What causes rain?
Cooling the air causes condensation, cloud
formation, and rain - the question is What causes the air to cool?
A. Low pressure systems • We already talked about these!
• Air over an entire region rises and then cools to dew point.
• These storms are large (including hurricanes) and rotate
counterclockwise.
• These systems drift across the globe.
• Most of Arizona’s winter rains are caused this way.
B. Local convection storms • Summer “Monsoons”
• Locally heated air rises rapidly after the sun
comes up.
• Storms usually dissipate after sunset.
Morning & afternoon in mid- to
late summer
C. Fronts -
• When one air mass moves in and replaces another, the
leading edge of the new air mass is a front.
• When cold air replaces warm air, you will have a cold front
(& v.v.)
• Both kinds of fronts bring rain, but in different ways.
D.Orographic (mountain) Rains • When wind encounters a mountain, the air must go up to
go over the mountain.
• As the air is pushed up over the mountain it cools to dew
point.
• When the air descends down the back side of the
mountain, it is heated by compression and the relative
humidity drops to very low.The back side is said to be in a
“rain shadow.”
• In California, the giant redwoods are on the windward
side of the Sierra Nevada, and Death Valley is in the lee.
Orographic Rain & Rain Shadow