How Clouds FormHow Clouds Form

Important TermsImportant 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 vapor

• At the dew point, water vapor condenses (becomes a liquid) and can form clouds, dew, fog, etc.

Cloud FormationCloud 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 coolso When air falls it compresses and heats

Dry Adiabatic RateDry Adiabatic Rate• When dry air rises it

cools • For every 1000 meters

you go up, air will cool 10 deg C and vice versa

• This is called the Dry Adiabatic Rate

Wet Adiabatic RateWet 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?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?How Do You Lift Air?• Lifting cools air so clouds are made• There are four ways to lift clouds

o 1. Orographic Liftingo 2. Frontal Wedgingo 3. Convergenceo 4. Convective Lifting

# 1Orographic # 1Orographic LiftingLifting

•Air runs into a

mountain a rises

high up one side

•It rains a lot on

one side and is

very dry on the

other.

#2 Frontal #2 Frontal WedgingWedging

•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 #3 ConvergenceConvergence

•Two winds

coming from two

different

directions collide.

•They can’t go

down so the air

goes up - fast

#4 Convective #4 Convective LiftingLifting

•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 ThoughtsLast Thoughts• Clouds only form at the dew point• They also need condensation nuclei to form

around.• Condensation nuclei can be dust, smoke, salt

particles from the ocean, etc.• Clouds are not vapor but actually water or ice

particles