dew point

How Clouds Form
Important Terms
• Humidity: The amount of water vapor in the air
• Humidity can be measured two ways
o Relative Humidity: how much water vapor the air is holding, expressed in
percent form. Ex., the humidity is at 60%
o Dew Point: The temperature of air when it is at 100% saturation with water
• At the dew point, water vapor condenses
(becomes a liquid) and can form clouds, dew, fog,
Cloud Formation
When air expands, it cools
When it compresses, it warms
If air cools enough it hits the dew point
Now the biggie:
o When air rises it expands and cools
o When air falls it compresses and heats
Dry Adiabatic Rate
• When dry air rises it
• For every 1000 meters
you go up, air will cool
10 deg C and vice
• This is called the Dry
Adiabatic Rate
Wet Adiabatic Rate
• If air rises high enough it
will hit the dew point
• If it continues to rise the
air will cool at only 5-9
deg C for every 1000
meters and vice versa
• This is called the wet
adiabatic rate
What Does it Mean?
• If you raise air high
enough it will cool until
it hits the dew point
• At that point you will
start making a cloud
• So the next question
is…how do you get the
air to rise?
How Do You Lift Air?
• Lifting cools air so clouds are made
• There are four ways to lift clouds
1. Orographic Lifting
2. Frontal Wedging
3. Convergence
4. Convective Lifting
# 1Orographic
•Air runs into a
mountain a rises
high up one side
•It rains a lot on
one side and is
very dry on the
#2 Frontal
•A mass of cold air
collides with a mass
of warm air
•They are like huge
armies meeting at
the front.
•The warm air rides
up and high over
the cold air.
#3 Convergence
•Two winds
coming from two
directions collide.
•They can’t go
down so the air
goes up - fast
#4 Convective
•Hot spots on the
land, like a huge
parking lot can
cause bubbles of
air to rise.
•These bubbles
are called
thermals and can
lead to big clouds.
Last Thoughts
• Clouds only form at the dew point
• They also need condensation nuclei to form
• Condensation nuclei can be dust, smoke, salt
particles from the ocean, etc.
• Clouds are not vapor but actually water or ice