Due to its large size and wide range of geographic features, the United States
contains every climate. The main influence on U.S. weather is the polar jet stream,
which brings in large low pressure systems from the northern Pacific Ocean. The
Cascade Range, Sierra Nevada, and Rocky Mountains pick up most of the moisture
from these systems as they move eastward. Greatly diminished by the time they
reach the High Plains, much of the moisture has been sapped by the orographic
effect as it is forced over several mountain ranges. However, once it moves over
the Great Plains, uninterrupted flat land allows it to reorganize and can lead
to major clashes of air masses. In addition, moisture from the Gulf of Mexico
is often drawn northward. When combined with a powerful jet stream, this can
lead to violent thunderstorms, especially during spring and summer. Sometimes
during late winter and spring these storms can combine with another low pressure
system as they move up the East Coast and into the Atlantic Ocean, where they
intensify rapidly. These storms are known as Nor'easters and often bring widespread,
heavy snowfall to the Mid-Atlantic and New England. The uninterrupted flat grasslands
of the Great Plains also lead to some of the most extreme climate swings in the
world. Temperatures can rise or drop rapidly and winds can be extreme, and the
flow of heat waves or arctic air masses often advance uninterrupted through the
plains.
The Great Basin and Columbia Plateau (the Intermountain Plateaus) are arid or
semiarid regions that lie in the rain shadow of the Cascades and Sierra Nevada.
Precipitation averages less than 15 inches (38 cm). The Southwest is a hot desert,
with temperatures exceeding 100°F (38°C) for several weeks at a time
in summer. The Southwest and the Great Basin are also affected by the monsoon
from the Gulf of California from July-September, which brings localized but often
severe thunderstorms to the region. Much of California consists of a Mediterranean
climate, with sometimes excessive rainfall from October-April and nearly no rain
the rest of the year. In the Pacific Northwest rain falls year-round, but is
much heavier during winter and spring. The mountains of the west receive abundant
precipitation and very heavy snowfall. The Cascades are one of the snowiest places
in the world, with some places averaging over 600 inches (1,520 cm) of snow annually,
but the lower elevations closer to the coast receive very little snow. Another
significant (but localized) weather effect is lake-effect snow that falls south
and east of the Great Lakes, especially in the hilly portions of the Upper Peninsula
of Michigan and on the Tug Hill Plateau in New York. The Wasatch Front and Wasatch
Range in Utah can also receive significant lake effect accumulations off of the
Great Salt Lake.
The climate is mostly temperate, but tropical in Hawaii and Florida, arctic in
Alaska, semiarid in the Great Plains west of the Mississippi River, Mediterranean
in coastal California, and arid in the Great Basin of the southwest; low winter
temperatures in the northwest are ameliorated occasionally in January and February
by warm Chinook winds from the eastern slopes of the Rocky Mountains.
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