An important influence on climate is mountainous
terrain, especially the long, high chains of mountains
that act as climatic divides. These obstacles deflect the
tracks of cyclones and block the passage of air masses
at the lower levels. If the pressure gradients are strong
enough to force the air masses over the mountains, the
forced ascent and descent modifies the air masses to a
great extent, thus modifying the climate on both the
windward and leeward sides of the range.
The orientation of the mountain range may block
certain air masses and prevent them from reaching the
lee side of the mountains. For example, the Himalayas
and the Alps, which have east-west orientations,
prevent polar air masses from advancing southward.
Therefore, the climates of India and Italy are warmer in
winter than are other locations of the same latitude. The
coastal ranges in North America, running in a
north-south line, prevent the passage of unmodified
maritime air masses to the lee side.
The most noted effect of mountains is the
distribution of precipitation. The precipitation values,
level for level, are much higher on the windward side
than on the leeward side.
In regions where the prevailing circulation flows
against a mountain barrier, the amount of precipitation
increases more or less uniformly with elevation on the
windward side of the range. This steady increase
normally occurs up to elevations of about 10,000 feet.
However, in the trade wind zone (such as at the
Hawaiian Islands), precipitation increases only to about
3,000 feet and then decreases gradually. Even with this
decrease in amount, more rain is received at 6,000 feet
than at sea level.
Another important topographical feature is the
presence of lakes. The lake effect can be notable for
large unfrozen bodies of water. The lee sides of lakes
show considerable diurnal and annual modification in
the form of more moderate temperatures; increased
moisture, clouds, and precipitation; and increased
winds (due to less friction) and land and sea breeze
Ocean currents play a significant role in controlling
the climate of certain regions. Ocean currents transport
heat moving cold polar water equatorward into warmer
waters and moving warm equatorial water poleward
into cooler waters.
Currents are driven by the major wind systems;
therefore, cold southward-moving currents flow along
the west coasts of continents, and warm northward
moving currents flow along the east coasts of
continents. This is true in both hemispheres. Basically,
this results in cooler climates along the west coasts and
warmer climates along the east coasts.
A brief explanation of the effects of ocean currents
is presented here.
Effects on the West Coasts
The northern portions of the west coasts of
continents generally have cool summers and warm
winters. The summers are cool because of the presence
of cold northern waters along their shores. However,
the winters are generally mild because of the transport
of warm ocean waters to these latitudes. For example,
the south and southeast coasts of Alaska and the west
coasts of Canada, Washington, and Oregon have
relatively warm currents flowing along their shores.
These currents are the Aleutian and North Pacific
northward-flowing Kuroshio Current. The currents
flow along the West Side of the Pacific high and bring
warm water into southern Alaska and the Pacific
As these currents merge and flow southward along
the British Columbia coast, they move into warmer
waters and become the cold California Current.
The southern portions of the west coasts of
continents generally have cooler climates than do the
east coasts of the same latitude. For example, during
summer, the cold California Current flows southward
along the shores of California. Due to the Pacific high,
the winds normally flow either across the cold current
toward shore (onshore) or parallel to the coastline. This
results in cool air being advected inland allowing cities
such as San Francisco and Seattle to enjoy relatively
cool summers. Unfortunately, when the warm, moist air
from the Pacific high does move over the underlying
cold current, extensive fog and stratus develop which
also move inland. This situation is typical along the
Another factor affecting west coasts is upwelling.
Upwelling is the process by which cold subsurface
waters are brought to the surface by wind. It occurs in
areas where the wind causes the surface water to be
transported away from the coast. The colder subsurface
water then replaces the surface water. In the Northern