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Winds blowing against mountain barriers tend to flatten out and go around or over them. If the barrier is broken by a pass or a valley, the air is forced through the break at considerable speed. When wind is forced through narrow valleys it is known as the funnel effect and is explained by Bernoulli’s theorem.

According to Bernoulli’s theorem, pressures are least where velocities are greatest; likewise, pressures are greatest where velocities are least. This observation is true for both liquids and gases. (See fig. 3-3-4.)

Bernoulli’s 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

There are two types of tertiary circulations produced by local cooling—glacier winds and drainage winds.

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 ex-tensive. Weak pressure results in a maximum of radiational cooling. 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 radiational 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 per-sistent, 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

Figure 3-3-4.—Strong wind produced by funneling.

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.

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