Download Cloud Formation

Cloud Formation
Review LCL & Dew Point
• The Sun’s radiation heats Earth’s surface, the surrounding air is heated due to
conduction and rises because of convection.
• Unstable air rises and contains water vapor
• Through adiabatic cooling, the temperature and pressure decrease with higher
altitudes. This causes the air parcel to expand & cool as it rises.
• Remember, as temperature drops, the air cannot hold the same amount of water
vapor causing it to condense
• At a certain altitude in the troposphere the rising air condenses. This is called the
‘lifting condensation level’.
• This occurs because the water vapor has reached its Dew Point.
• The Dew Point is when the temperature of water vapor condenses into water
droplets and forms a cloud
• This is why a cloud appears to be flat at the base
Why Clouds Form
• Relative Humidity has gone over 100% (supersaturated)
• Cloud Condensation Nuclei (CCN)
• Solid particles suspended in the atmosphere where water vapor
• condenses around to form a cloud
• Particles come from various sources
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Car exhausts
Volcanic activity
Forest fires
Sea salt (ocean spray)
Soil
Dust
Sand
Fossil fuel combustion.
• They attract water droplets above the LCL
• Nearly all cloud droplets or ice particles form around a CCN
Warm Clouds
• Collision and Coalescence
Mechanism
• A warm cloud form when
temperature is above 0⁰C
• Cloud droplets are carried through
the air in air currents within the cloud
itself
• These droplets bump into each other
– This is called a collision
• As droplets collide, they stick
together – This is called Coalescence
• When this occurs, the cloud droplet
get bigger
• When the cloud gets heavy enough,
rain will occur
Cold Clouds
• Bergeron Process• Supercooling
• Water in the liquid state that is below 0⁰C
• Supercooled water will freeze if it touches a
solid object (such as Condensation Nuclei)
• Supersaturation
• When relative humidity is over 100%
• If liquid water is 100%, then ice in
supersaturated (over 100%)
• Ice Crystals cannot coexist with water droplets
• Excess water vapor in the air become ice
crystals, which lowers the relative humidity
near water droplets
• This causes surrounding water droplets to
evaporate into water vapor creating and in turn
creating more ice crystals
• This is how snowflakes are made in clouds
Types of Clouds
• Clouds are classified by their form and height
• Clouds form at different Altitudes
• High Clouds (~6000m)
• Middle Clouds (~4000m)
• Low Clouds (~2000m)
• Forms
• Cirrus- “curl of hair”; High clouds, white, thin
• Cumulus - “a pie”; rounded individual cloud masses ; flat base, dome
structured
• Stratus- “a layer”; look like sheets or layers that cover the sky; no distinct
breaks in-between clouds
Development of a Thunderstorm
• Thunderstorms form when warm, humid air rises in an unstable
environment.
• 3 stages
• Cumulus Stage
• Mature Stage
• Dissipating Stage
• Updraft- Rising air
• Downdraft- sinking air
Stage 1- Cumulus Stage
• Warm, moist air rises, and the water
vapor within the air condenses to form
a cumulus cloud
• Cloud grows above the freezing point in
the atmosphere
• creating both warm and cold cloud
Stage 2- Mature Stage
• Condensation continues as the
cloud rises and becomes a dark
cumulonimbus cloud
(thunderstorm cloud)
• Amount of water held in cloud
become too much for the
updraft (upward movement of
air) to support
• Heavy, torrential rain and hail
storms begin to fall from the
cloud
Stage 3- Dissipating Stage
• Strong downdrafts (downward
movement of air) stop air currents
from rising
• Cooling effect from falling
precipitation and colder air high
above cause the storm to die down
• Life span of a cumulonimbus cloud
within a thunderstorm is only
about an hour
• As storm moves, new warm moist
air generate new clouds which
causes storms to last longer than
an hour