Precipitation From Clouds

The Bergeron Process

• The Bergeron Process

• Named for Swedish meteorologist Tor Bergeron• Upper troposphere temperatures can drop as

low as -50° C (-58° F) • Two important properties of water:– Water does not freeze at 0°C (32° F)– Pure water suspended in air does not freeze until it

reaches a temperature of nearly -40°C (-40°F).

The Bergeron Process

• In this state, water is said to be “supercooled”.

• Supercooled water will freeze if it impacts an object.– This is why aircraft experience icing conditions when

they pass through a cloud made up of supercooled droplets.

– This is also why the stuff we call freezing rain or glaze falls as a liquid and then forms a sheet of ice when it strikes pavement or a tree branch.

The Bergeron Process

• Additionally, supercooled droplets will freeze on contact with solid particles that have a crystal form closely resembling ice (like silver oxide- the chemical compound used in cloud seeding).

• These types of materials are referred to as frozen nuclei.

• Freezing nuclei in the atmosphere are sparse and do not become active until the temperature reaches -10°C (14°F).

The Bergeron Process

• At a temperature between 0° and -10°C, clouds consist mainly of supercooled water droplets.

• Between -10° and -20°C, liquid droplets and ice crystals coexist.

• Below -20°C (-4°F), clouds are composed mainly of ice crystals. (Like high altitude cirrus clouds).

The Bergeron Process

• The second important property of water.• The saturation vapor pressure above ice crystals is

somewhat lower than above supercooled water droplets.

• This is because ice crystals are solid which means that the individual water molecules are held together more tightly than those forming liquid droplets.

• This makes it easier for these water molecules to escape from supercooled water droplets.

Precipitation

• When air is saturated (100% relative humidity), with respect to liquid droplets, it is supersaturated with respect to ice crystals.

• See table 5-2

Precipitation

Precipitation

• So, how does precipitation form?• Picture a cloud at a temperature of -10° C

(14°F)• Each ice crystal is surrounded by thousands of

liquid droplets.• Because the air was initially saturated 100%

with respect to liquid water, it will be supersaturated (over 100%) with respect to the newly formed ice crystals.

Precipitation

• As a result of this supersaturated condition, ice crystals continue to gather water moisture and they grow.

Precipitation and the Bergeron Process

Precipitation and the Bergeron Process

• Because the level of supersaturation with respect to ice can be great, the growth of snow crystals continues and generates crystals large enough to fall.

• These crystals in turn are enlarged as they pass through the cloud by collision with water droplets.

• Air movement in the cloud can cause some of these crystals to break apart, and they become freezing nuclei for other liquid droplets.

Precipitation and the Bergeron Process

• A chain reaction develops and produces many snow crystals.

• These snow crystals, through a process called accretion, form into larger masses called snowflakes.

• Large snowflakes may consist of 10 to 30 individual crystals.

Precipitation and the Bergeron Process

• In summary, the Bergeron process can produce precipitation throughout the year in the middle latitudes so long as the upper portions of the clouds are cold enough for the generation of ice crystals.

• The type of precipitation that reaches the ground (rain, snow, sleet, freezing rain) depends on temperature in the lower few kilometers of the atmosphere.