LOCAL WINDS 
tains, well known to every mountain climber. These 
winds start one fourth to three fourths of an hour 
after sunrise, and blow uphill in daytime. They 
COMPENSATION 
CURRENT 
SLOPE WIND 
=> MOUNTAIN WIND SYSTEM 
=» GENERAL WIND SYSTEM 
--- NEUTRAL LAYER 
Fie. 7—Schematic illustration of the air circulation dur- 
ing daytime in a cross section through the Alps. (After Burger 
and Ekhart [12].) 
reach their greatest intensity at the time of maximum 
insolation and reverse their direction in the evening 
(about one fourth to three fourths of an hour after sun- 
set). Because of the stronger insolation, they are es- 
pecially well developed on the southern slopes and are 
weaker or almost nonexistent on the northern slopes. 
This wind prefers the ravines and gullies of the usually 
eroded slopes and is hardly noticeable on the projecting 
ridges. Numerous pilot-balloon observations in the up- 
and-down drafts on the slopes of the mountains north 
of the Inn valley near Innsbruck [48-45, 65] have 
clearly demonstrated the existence of such currents. 
Here the thickness of the slope wind layer, as meas- 
ured perpendicular to the slope, varies periodically with 
the wind intensity. Maximum values up to 260 m have 
been measured. However, as a rule, the thickness of the 
layer lies between 100 and 200 m. The thickness is less 
for the nocturnal downslope wind. The uphill wind 
continuously entrains, along its path, air from the 
space over the valley, so that the thickness of the 
affected layer is steadily increased in the direction of 
the uphill flow, and the layer becomes wedge-shaped. 
Naturally, the intensity of these slope winds varies 
greatly with the local differences in the slope and its 
exposure. Also, these winds can seldom be observed in 
their pure form and are often weakened or strengthened 
by extraneous wind conditions. On the average, the 
slope-wind speeds in the direction of the slope amount 
to about 2-4 m sec , according to measurements. 
Projecting parts of the slope cause a detachment of this 
current from the slope and thereby an increased vertical 
movement which can be utilized by soaring pilots to 
gain altitude. In fact, the entire phenomenon of thermal 
slope winds is of greatest importance in soaring. The 
existence of thermal slope winds is often indicated by 
isolated cumulus clouds over summits or chainlike cu- 
mulus formations along ridges. Velocities of 13 m sec ~ 
have been measured in these updrafts. The nocturnal 
downslope wind shows a lesser vertical extent and 
lower velocities. 
The highest velocity of these slope winds does not 
occur close to the slope surface, but at a definite dis- 
tance from it. Higher up, it rapidly decreases again or is 
supplanted by other wind conditions. The current is 
extremely sensitive to changes in the insolation, and a 
663 
temporary shading of the slopes will cause an im- . 
mediate response by the wind. Also the difference in the 
starting time of the current is determined by the vary- 
ing time at which insolation begins on slopes of different 
exposure. Thus, the phenomenon is often weakened at 
the time of the maximum temperature because of shad- 
ows cast on the slopes by extensive cloud formations. 
Little is known about the thermal structure of the 
affected layer, because temperature measurements nor- 
mal to the slope would be necessary for its actual 
determination. Such measurements have been made 
only up to about 20 m above the slope, since greater 
heights are difficult to reach. It is known that the layer 
of air adjacent to the slope shows strong superadiabatic 
gradients. During the cool downdraft at night, on the 
other hand, a strong increase of potential temperature 
(inversion) is often noticeable. The time span during 
the reversal of the wind is characterized by an iso- 
thermal air layer near the ground. At present, we have 
only a theoretical picture of the structure of the entire 
slope-wind layer, which, however, is probably very 
close to reality (see Figs. 11 and 12). 
A variant of the thermal slope wind is the glacier 
wind. This wind, a shallow downdraft along the icy 
surface of the glacier, continues all day regardless of 
insolation. Its thermal cause is the continuously present 
temperature difference between the glacier surface and 
the free air at the same altitude. For this reason, the 
glacier wind has no diurnal period as does the slope 
wind. Since the wind is a function of this temperature 
difference, it reaches its maximum intensity and greatest 
vertical extent (between 50 m and 400 m) in the early 
afternoon, according to measurements by Tollner [69] 
and Ekhart [21]. The glacier wind always appears, even 
on cloudy days. In daytime, it fades out soon after 
leaving the glacier because its kinetic energy is dis- 
persed by the ground friction. The glacier wind often 
collides with the upvalley wind and then slides under 
it. At night, after leaving the glacier, it blends into the 
mountain wind which has the same direction. A char- 
acteristic of the glacier wind is its strongly turbulent 
‘flow which, in a more moderate form, is also a feature 
of the nocturnal slope wind. 
The Mountain and Valley Winds. The phenomenon 
of the daily wind change along the axis of large valleys 
is known in all mountainous countries. In daytime, 
from about 0900 to 1000 until sunset, an upvalley, or 
so-called valley wind blows. At night an opposite down- 
valley or so-called mountain wind appears which con- 
tinues into the early morning hours after sunrise. Nu- 
merous investigations of this phenomenon, including 
soundings of its vertical structure, have been made in 
many mountainous countries [19]. Mountain and valley 
winds are best developed in the wide and deep valleys 
of the Alps. The shape of the valley’s cross section and 
the inclination of the valley bottom are of little in- 
fluence on these winds. As a matter of fact, in fairly 
level valleys, such as the valley of the Inn River in 
Tirol, these winds are particularly well developed. They 
occur most frequently during persistent high-pressure 
situations in summer and are thus a typical fair-weather 
