Chapter 5
Forms of Condensation
and Precipitation
Notes
Cumulonimbus Cloud
Cloud Formation
Clouds – visible aggregate of minute droplets of water, or tiny crystals of ice, or a
mixture of both.
Clouds are a form of condensation produced when water vapor condenses in the
atmosphere. The cloud formation process includes adiabatic cooling, and
condensation of water vapor as a parcel of air rises above the lifting condensation
level.
Condensation occurs when two conditions are met:
1) The air is cooled to it dew point.
2) A surface is available on which the water vapor can condense
For condensation to occur aloft, tiny particles known as cloud condensation nuclei
must be present.
Hygroscopic nuclei – particles that are the most effective sites for condensation
Hydrophobic nuclei – water repelling particles
Dust storms, volcanic eruptions, and pollen are major sources of cloud condensation
nuclei. Also, by-products of combustion act as hygroscopic particles.
Cloud Types
Cloud Classification
Clouds are classified on the basis of two criteria: form and height.
Three basic cloud forms are recognized:
1) Cirrus – high, white, and thin clouds. They appear as delicate veil-like patches,
extended wispy fibers
2) Cumulus – consist of globular individual cloud masses. They exhibit a flat base
and appear as rising domes or towers.
3) Stratus – appear as sheets or layers that cover much or all of the sky. There are
no distinct individual cloud units.
All clouds have one of these three basic forms or combinations or modifications of
them.
Three height levels are used to classify clouds:
1) High clouds – normally have bases above 6,000 meters (20,000 feet)
2) Middle clouds – occupy heights from 2,000 to 6,000 meters
3) Low clouds – form below 2,000 meters
Clouds of vertical development – clouds that extend vertically to span more than
one height range.
Advection Fog in San Francisco
Types of Fog
Fog – a cloud with its base at or very neat the ground.
Physically, the is no difference between a fog and a cloud; their appearance and
structure are the same. The difference is the method and place of formation.
Clouds result when air rises and cools adiabatically. Fog results from cooling or
by the addition of enough water vapor to cause saturation.
There are three types of fog:
1) Radiation fog – results from radiation cooling of the ground and adjacent air. It
is a night time phenomenon requiring clear skies and a fairly high relative
humidity.
2) Advection fog – when warm and moist air is blown over a cold surface, it
becomes chilled by contact. If cooling is sufficient, the result will be a blanket of
fog. Advection refers to air moving horizontally, so warm, moist air moving
horizontally across a cold surface will lose heat.
3) Upslope fog – created when relatively humid air moves up a gradual sloping
plain, or up steep mountain slopes. Because of the upward movement, air
expands and cools adiabatically. If the dew point is reached an extensive layer of
fog may form.
Other fogs
When saturation occurs primarily because of the addition water vapor, the
resulting fogs are called evaporation fogs. Two types of evaporation fogs are
recognized:
1) Steam fog – When cool air moves over warm water, enough moisture may
evaporate from the water surface to saturate the air immediately above. As the
rising water vapor meets the cold air, it condenses and rises with the air that is
being warmed from below.
2)
Frontal fog – when frontal wedging occurs, warm air is lifted over colder air. If
the resulting clouds produce rain, and the cold air below is near the dew
point, enough rain can evaporate to produce fog. This fog is more or less a
continuous zone of condensed water droplets reaching from the ground up
through the clouds.
Steam Fog Rising from a Lake
Days per Year with Heavy Fog
Dew and Frost
Dew – the condensation of water vapor on objects that have radiated
sufficient heat to lower their temperature below the dew point of the
surround air. Because different objects radiate heat at different rates, dew
may form on some surfaces but not on others.
White frost – forms when the dew point of the air is below freezing. Frost
forms when water vapor changes directly from a gas into a solid without
entering the liquid state.
Raindrops Are much Larger than Cloud Droplets
Although all clouds contain water droplets not all clouds produce
precipitation.
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Clouds droplets are very small – 20μm (0.02mm)
Clouds droplets result from the abundance condensation nuclei
Because of their small size, droplets would fall very slowly and
would evaporate before is fell a few meters from the cloud
A typical raindrop has a diameter of about 2000μm (2.0mm)
The volume of atypical raindrop is a million times that of a cloud
droplet
Snowflake Being Formed by the Bergeron Process
Precipitation from Cold Clouds – The Bergeron Process
The Bergeron Process is the process that generates much of the
precipitation in the middle latitudes.
• The upper portions of the clouds must be cold enough to produce ice
crystals
• As the ice crystals fall through the cloud they enlarge by collecting cloud
droplets
• The type of precipitation that reaches the ground depends on the
temperature profile in the lower few kilometers of the atmosphere
Precipitation from Warm Clouds
The Collision-Coalescence Process
Precipitation from warm clouds (clouds
located below the freezing level) do not
form by the Bergeron Process.
Clouds made entirely of water
droplets—no ice crystals—must contain
some droplets larger than 20μm if
precipitation is to form. These large
droplets can form when “giant”
condensation nuclei are present, or
when hygroscopic particles exist. The
large droplets fall rapidly through the
cloud.
As the droplets fall they collide with
smaller, slower droplets and coalesce.
The larger the droplet becomes, the
faster they fall and the greater their rate
of growth.
To get droplets of sufficient size to fall
as precipitation a tremendous number
of collisions must occur. So, the cloud
must have great vertical thickness and
contain large cloud droplets to have a
chance of producing precipitation.
Updrafts within the cloud help the
collision-coalescence process because
they allow the droplets to move up and
down repeatedly in the cloud allowing
for more collisions.
The collision-coalescence process is
most efficient in the tropics where the
air is very humid and very few
condensation nuclei are present.
Raindrop Forming by
Collision and Coalescence
Temperature Profiles for Rain, Snow, Sleet and Glaze (Freezing Rain)
Forms of Precipitation
Because atmospheric conditions vary greatly both geographically and seasonally,
several different forms of precipitation are possible.
Rain
•Defined meteorologically as drops of water that fall form a cloud and have a
diameter of at least 0.5 millimeter.
•Most rain originates in either nimbostratus clouds or in towering cumulonimbus
clouds
•Drizzle and mist are not considered forms of rains because of the small size of the
water droplets that form them
Snow
•Precipitation in the form of ice crystals (snowflakes) or aggregates of ice crystals
Sleet and Glaze
•Sleet is a wintertime phenomenon and refers to the fall o9f small particles of ice
that are clear to translucent.
•Glaze forms when raindrops become supercooled as it falls through cold air and
turns to ice.
Hail
•Hail is precipitation is the form of hard, rounded pellets or irregular lumps of ice.
•Most hailstones have diameters between 1.0 centimeter and 5.0 centimeters.
Rime
•A deposit of ice crystals formed by the freezing of supercooled fog or cloud
droplets on objects whose surface temperature is below freezing.
Doppler Radar Image of a Band of Heavy Precipitation
Measuring Precipitation
Instruments:
• Standard rain gauge, tipping-bucket gauge, weighing gauge
Measuring snowfall:
Two measurements taken – snow depth and water equivalent