822 



SCIENCE 



[N. S. Vol. XL. No. 1040 



NOTES ON METEOnOLOGY AND 



CLIMATOLOGY 



MOUNTAINS AND WINDS 



A RECENT paper by Th. Hesselberg and H. 

 TJ. Sverdrup, entitled, "tJber den Einfluss der 

 Gebirge auf die Luftbewegung langs der 

 Erdoberflache und auf die Druckverteilung," ^ 

 includes studies of the Appalachians, Alps 

 and Apennines as influencing winds. For the 

 United States 51 weather maps in 1906 were 

 chosen for study. The winds of the Appa- 

 lachian region exhibit three simple types of 

 influence. (1) With a general flow of air from 

 the northwest there is divergence on the wind- 

 ward side and convergence on the leeward 

 side of the mountains. (2) With a southeast 

 wind there is the same general flow of air 

 around the mountain mass but locally there 

 are converging winds on the windward side. 

 This is said to indicate an air whirl on a hori- 

 zontal axis there. (3) When there is a general 

 southwest wind parallel with the mountain 

 chain, the winds over the mountains are 

 usually across them from the west. But when 

 the flow is from the northeast, there are no 

 such corresponding winds crossing the moun- 

 tains. These cross west winds are apparently 

 due to the influence of the upper prevailing 

 westerlies. 



Stronger and more complex, mountain in- 

 fluence on crosswise and lengthwise winds 

 was found in Europe, both because of the 

 greater altitude of the mountains and because 

 of more observing stations. With strong 

 cross-mountain flow of air the atmospheric 

 pressure is raised to windward and lowered to 

 leeward, in addition to changes wrought in 

 the wind direction. Cool, wet weather to 

 windward and warm dry weather to leeward 

 followed as a result of the dynamic changes 

 in temperature experienced by the wind in 

 crossing the mountains. In winter locally the 

 mountains become high-pressure, divergence 

 points; while over the valleys low pressure 

 and convergence is the rule. In summer the 

 weather maps indicate a partial reversal of 

 these conditions, since frequently by the time 



1 VerofE. des Geophys. Inst. d. Univ. Leipzig, 

 2te Serie, heft 4, Leipzig, 1914, pp. 102-116, 2 pi. 



the observations are taken, the diurnal heat- 

 ing of the valleys has already started the up- 

 valley breezes. Thus the wind directions in 

 and about a mountain region are strongly 

 controlled by the presence of the mountains. 



THE INFLUENCE OF METEOROLOGICAL CONDITIONS 

 ON THE PROPAGATION OF SOUND 



This study by Dr. H. Bateman^ includes 

 the effects of wind, temperature, moisture and 

 air composition on the propagation of sound. 

 The general influence of any wind is to re- 

 duce the audibility of sounds. The usual 

 greater range of a sound with the wind than 

 against it is ascribed to the increase of wind 

 velocity with altitude, which bends upward 

 the sound waves traveling against the wind 

 and downward those going vsdth the wind. 

 On the other hand, if the upper wind is oppo- 

 site to the lower one, a sound refracted up- 

 ward in traveling against the surface wind 

 may be bent to the earth again on entering 

 the other current. This may account for the 

 peculiar regions of silence and sound often 

 observed in easterly surface winds. If the 

 transition from one air current to another is 

 sharp, the boundary may become a reflecting 

 surface. 



The temperature effect on sound is much 

 like that of the wind. In the daytime, the 

 normal vertical decrease in temperature leads 

 to refraction of sound waves upward, and the 

 lack of thermal homogeneity of the air aids 

 in dispersing sounds. At night or in cloudy 

 weather, when the temperature is more uni- 

 form, sounds are more easily heard. Appar- 

 ently the lower surface of the stratosphere 

 acts as a reflecting surface which returns to 

 earth heavy sounds such as from artillery fire 

 or volcanic explosions, making spots where 

 such sounds are heard far beyond the limits 

 of direct audibility. 



Sounds entering moist masses of air are 

 weakened by " stifling," refraction, scatter- 

 ing and perhaps reflection. Eog usually pro- 

 duces peculiar sound effects, probably on ac- 

 count of temperature differences in the fog 



2 Monthly Weather Review, May, 1914, pp. 258- 

 265, 1 fig. 



