AEROGRAPHER'S MATE 3 & 2 



dewpoint. Condensation occurs, and a cloud forms 

 above the mountain with possible precipitation 

 on the windward side of the mountain. During 

 the descent of the air on the leeward side of 

 the mountain, heating takes place, due to com- 

 pression, again at the dry adiabatic lapse rate. 

 These winds are characteristic of the Alps, 

 and also of the Rockies, where they are known 

 as Chinook winds. 



FUNNEL EFFECT 



Winds blowing against mountain barriers tend 

 to flatten out and go around them. If the barrier 

 is broken by a pass or a valley, the air is 

 forced through the break with considerable speed. 

 Such forcing of wind through narrow valleys 

 is known as the funnel effect. A good example 

 of the funnel effect is the Santa Ana wind of 

 Southern California. This type of wind is dis- 

 cussed in chapter 12 in the section entitled 

 "Bernoulli's Theorem." 



GLACIER WINDS 



Glacier winds, or fall winds as they are 

 sometimes called, occur in a great variety 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, 

 permitting a maximum amount of radiational 

 cooling and consequently cooling of the sur- 

 rounding air coming in contact with the cold 

 surfaces. This cooling effect makes the air 

 denser, therefore heavier than the surrounding 

 air. When this air is set in motion, it flows 

 down the sides of the glacier or plateau. Drainage 

 winds of this variety cover a wide range — 

 from light local breezes that descend through 

 the valleys from small isolated glaciers to the 

 violent outbreaks of cold air that rush down the 

 lee side of a continental ice plateau. The latter 

 is caused by the development of a gradient 

 wind that sets in motion the reservoir of extremely 

 cold air from the high-level snowfields. These 

 winds in many areas occur as violent squalls 

 of short duration when a mass of cold air is 

 released over the edge of a cold plateau and 

 plunges down through an adjacent valley or 

 fjord to sea level. 



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 so that 

 the cold air rushes down to sea level along a 

 front which may extend for hundreds of miles. 

 This condition occurs in winter on a large scale 

 along the edge of the Greenland icecap, where 

 in some places the wind attains a velocity in 

 excess of 90 knots for days at a time and 

 reaches more than 150 miles out to sea. 



Since all of the drainage winds are heated 

 adiabatically in their descent, they are pre- 

 dominantly dry. Occasionally, the glacier winds 

 pick up moisture by falling precipitation when 

 they underride warm air. All glacier or fall 

 winds are essentially cold winds, even with the 

 adiabatic heating which they undergo, because of 

 the extreme coldness of the air in its source 

 region. Contrary to all other descending winds 

 which 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. 



EDDIES AND TURBULENCE 



Turbulence is the irregular motion of the 

 atmosphere when the air flows over an uneven 

 surface, or when two currents of air flow past 

 each other in different directions or at different 

 speeds. The main source of turbulence is the 

 friction along the surface of the earth. This is 

 called mechanical turbulence. Turbulence is also 

 caused by irregular temperature distribution. 

 The warmer air rises and the colder air 

 descends, causing an irregular vertical motion 

 of air; this is called thermal turbulence. 



Mechanical turbulence is intensified in un- 

 stable air and is weakened in stable air. These 

 influences cause fluctuations in the wind with 

 periods ranging from a few minutes to more 

 than an hour. If these wind variations are 

 strong, they are called wind squalls and are 

 usually associated with convective-type clouds. 

 They are an indication of approaching towering 

 cumulus or cumulonimbus clouds. 



Gustiness and turbulence are more or less 

 synonymous. Gustiness is the irregularity in 

 the wind speed which creates eddy currents 

 disrupting the smooth air flow. Thus, the term 



310 



