668 
is markedly warmer and drier than the air that was 
there before. This type of foehn, called north foehn 
[25; 40; 29, pp. 48-53], is relatively rare. 
LOCAL CIRCULATIONS 
lies in the valleys and in most cases shows up as a 
sharp inversion and decrease in humidity. The descend- 
ing motion is very slow and must be ascribed to a 
Fic. 13.—Windward and leeward effects of mountains on the isobaric pattern at sea level (schematic): (a) south foehn in the 
European Alps, (6) north foehn in the European Alps, and (c) southeast current over the Scandinavian mountains. 
In the case of a north-south orientation of a moun- 
tain range, the development of a foehn requires low 
pressure to the north and high pressure to the south, or 
vice versa. Then, a west-east current passes across the 
mountains (e.g., the Rocky Mountains, the Andes, 
or those of Greenland, New Zealand, and Scandinavia) 
(see Fig. 13c). On the lee side, considerable tempera- 
ture differences in the meridional direction occur; for 
example, in western Canada the broad warm foehn 
current, after passing the Rocky Mountains, meets the 
extraordinarily cold Canadian polar air. The result is 
increased cyclogenesis or rapid deepening of existing 
cyclones, which must be considered a consequence of 
the foehn process. This phenomenon, which is to a 
much lesser extent associated with the south or north 
foehn, has perhaps an analogy in the formation of the 
Genoa cyclone south of the Alps and the Apennines. 
The thermal pressure effects during foehn cause in 
all cases a great increase of the horizontal pressure 
gradient in the direction of the mountaims (as much as 
9 mb per 100 km at sea level or at the level of the 
valleys). The mountain barrier prevents the transfor- 
mation of such gradients into air motion; or, in reverse, 
the establishment of such gradients is made possible 
only by the mountain barrier. Although this gradient 
is less strong at the level of the mountain ridges, it 
cannot be explained by the thermal contrast alone. 
The factors determining the various types of foehn 
are the general synoptic situation, the orientation of 
the mountain ranges, and the type of air masses passing 
over the mountains. In turn, the thermodynamic foehn 
process causes changes in the pressure field and in the 
properties of the air masses involved, which affect the 
general weather situation. 
The warming and drying of air, observed in anti- 
cyclones, bear great similarity to the foehn and are 
caused by the dynamic heating during the descent of 
air from aloft. As with foehn, clouds dissolve and fair 
weather without precipitation sets in. In these cases, 
warm air is always present aloft as is evident from 
observations at mountain stations or from aerological 
soundings. This warm air rests on the cold air that 
divergent flow at the surface, directed away from the 
mountains. This phenomenon is called high foehn or 
free foehn in the literature [28, pp. 53-58]. Since an 
anticyclonic situation usually precedes the south foehn, 
the free foehn over the mountains often forebodes the 
subsequent development of a foehn current. 
The warm foehn current on the lee of the mountains 
descends into the lowlands rather than ascends as one 
would expect. The cause of this descent into the valleys 
is a meteorological problem. Following older concepts, 
v. Ficker [28, pp. 34-37] has answered this question by 
resolving the foehn development into several phases 
(preliminary phase, anticyclonic phase, stationary foehn 
phase). A period of foehn is generally preceded by an 
anticyclonic weather situation as the preliminary phase, 
with very stable vertical temperature distribution. Cold 
air lies in the valleys, with dry warm air above it and 
separated from it by an anticyclonic subsidence inver- 
sion. Before the onset of the foehn, the cold air moves 
out of the valleys and away from the mountains under 
the influence of the pressure distribution. The upper 
boundary of the cold air is thereby lowered, and warm 
air from aloft supplants it. When this warm air reaches 
a station, the anticyclonic phase sets in for this station. 
The temperature rise and humidity drop connected 
with very little air motion indicate the anticyclonic 
foehn. Thus, the warm air does not actually break 
through the cold air to the valley floor, but follows the 
cold air which must first flow out of the valley into 
the plains while its upper boundary subsides. Only 
when the foehn wall forms on the windward side of the 
mountains and the horizontal pressure gradient is in- 
creased in the direction of the mountains, does the 
stationary foehn phase set in. In this phase, the foehn 
proceeds as explained by Hann. 
The foehn does not always reach the valley floor 
aS a warm current. If a remnant of shallow cold air 
remains in the lowlands, the foehn current moves over 
it. Occasionally, some cold air remains in certain parts 
of the valley, and the warm air reaches the valley floor 
only at certain foehn islands which are of great climatic 
significance. Also at night, when the intensity of the 
