Why Does it Rain?

Splashing Water Drops On Road
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Rain. It ruins our parades and gives us the blues. And while you might think rain forms solely to be a nuisance to you, the truth is precipitation forms when millions of tiny water droplets inside of clouds collide and join together. 

There are two methods that produce cloud droplets that grow into raindrops: the Bergeron process and the collision coalescence process.

Collision Coalescence

Collision coalescence describes how rain forms in "warm clouds" -- clouds located well below the freezing levels of the upper atmosphere.

In it, relatively large liquid cloud droplets form thanks to the presence of "giant" condensation nuclei such as sea salt. These larger drops fall at fairly fast speeds through the cloud and collide with the smaller, slower droplets. As this happens, they then coalesce, or join together, and become larger. This larger, blended drop then falls even faster and picks up more of its slow-moving neighbors. This cycle continues on and on until roughly a million or so cloud droplets have been collected. At that point, the conglomerate drop is finally large enough to fall out of the cloud and journey to the ground without evaporating before it reaches earth's surface.        

Related: What Shape are Raindrops?

The Bergeron or "Cold Rain" Process

Collision coalescence isn't the only way to make it rain. The Bergeron process explains how precipitation is produced in the frigid upper portions of clouds where temperatures are considerably below freezing.

Much of the rain that results from the Bergeron process starts off as snowflakes (hence, why it is sometimes called the "cold rain" process).

Named for Tor Bergeron, a Swedish meteorologist, it describes how supercooled water droplets interact with ice crystals to grow snowflakes. How can water remain a liquid at below freezing temperatures, you ask?

As contrary to common sense as it sounds, when pure water is suspended in air it actually doesn't freeze at 32 °F (0 °C). (It won't freeze until it reaches a temperature of nearly -40 degrees.) Back to our cloud...it contains ice crystals surrounded by many thousands of liquid droplets. The ice crystals collect more water molecules than they lose from sublimation. And so, as the liquid drops evaporate, the ice crystals grow from the water vapor. As this cycle continues, it produces snow crystals that are large enough to fall. As the crystals fall through the cloud, they meet cloud drops that freeze on them and as a result of this, they enlarge. A chain reaction occurs and produces many snow crystals. These soon clump together into larger masses called snowflakes!

If temperatures throughout the cloud and down to the surface remain below freezing, these snowflakes will stay frozen and fall as snow. However, if temperatures at lower levels inside the cloud rise above freezing, or if there's a deep layer of above-freezing air down to the surface, the snowflakes will melt and fall as rain.     

More precipitation forms by the Bergeron process than from collision coalescence.

Why Don't All Clouds Make Rain? 

We've just explored how raindrops are made when tiny cloud droplets bump into other droplets and grow bigger.

But if this is true, and all clouds contain water, why do some clouds produce rain and snow and others don't?

Yes, all clouds are made up of very tiny droplets of water, but because of their small size, these droplets would evaporate shortly after falling out of the cloud base into the relatively dry air below it. In able to make the journey to the ground, the droplet must grow about 1 million times in size. But only certain clouds . For the Bergeron process to work, a cloud needs to contain both liquid water droplets and ice crystals. Both only coexist within clouds having temperatures between -10 and -20 °C.  

Similarly, the collision coalescence process can only work when clouds contain some liquid droplets that are larger than the average cloud droplet size of 0.02 millimeters across. Because not all clouds do, not all are capable of producing precipitation by collision coalescence.

Clouds that are shallow or thin aren't ideal for supporting collision coalescence either, since they won't offer a long enough distance for the raindrops to hit others and grow to a sufficient size as they fall through the cloud's interior. Clouds with a deep vertical extent work best.     

Which Clouds are Rainclouds?

Now that we know all clouds aren't precipitation makers and why this is, let's take a look at which cloud types are well-known rainmakers:

  • Altostratus clouds are known to produce very light precipitation.
  • Stratus are associated with drizzle.
  • Nimbostratus clouds are one of the chief precipitation producers. After all, the "nimbus" in it's name is Latin for rain cloud! 
  • Cumulonimbus clouds are thunderstorms and heavy rainfall. 

To learn more about these clouds, see photos, and what seeing them for your forecast, read The 10 Basic Types of Clouds and How to Recognize Them in the Sky

Now that you know what causes rain to form, why not find out the real shape of raindrops or the temperature of rainwater.

Yes, all clouds are made up of very tiny droplets of water, but because of their small size, these droplets would evaporate shortly after falling out of the cloud base into the relatively dry air below it. In able to make the journey to the ground, the droplet must grow about 1 million times in size. But only certain clouds . For the Bergeron process to work, a cloud needs to contain both liquid water droplets and ice crystals. Both only coexist within clouds having temperatures between -10 and -20 °C.  

 

Resources & Links:

Lutgens, Frederick K., Tarbuck, Edward J. The Atmosphere, 8th ed. Upper Saddle River: Prentice-Hall Inc., 2001.  

Why Raindrops are Different Sizes, The USGS Water Science School.