Chapter 5 Forms of Condensation and Precipitation The Atmosphere 10e Lutgens & Tarbuck Power Point by Michael C. LoPresto

Cloud Formation Clouds – visible aggregate of minute droplets of water, or tiny crystals of ice, or a mixture of both. Clouds are important to meteorologists because they indicate what is going on in the atmosphere.

The most important cloud-forming process is adiabatic cooling. When an air mass reaches it’s lifting condensation level, the ascending parcel has cooled to its dew point, and clouds form.

All types of condensation (fog, dew, clouds) form when two conditions are met. The air must be saturated. There must be a surface on which the water vapor can condense.

Cloud condensation nuclei – particles in the atmosphere that water vapor can condense on. Examples: dust, volcanic ash, smoke, salt

Growth of cloud droplets Hygroscopic nuclei – water seeking particles like salt, smoke from combustion (fire). Hygroscopic nuclei tend to form very large rain drops. Hydrophobic nuclei – water-repelling particles like dust. Hydrophobic nuclei tend to form much smaller rain drops.

Interesting Fact: It takes about a million cloud droplets to form a single rain droplet.

Size of Raindrops Compared to Cloud Droplets

How Precipitation Forms Facts: Cloud Droplets are very tiny (.02 millimeters in diameter). Approx. ¼ the diameter of a human hair. Cloud droplets fall very slowly. All cloud droplets evaporate before they reach the ground.

Cloud droplets turn to precipitation in one of two ways. Bergeron Process (cold clouds) Collision-Coalescence Process (warm clouds)

1. Bergeron Process – Cold cloud precipitation formation 1. Bergeron Process – Cold cloud precipitation formation. Named for Swedish meteorologist Tor Bergeron. Cloud droplets do not freeze at 0 degrees Celsius as expected. Ice crystals coming in contact with cloud droplets freeze those cloud droplets.

Bergeron Process

These two facts lead to the development of precipitation in cold clouds. The type of precipitation that reaches the ground depends on the atmospheric conditions near the ground.

When the surface temp is above 4 degrees Celsius, snowflakes will usually melt before they reach the ground. Even in the middle of summer, rainfall may have started out as snowflakes.

2. Collision-Coalescence Process Takes place in warm clouds – clouds with temperatures below the freezing level. Large cloud droplets fall faster than typical cloud droplets. As they fall, they collide and combine with other cloud droplets until they are large enough to reach the ground.

The Collision- Coalescence Process

In large cumulonimbus clouds, both processes are often at work. At the top of the cloud, freezing is taking place forming snowflakes. As the snowflakes travel down in the cloud, they melt and collide with other cloud droplets forming very large raindrops.

Cloud Classification Clouds are classified by two criteria: Height Form

Three basic cloud forms are recognized. Cirrus – High, white, and thin – often appear feathery or wispy. Cumulus – consist of globular individual cloud masses. Look like cauliflower. Often have a flat base and a rising dome. Stratus – sheets or layers. No distinct individual clouds.

Three levels are recognized. High clouds – bases above 6000 meters. Middle clouds – 2000 to 6000 meters. Low clouds – form below 2000 meters. Clouds of vertical development – extend through multiple height ranges – cumulonimbus clouds are in this category.

Cloud Types

Fog – a cloud with its base at or near the ground. How is fog different from a cloud? The method of formation.

How does fog form? Cooling (not adiabatically) or by adding water vapor.

Fogs formed by cooling: Radiation Fog – happens at night with clear skies and high relative humidity. The ground and air above it cool rapidly causing the air to reach its dew point producing fog. The fog will disappear within a few hours of sunrise as the ground and air warm.

Valley Fog

Advection Fog – warm moist air moves over a cold surface Advection Fog – warm moist air moves over a cold surface. Often, this fog is very thick and persistent. Example: Golden Gate Bridge

Advection Fog

Upslope fog – fog formed when air moves up a hill or mountain Upslope fog – fog formed when air moves up a hill or mountain. This is the only type of fog that forms adiabatically.

Fogs formed by evaporation: Steam Fog – when cool air moves over warm water, enough water may evaporate to saturate the air and form fog.

Steam Fog

Frontal or Precipitation Fog – when warm air is lifted over colder air and precipitation takes place, fog may form if the cold air parcel is near the dew point.

Dew – condensation of water vapor on objects that have cooled to a temperature below the dew point of the surrounding air mass. White frost – not frozen dew…formed when water vapor below the freezing point turns directly into ice (deposition).

MIST

DRIZZLE

RAIN

SLEET

GLAZE

RIME

SNOW

HAIL

GRAUPEL

Cirrus Clouds

Cirrostratus

Cirrocumulus

Altocumulus Altostratus

Nimbostratus

Cumulus Clouds

Aircraft Contrails

Cumulonimbus

Lenticular Clouds