization Studio A Drop is Not a Drip “A Semi-Empirical Determination of the Shape of Cloud and Rain Drops”. NASA explains the water cycle, then takes a closer look at raindrop shapes and the instruments scientists use to view them: Meteorologists use ground radar and satellites to view raindrop sizes and predict precipitation. So, scientists use high-speed photography to image raindrops. The human eye typically forms 10 to 12 images per second, so falling rain appears more as lines than drops. Supersized raindrops are believed to form when droplets condense onto large smoke particles and then collide with each other. In 1999, raindrops of the same size fell near the Kwajalein Atoll, again from a cumulus congestus cloud. They fell in 1995 in northern Brazil from cumulus congestus clouds. In nature, the largest recorded raindrops were about 8.8 mm in diameter.
These drops had the distorted umbrella shape and a volume equal to that a 4.5 mm sphere. However, researchers have made and levitated 10 mm drops in a wind tunnel. Supersized RaindropsĪt one time, scientists thought the maximum size of a raindrop was about 4 millimeters.
The bottom rim of the droplet is thicker. Any additional pressure, like from a wind gust, tears the fragile shape apart.
#Raindrop shape skin
As the droplets split, the central depression becomes a thin skin of water molecules.
Raindrops about 3 mm in diameter resemble jellybeans or deformed hamburger buns.Ī massive raindrop resembles an umbrella, parachute, or jellyfish. A depression forms in the bottom center of the droplet. When a raindrop is large enough to achieve terminal velocity, it starts to split into two drops. There is less pressure on the top of the drop, so it stays round. Air pushing past the falling drop exerts pressure on its base, flattening it out and increasing its diameter. The top and bottom of the drop experience pressure differences. The larger a droplet is, the faster it falls. As the droplets gain mass, gravity draws them downward. Raindrops grow as more water vapor condenses and by merging with other droplets. When the drops are very tiny, the surface tension and cohesive force of the water molecules forms spheres that aren’t readily distorted by wind or gravity. Within clouds or very humid air, water vapor condenses into liquid water. Depending on their size, raindrops range in shape from tiny spheres to hamburger bun shapes to jelly beans to umbrellas.
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