How do drops of rain form

Technical Announcements. Employees in the News. Emergency Management. Survey Manual. The air is full of water, even if you can't see it. Higher in the sky where it is colder than at the land surface, invisible water vapor condenses into tiny liquid water droplets—clouds. When the cloud droplets combine to form heavier cloud drops which can no longer "float" in the surrounding air, it can start to rain, snow, and hail Note: This section of the Water Science School discusses the Earth's "natural" water cycle without human interference.

Precipitation is water released from clouds in the form of rain , freezing rain, sleet, snow, or hail. It is the primary connection in the water cycle that provides for the delivery of atmospheric water to the Earth.

Most precipitation falls as rain. The clouds floating overhead contain water vapor and cloud droplets, which are small drops of condensed water. These droplets are way too small to fall as precipitation, but they are large enough to form visible clouds.

Water is continually evaporating and condensing in the sky. If you look closely at a cloud you can see some parts disappearing evaporating while other parts are growing condensation.

Most of the condensed water in clouds does not fall as precipitation because their fall speed is not large enough to overcome updrafts which support the clouds. For precipitation to happen, first tiny water droplets must condense on even tinier dust, salt, or smoke particles, which act as a nucleus. Water droplets may grow as a result of additional condensation of water vapor when the particles collide.

If enough collisions occur to produce a droplet with a fall velocity which exceeds the cloud updraft speed, then it will fall out of the cloud as precipitation. This is not a trivial task since millions of cloud droplets are required to produce a single raindrop.

A more efficient mechanism known as the Bergeron-Findeisen process for producing a precipitation-sized drop is through a process which leads to the rapid growth of ice crystals at the expense of the water vapor present in a cloud. These crystals may fall as snow, or melt and fall as rain. You might be surprised at the number of gallons of water that fall from the sky in even a small but intense storm.

One inch of rain falling on just a single acre results in 27, gallons of water on the landscape. If you'd like to know how much water falls during a storm, use our Interactive Rainfall Calculator English units or Metric units to find out - you just enter an area size and rainfall amount and see how many gallons of water reach the ground.

It's obvious that I'm a raindrop, right? After all, all of you know that raindrops are shaped, well As proof, you've probably seen me on television, in magazines, and in artists' representations.

Truth is, I'm actually shaped more like a drip falling from a water faucet than a raindrop. The common raindrop is actually shaped more like a hamburger bun!

When an ice crystal falls through a cloud, it may collide with and collect supercooled water droplets. This process is called accretion and is a mechanism to quickly form large particles. How big a droplet or crystal grows depends on how long it stays in the cloud. The longer a particle is in the cloud, the more particles it can collect and the larger it grows. The strength of the vertical motions and the thickness of the cloud determine how long it stays in the cloud.

It is typically between 0. These droplets are too light to fall out of the sky. When these droplets become bigger and heavy enough, they will fall out of the sky as rain. As these cloud droplets collide into each other, they get bigger. Think of it as rolling a snowball around in the yard. When the droplet reaches a size of 0. A variety of factors influence the size of raindrops as it falls from the cloud. Turbulence and wind is a big factor when it comes to raindrop sizes.

A turbulent atmosphere means raindrops are blowing around and colliding with other raindrops. When two raindrops crash into each other, it creates a bigger raindrop. Therefore you tend to notice bigger raindrops during a thunderstorms when there is a lot more turbulence in the atmosphere. Towering storm clouds tend to toss raindrops around like there are inside a washing machine and spit it out as hail or bigger water droplets.

While we know raindrops can be different sizes it takes a good scientist to prove it. If so, it would turn into a large droplet that more easily coalesces with smaller ones. The ice crystals get the droplet to grow initially. Then coalescence takes over. Well, not exactly. There's always a problem, isn't there? Water droplets don't form ice just because the temperature is at 32 degrees Fahrenheit or 0 degrees Celsius.

If the droplets are microscopic, they can stay in liquid form at temperatures as low as 40 below; which, by the way, is where the Fahrenheit temperature scale matches the Celsius one. Clouds aren't necessarily cold enough. Ice may be just what we need for droplet growth, but there appears to be a shortage of ice within clouds. But come on! It does rain. So something else must be going on. Fortunately for reservoirs and wells, that below-freezing threshold only exists for pure water?

Thanks to suspended particles, the temperature at which liquid droplets freeze within clouds is closer to 12 below Celsius or 10 degrees Fahrenheit, which is far more attainable at cloud elevation. So then it rains! But if the atmosphere were absolutely crystal clear and pure, it simply would never rain. The first person to break the ice on this theory was Alfred Wegener, of continental-drift fame, in the early s.

In the s, it was refined by Swedish meteorologist Tor Bergeron.