Altitude and Mountain Ranges
All else being equal, air temperature decreases at higher altitudes. A town at 3000 meters
in the mountains will be much cooler, on average, than a town at the base of the same
mountains. Gravity pulls air molecules closer together at sea level than at higher altitudes.
The closer molecules are packed together, the more likely they are to collide. Collisions
between molecules give off heat, which warms the air. At higher altitudes, the air is less
dense and air molecules are more spread out and less likely to collide.
Mountain ranges have two effects on the climate of the surrounding region. One is the
rainshadow effect. As moist air rises over a mountain, it cools and drops precipitation. The
air then descends down the leeward side of the range. This process creates a high pressure
region on the back side of the mountain where evaporation exceeds precipitation. The result
is that the windward side of the mountain range is wet but the leeward side is dry. The
other effect occurs when the mountain range separates the coastal region from the rest of the
continent. In this case, the ocean can only influence the coastal area before the mountain
range. The coastal area near the ocean will have a maritime climate but just over the
mountain, the inland area will have a continental climate.
California has two mountain ranges that exhibit both effects on climate: the Coast Range
right along the coast and the Sierra Nevada, further east. The predominant winds here
are the westerlies, winds that blow from the west over the ocean onto the continent. Both
ranges trap cool air from the Pacific so that it has a difficult time moving eastward. As
this moist ocean air rises over the Coast Range, it drops a lot of rain on the windward side.
A rainshadow is created as the air then descends into the Central Valley, some of which
receives so little rainfall it is classified as a desert. As the air continues eastward, it rises
over the Sierra Nevada Mountains and drops more rain and snow on the west side of these
mountains. On the leeward side, the air then descends into Nevada. The Sierra Nevada rain
shadow creates the Great Basin desert, covering Nevada, western Utah and a small part of
southeastern Oregon (Figure17.6).
Lesson Summary
- Many factors influence the climate of a region, all of them somehow related to the
region’s position. - Latitude determines how much solar energy a particular place receives during a day or
a year. - Latitude is directly related to location within one of the global wind belts, therefore
latitude determines if a location is beneath a high or low pressure cell, where winds
are low. - If a region is near a large water body, its climate will be influenced by that water body.
- Mountain ranges can separate land areas from the oceans and can create rainshadow
effects, which also influence climate.