One Million Times Larger!

Finally! We have a cloud comprised of billions of cloud droplets.

The cloud droplets remain suspended in the air by slight upward air currents and some heavier droplets actually fall out of the bottom of the cloud evaporating very quickly in the drier air beneath the cloud.

Now we can take a look at how a humble little cloud droplet grows to be 1 million times larger!

It is not easy for a cloud droplet to continue to grow into a form of precipitation. Several conditions must exist within the cloud in order for the droplet to grow large enough to fall to the ground as precipitation. We will talk about these conditions as they apply to the 2 major mechanisms for precipitation development:

The 2 major mechanisms that explain how cloud droplets become large enough to fall to the ground without evaporating are:

1) The Bergeron or "Ice Crystal" Process
2) Growth by Collision and Coalescence

The Bergeron Process, named after its discoverer, Tor Bergeron, involves supercooled water droplets. The term supercooled, refers to water that is still a liquid at temperatures below the melting point of 0 degrees Celsius or 32 degrees Fahrenheit. Sounds impossible right?

Actually, studies have shown that water in very small drops, such as the size of a cloud droplet, can exist at temperatures well below freezing (as low as -40 C)!

HERE IS SOME ADDITIONAL INFORMATION YOU WILL NEED TO KNOW BEFORE WE LEARN ABOUT THE BERGERON PROCESS:

An interesting and useful fact is that these supercooled water droplets will freeze almost instantly if they come into contact with a solid particle that resembles an ice crystal. This solid particle is called a freezing nucleus.

Also they will freeze if agitated; sometimes just by coming into contact with one another causes ice crystals to form.

Many of the puffy cumulus clouds that you see in the sky may be made up entirely of supercooled liquid water droplets!

Clouds at certain temperatures can also be comprised of both supercooled water and ice crystals and are classified as Mixed Clouds:

Cloud Temperature	Droplets?
Above 0 C (32 F)	Liquid Water

-10 to 0 C (12-32 F)	Supercooled Water

-40 to -10 C (-4-14 F)	Supercooled Water and Ice Crystals Coexist (mixed clouds)

Below -40 C (-4 F)	Mainly Ice Crystals (glaciated clouds)

When a cloud is comprised of both Supercooled Water and Ice Crystals, The Bergeron Process can begin!

The Bergeron Process explains how ice crystals grow at the expense of liquid cloud droplets within a mixed cloud:

There are more water molecules surrounding the water droplets than there are surrounding the ice crystals. This occurs because the saturation vapor pressure over a water surface is greater than that over an ice surface at the same [subfreezing] temperature. Saturation vapor pressure describes how much water vapor is needed to make the air saturated at any given temperature and in effect, is the pressure that the water vapor would exert if the air were saturated with respect to a given temperature. The supercooled liquid droplets are more readily able to evaporate and contribute to the vapor pressure in the surrounding air than the ice crystals are able to sublimate and contribute to the vapor pressure. Therefore, when ice and liquid coexist within a cloud, water vapor must evaporate from the drop and flow toward the ice crystal in order to maintain equilibrium. As this water vapor diffuses toward the ice crystal, the droplet must evaporate more in order to keep the vapor pressure in equilibrium with its surroundings. Therefore, what happens, is a viscious cycle of water vapor evaporating from the drop, collecting on the ice crystal, and freezing so that the crystal continuously grows at the water droplet's expense.

A mixed cloud may contain a million water droplets to every ice crystal, but given the right conditions, this ice crystals can grow and multiply very rapidly!

This picture shows supercooled droplets and ice crystals surrounded by millions of water molecules [shown in red].

Click here to continue this lesson in sequence and learn about the 2nd VERY IMPORTANT mechanism for precipitation formation; The Collision and Coalescence Process!

OR Click here for the Main Precipitation Page!