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.
1. The air must be saturated.
2. 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:
1. Cloud Droplets are very tiny (.02 millimeters
in diameter). Approx. ¼ the diameter of a
human hair.
2. Cloud droplets fall very slowly. All cloud
droplets evaporate before they reach the
ground.
•
Cloud droplets turn to precipitation in one
of two ways.
1. Bergeron Process (cold clouds)
2. Collision-Coalescence Process (warm clouds)
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
CollisionCoalescence
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.
1. Cirrus – High, white, and thin – often
appear feathery or wispy.
2. Cumulus – consist of globular individual
cloud masses. Look like cauliflower.
Often have a flat base and a rising dome.
3. Stratus – sheets or layers. No distinct
individual clouds.
•
1.
2.
3.
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. 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. 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