Bernoullis theorem is frequently used to forecast
tertiary winds in the mountainous western United
States. The famous Santa Ana winds of southern
California are a prime example. Winds associated with
high pressure situated over Utah are funneled through
the valley leading into the town of Santa Ana near the
California coast. Low pressure develops at the mouth of
the valley and the end result is hot, dry, gusty and
extremely dangerous winds. When the Santa Ana is
strong enough, the effects are felt in virtually every
valley located along the coast of southern California.
Visibility is often restricted due to blowing sand. It is
common to see campers, trailers, and trucks turned over
by the force of these winds. When funneled winds
reach this magnitude, they are called jet-effect winds,
canyon winds, or mountain-gap winds.
Winds Due to Local Cooling and Heating
There are two types of tertiary circulations
produced by local coolingglacier winds and drainage
GLACIER WINDS.Glacier winds, or fall
winds (as they are sometimes called) occur in many
varieties in all parts of the world where there are
glaciers or elevated land masses that become covered
by snow and ice during winter. During winter, the area
of snow cover becomes most extensive. Weak pressure
Consequently the air coming in contact with the cold
snow cools. The cooling effect makes the overlying air
more dense, therefore, heavier than the surrounding air.
When set in motion, the cold dense air flows down the
sides of the glacier or plateau. If it is funneled through a
pass or valley, it may become very strong. This type of
wind may form during the day or night due to radiation
cooling. The glacier wind is most common during the
winter when more snow and ice are present.
When a changing pressure gradient moves a large
cold air mass over the edge of a plateau, this action sets
in motion the strongest, most persistent, and most
extensive of the glacier or fall winds. When this
happens, the fall velocity is added to the pressure
gradient force causing the cold air to rush down to sea
level along a front that may extend for hundreds of
miles. This condition occurs in winter on a large scale
along the edge of the Greenland icecap. In some places
along the icecap, the wind attains a velocity in excess of
90 knots for days at a time and reaches more than 150
nautical miles out to sea.
Glacier winds are cold katabatic (downhill) winds.
Since all katabatic winds are heated adiabatically in
their descent, they are predominantly dry. Occasionally,
the glacier winds pick up moisture from falling
precipitation when they underride warm air. Even with
the adiabatic heating they undergo, all glacier or fall
winds are essentially cold winds because of the extreme
coldness of the air in their source region. Contrary to all
other descending winds that are warm and dry, the
glacier wind is cold and dry. It is colder, level for level,
than the air mass it is displacing. In the Northern
Hemisphere, the glacier winds descend frequently from
the snow-covered plateaus and glaciers of Alaska,
Canada, Greenland, and Norway.
DRAINAGE WINDS.Drainage winds (also
called mountain or gravity winds) are caused by the
Consequently, the air becomes heavy and flows
downhill, producing the MOUNTAIN BREEZE.
Drainage winds are katabatic winds and like glacier
winds; a weak or nonexistent pressure gradient is
required to start the downward flow. As the air near the
top of a mountain cools through radiation or contact
with colder surfaces, it becomes heavier than the
surrounding air and gradually flows downward (fig.
3-27). Initially this flow is light (2 to 4 knots) and only a
few feet thick. As cooling continues, the flow increases
achieving speeds up to 15 knots at the base of the
mountain and a depth of 200 feet or more. Winds in
excess of 15 knots are rare and only occur when the
mountain breeze is severely funneled.
Drainage winds are cold and dry. Adiabatic heating
does not sufficiently heat the descending air because of
the relative coldness of the initial air and because the
distance traveled by the air is normally short. Drainage
winds have a very localized circulation. As the cold air
enters the valley below, it displaces the warm air.
Temperatures continue to fall. If the flow achieves
speeds of 8 knots or more, mixing results between the
warm valley air and the cold descending air that results
in a slight temperature increase. Campers often prefer
to make summer camps at the base of mountains to take
advantage of the cooling effect of the mountain breeze.
VALLEY BREEZES.The valley breeze is the
anabatic (uphill) counterpart of the mountain breeze.
When the sun heats the valley walls and mountain
slopes during the morning hours, the air next to the
ground is heated until it rises along the slopes. Rocky or