Important Terms • Humidity: The amount of water vapor in the air • Humidity can be measured two ways • Relative Humidity: how much water vapor the air is holding, expressed in percent form. Ex., the humidity is at 60% • 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 Formation • When air expands, it cools • When it compresses, it warms • If air cools enough it hits the dew point • Now the biggie: • When air rises it expands and cools • When air falls it compresses and heats
Dry 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 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 Lifting 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 Wedging 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 different directions collide. They can’t go down so the air goes up - fast
#4 Convective Lifting 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 around. • Condensation nuclei can be dust, smoke, salt particles from the ocean, etc. • Clouds are not vapor but actually water or ice particles