Clouds and
Cloud
Formation
What is a cloud?
• A cloud is a collection of small water droplets
or ice crystals suspended in the air.
• They are visible because the water droplets
and ice crystals reflect light
• Clouds don’t always mean precipitation
Cloud Formation
• Clouds form when water vapor in the air
condenses
• For Condensation:
– Air must be cooled to its Dew Point
– There must be a solid particle for the water
vapor to condense on
Process
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The sun warms a water source
Evaporation occurs creating warm, moist air
As the warm air rises, it cools
When it cools to its Dew Point and below, the
water vapor in the air condenses onto tiny
particles called Cloud Condensation Nuclei
• Cloud condensation nuclei can be smoke,
dust, dirt, salt…
• Most clouds are composed of water droplets,
but very high altitude clouds are composed of
ice crystals
How do clouds form?
DEW POINT – Condensation Occurs on Nuclei
Air cools as it rises
Radiation
Warm, moist air rises
Water Source
Evaporation
Frontal Lifting
Orographic Lifting
Prevailing winds
Warm, moist air
rises up slope
3 classes of clouds based on shape:
• Stratus Clouds– thin and flat with undefined edges
– Stratus means “layer”
– Often gray
– Light mist or drizzle may come from stratus clouds
– Fog is a type of stratus cloud that forms near the
ground
• Cumulus Clouds– Thick and puffy on top, flat on bottom
– Have well defined edges and change shape rapidly
– In fair-weather, they are bright and white
– They can become dark and bring thunder and
lightening and heavy rain
– Cumulus means “heap”
• Cirrus Clouds– Appear feathery and their ends curl
– They form high in the atmosphere where
temperatures are very cold
– They are made of ice crystals
– Do not produce precipitation that reaches the
Earth
Fog
• Forms when moist air near the ground cools to its
dew point
• Ground Fog (Radiation Fog)- forms in low-lying
areas on clear calm nights; Earth’s surface cools and
moist air near the ground reaches its dew point
• Sea Fog (Advection Fog)- warm moist air moves
over colder water and cools to its dew point
• Steam Fog- evaporation takes place into cold air
from warmer water; the air nearest the water is
warmer, and as it rises, it cools to its dew point;
seen a lot in the fall as the water cools more slowly
than the air