Forms of Condensation &

Precipitation

Forms of Condensation &

Precipitation

1. Condensation occurs when water changes from vapor to liquid, to produce dew, fog or clouds

2. The air must be saturated

3. There must be a surface on which condensation can occur - e.g. blades of grass (dew) and condensation nuclei

Forms of Condensation &

Precipitation - 2

1. Condensation nuclei - microscopic dust particles, smoke, salt particles

2. Need condensation nuclei in order to get condensation when RH is about 100%

3. The most effective nuclei are hygroscopic(water absorbent) - e.g., crystals of sulfates & nitrates

4. Cloud formation depends on adiabatic cooling as a parcel of air ascends (adiabatic = no heat added or lost)

Condensation Trails (aircraft

contrails)

1. Consist of ice crystals

2. Form above 9 km, where the air temperature is -50C

3. Engine exhausts contain hot humid air and condensation nuclei such as sulfates

4. Trails last longer if the air is nearly saturated, and there are no strong winds

5. Trails do not start immediately behind the engines – why?

Clouds

1. Clouds are visible aggregates of minute

droplets of water or tiny crystals of ice

2. Cloud classification by form : cirrus,

cumulus, stratus

3. Cloud classification by height: high (bases

above 6000 m), middle (2000 to 6000 m), low

(below 2000m), clouds of vertical

development (more than one height range)

Clouds - 2

1. High clouds - cirrus, cirrostratus,

cirrocumulus; not usually precipitation

makers

2. Middle clouds - altocumulus, altostratus

3. Low clouds - stratus, stratocumulus,

nimbostratus (rain clouds)

Cirrus

Cirrostratus

Cirrocumulus

Altocumulus

Altostratus

Nimbostratus

Summer cumulus

Clouds - 3

1. Clouds of vertical development -

cumulus (fair weather), cumulonimbus

(storm clouds)

2. Lenticular clouds often form on leeward

side of mountains

Lenticular Cloud

Fog

1. Fog is a cloud with its base at or very

near the ground

2. Fogs can be formed by cooling the air, or

by adding water vapor

Fogs caused by Cooling

1. Condensation produces fog when the temperature of the layer of air in contact with the ground falls below its dew point

2. Radiation fog - results from radiation cooling of ground & adjacent air

3. Radiation fog occurs at night under clear skies

4. Radiation fog is thickest in valleys (cold air), burns off 1 to 3 hours after sunrise

Fogs caused by Cooling - 2

1. Advection fog - caused when warm moist air

passes over a cold surface

2. Advection fogs are frequently very thick

3. Upslope Fog - created when relatively humid

air moves up a gradually sloping plain, or

steep slopes of mountains

Radiation Fog

Advection Fog

Occurrence of Fog

• Fig 5-12 shows average number of days per year with fog.

• Pacific Northwest, California, New England

• Cold ocean climates lower the temperature of the air, increasing the RH to 100%, so water vapor condenses out

Heavy fog days/year

Fogs formed by Evaporation

1. Evaporation fogs are caused by the addition

of water vapor

2. Two types - steam fog & frontal (precipitation)

fog

3. If cool air moves over warm water, enough

water may evaporate to saturate the air

immediately above. As the rising vapor meets

the cold air, it condenses - steam fog

Fogs … by Evaporation - 2

1. Steam fog is common over lakes & rivers when

the water is warm and the air is cold

2. When frontal wedging occurs, warm air is lifted

over cold air. If the resulting clouds yield rain,

and the cold air below it is near the dew point,

enough rain can evaporate to produce frontal

or precipitation fog

Steam Fog

Dew & Frost

1. Dew is water vapor condensed on

objects that have radiated enough

energy to drop their temperature below

the dew point of the air

2. White frost forms when the dew point of

the air is below freezing

How Precipitation Forms

1. Cloud droplets are very small - 20 μm.

Numerous condensation nuclei share the

available water vapor

2. Because they are small, cloud droplets

fall very slowly. Probably evaporate.

3. Raindrops have diameters around 2000

μm (2 mm)

How Precipitation Forms - 2

1. To form a raindrop, cloud droplets must

increase by a million times to produce

"massive" rain drops

2. "Massive" rain drops are formed by the

Bergeron process and by collision-

coalescence