air up from the valley. This uphill airflow is called avalley breeze. When the Sun goes
down, the mountain slopes cool more quickly than the air in the nearby valley. This cool air
sinks, which causes amountain breezeto flow downhill.
Katabatic Winds
Katabatic windsalso move up and down slopes, but they are stronger mountain and
valley breezes. Katabatic winds form over a high land area, such as on a high plateau. The
plateau is usually surrounded on almost all sides by mountains. In winter, the plateau grows
cold, making the air above it extremely cold. This dense air sinks down from the plateau
through gaps in the mountains. Wind speeds depend on the difference in air pressure over
the plateau and over the surroundings. If a storm, which has low pressure, forms outside the
plateau, there is a big difference in wind pressure and the winds will race rapidly downslope.
Katabatic winds form over many continental areas. Extremely cold katabatic winds blow
over Antarctica and Greenland.
Foehn Wind (Chinook Winds)
Foehn windsor Chinook windsdevelop when air is forced up over a mountain range.
This takes place, for example, when the westerly winds bring air from the Pacific Ocean over
the Sierra Nevada Mountains in California. As the relatively warm, moist air rises over the
windward side of the mountains, it cools and contracts. If the air is humid, it may form
clouds and drop rain or snow. When the air sinks on the leeward side of the mountains, it
forms a high pressure zone. The windward side of a mountain range is the side that receives
the wind; the leeward side is the side where air sinks.
The descending air warms and creates very strong, dry winds. Foehn winds can raise tem-
peratures more than 20oC (36oF) in an hour and cause humidity to decrease. If there is snow
on the leeward side of the mountain, it may disappear by quickly melting and evaporating in
the dry winds. If precipitation falls as the air rises over the mountains, the air will be very
dry as it sinks on the leeward size of the mountains. This dry, sinking air causes arain-
shadow effect(Figure15.22). Many deserts are found on the leeward side of mountains
due to rainshadow effect.
The name of these winds is a bit confusing. Some people refer to all of these winds as Foehn
winds, others as Chinook winds, and still others as orogenic winds. The names Foehn and
Chinook are sometimes used for any of these types of winds, but are also used regionally.
Foehn winds are found in the European Alps, and Chinook winds are found in the Rocky
MountainsofNorthAmerica. Althoughthedescriptionisapt, Chinookdoesnotmean“snow
eater”.