LOCAL WINDS 
high (the height of the valley) is completely replaced 
in a closed circulation by heated slope air in 41% hr. 
Thus, if we assume the heating to begin at 0830, the 
maximum pressure gradient between plains and valley 
would occur at about 1300. This time would be the 
beginning of the main phase of the valley-wind de- 
velopment. A comparison with the time of the valley- 
wind maximum at Innsbruck (between 1500 and 1600) 
proves satisfactory if we consider the distance traveled 
by the air from the valley entrance to Innsbruck. 
These results confirm the importance of the return of 
the upslope wind to the valley center for the theory of 
the mountain and valley winds, as postulated by Wag- 
ner. 
DISTURBANCES OF THE WIND THROUGH 
OROGRAPHIC INFLUENCES 
The Foehn. In almost all mountain areas, con- 
spicuous local winds can be observed which blow from 
the ridges down into the leeward lowlands. At times 
these winds can assume gale intensity. Their chief 
characteristics, after reaching the plains, are abnormal 
temperature rise and dryness, which are of climatic 
significance over a more or less wide belt in the lee of 
the mountains. These local winds have become known 
under various names; however, today the general term 
foehn is used. 
Although particularly well developed in the European 
Alps, especially in Switzerland and Tirol, such foehn 
winds blow also in Greenland from the inland ice over 
the mountain ranges down to the coast. In North 
America the foehn, known as the chinook, has a climatic 
influence on a very wide belt east of the Rocky Moun- 
tains. In Argentina, this wind is known as the zonda 
and blows down from the Andes. Local winds with 
foehn characteristics are also well known in Japan, 
New Zealand, and in eastern and central Asia. There 
is hardly a mountain range where the foehn is com- 
pletely lacking. After all, the foehn will appear wherever 
prevailing winds must pass over a mountain barrier. 
Such barriers thereby become great divides of weather 
and climate. 
The foehn is also noted for characteristic weather 
elements other than high temperature and low hu- 
midity. There is always extraordinarily good visibility; 
the mountains appear unnaturally close and clear and 
assume steel-blue to purplish hues. The clouds as- 
sociated with the foehn are lens-shaped (lenticularis) at 
medium altitudes, and elongated banks often suggest 
wave formation. The peaks of the mountains and the 
upper part of the windward slope are shrouded in 
rather flat, cumuliform clouds. These clouds, the so- 
called ‘“foehn wall,” are in a continuous process of 
forming and dissolving, because they remain stationary 
in spite of strong winds. 
The foehn is a gusty wind, and temperature and 
humidity curves therefore always show irregularities to 
a varying degree during a foehn. In the Alps the foehn 
is always strongest where north-south valleys open 
into the plains or into large east-west cross valleys. 
This latter form is characteristic, for instance, at Inns- 
667 
bruck. It was there that the investigation of this phe- 
nomenon was most intensively pursued (see, for exam- 
ple, the work by Trabert, v. Ficker, A. Defant, Ekhart, 
and Pernter [14, 20, 24, 26, 60)). 
During the winter, the great increase in temperature 
causes a rapid melting of the snow. However, floods 
seldom occur because the melting is very localized and 
decreases rapidly with altitude. Also, during a foehn, 
evaporation is very rapid because of the low relative 
humidity, and precipitation is confined to the passes, 
the peaks, and the windward side of the mountains. 
This distribution of precipitation is the chief character- 
istic of the foehn and has decisive climatic consequences 
for the areas on both sides of the range. 
The thermodynamic explanation of the foehn is es- 
sentially due to Hann [32]. Since the theory belongs to 
the basic principles of theoretical meteorology, the 
reader is referred to pertinent textbooks. In general, 
it can be said that observations are in good agreement 
with Hann’s theory: On the windward side, cloud 
formation (particularly the stationary foehn wall) and 
precipitation must occur at a certain altitude as a 
result of the condensation. This removal of moisture 
together with the compression of the air during its 
descent on the lee must cause dissolution of clouds, 
warmth, and dryness there. 
When air flows across a mountain range it is sub- 
jected to the above-mentioned foehn processes, as ex- 
plained by Hann, and exhibits foehn characteristics on 
the lee side of the mountain. Such an air flow normal 
to the mountain ridge can be maintained only by a 
pressure gradient that is parallel to the ridge. Accord- 
ingly, such an air flow exists only when the general 
weather situation has a very definite pressure pattern, 
as shown schematically in Fig. 13. The sinusoidal de- 
formation of the isobars, characteristically produced 
by the thermal pressure effect, forms a bulge of the 
isobars toward the high pressure on the lee side of the 
mountains and toward the low pressure on the wind- 
ward side (often called the ‘‘foehn nose’’). 
Depending on the orientation of the mountains, the 
air currents, after passing through the thermodynamic 
process, show the foehn phenomena to a greater or 
lesser extent. In the case of mountain ranges oriented 
from west to east (e.g., the European Alps or the 
Pyrenees) a south wind will blow across the mountains 
if the low pressure is to the west and the high pressure 
to the east (see Fig. 13a). Because of its original warmth, 
the air from the south is well suited to the development 
of very marked foehn phenomena. For this reason, the 
south foehn is the most striking type of foehn from the 
viewpoint of the meteorologist as well as of the layman 
(see v. Ficker [28, pp. 25-37]). If low pressure lies to 
the east and high pressure to the west, air from the 
north is transported across a mountain range oriented 
west-east (see Fig. 13b). The cold air piles up to the top 
of the mountains and then descends on the lee side 
while undergoing the thermodynamic process. This air, 
however, exhibits foehn properties only when the foehn 
process changes its cold-air character to such an extent 
that, upon arriving on the lee side of the mountains, it 
