Geoscience Reference
In-Depth Information
f igu r e 3 . Monument Valley, southern Utah. (Photo by B. Lynn Ingram.)
water and energy with the atmosphere. h e atmosphere receives moisture
from the ocean through the process of evaporation, and some of that mois-
ture is eventually carried away from the ocean and delivered on the conti-
nent as rain or snow. Just how and when that evaporated seawater drops as
rain or snow over the West depends on the patterns of ocean currents and
atmospheric winds and on the interaction of moisture-laden air with the
landscape as it advances inland.
Weather satellite images of the West Coast vividly show the movement
of the atmosphere over the Pacii c Ocean: large, counterclockwise swirls
of white, moisture-laden air that drit toward the coast and i nally over
the land. h e topography of this region, especially the high mountain
ranges, disrupts this l ow of air, forcing it to rise up the slopes. As the air
moves to higher altitudes, the moisture condenses, because rising cooling
air cannot hold as much water vapor. h e major mountain chains of the
West—the Cascades in the northwest, the Sierra Nevada in California,
and the Rockies farther east—are oriented north-to-south or northwest-
to-southeast, which are the ideal orientations for intercepting the Pacii c
winter storms that l ow from west to east. As these storms are forced up
and over these mountain chains, much of the moisture is wrung out on
