664 
phenomenon of summer. Nevertheless, mountain and 
valley winds may also occur on cloudy days and in 
winter when they become manifest mostly in a modifica- 
tion of the general wind. The vector diagram of the 
winds in the Inn river valley near Innsbruck in Fig. 
8 shows a good example of mountain and valley winds, 
SSS 
(0) I 2 Si 5 
M SEC7! 
Fie. 8 —Vector diagram of the mountain and valley winds 
in the Inn valley (Innsbruck) on June 19, 1929, 100 m above the 
valley bottom. The dash-dotted lines indicate the axes of the 
Inn valley; upstream is to the left. Numbers indicate local 
mean time. (After Ekhart [19].) 
This diagram of the wind conditions in the course of a 
clear day, June 19, 1929, shows that the maximum of 
the valley wind with speeds of 5-6 m sec occurs 
-shortly after 1500. The maximum of the mountain wind 
with speeds of 3-4 m sec in the opposite direction 
occurs at around 0700. In general, the winds blow in 
the longitudinal direction of the valley’s axis. 
As an example of the height and intensity of the 
mountain and valley winds at Innsbruck, an isopleth 
diagram by Ekhart is reproduced in Fig. 9. The wind 
HEIGHT (KM) 
0600 
1200 1800 
LOCAL MEAN TIME 
Fie. 9 —Mean velocity isopleths of the mountain and valley 
winds at Innsbruck (numbers are wind speed components 
(m sec™!) in the direction of the valley; + upvalley, — down- 
valley). (After Hkhart in Hann-Stiring, Lehrbuch der Meteor- 
ologie, 5. Aufl., p. 652.) 
velocity distribution shows the characteristic transition 
from mountain to valley wind. The values are averages 
of several fair summer days in 1929 and 1931. Accord- 
ing to the diagram, the wind maximum is to be found at 
an altitude of about 200-400 m, while the velocities 
near the ground are reduced because of the influence of 
friction. This increase of the valley wind with altitude 
is characteristic of all valley winds, as is the fact that 
their average velocities are higher than those of the 
nocturnal mountain wind. Furthermore, the diagram 
shows that the valley wind starts almost simultaneously 
in all layers up to relatively high altitudes. The starting 
time of the valley wind is closely related to the width 
LOCAL CIRCULATIONS 
of the valley and thus to the size of the mass of air 
involved. This starting time changes with the season, 
that is, with the magnitude of the diurnal temperature 
variations. 
The height of the valley wind usually reaches to, or 
somewhat above, the flanking mountain ridges. The 
more stably stratified mountain wind, on the other 
hand, is confined to lower levels. The upper boundary 
of the mountain and valley wind system usually arches 
several hundred meters over the mountain crests. 
There, the transition into the general wind system 
usually takes the form of an abrupt wind shift or a 
calm. 
The Maloja Wind. The Maloja wind, named after 
the windshed between Engadine and Bergell, Switzer- 
land, where it appears particularly well developed, is a 
variation of the mountain and valley wind [4, 6, 7, 10, 
35, 36, 39, 50, 58, 59, 72, 74]. It is a mountain wind 
which blows downvalley both day and night. The phe- 
nomenon must be attributed to the fact that the valley 
wind of one valley reaches over a pass into another 
valley. In the mountain and valley wind system there, 
it acts as a disturbance, or, more specifically, as an 
abnormal development of the valley wind. The de- 
velopment of this anomaly is decisively determined by 
which one of the valleys involved has the larger diurnal 
temperature amplitude and thus effectively extends its 
circulation into the other valley across the pass. The 
question has not been satisfactorily answered whether 
this phenomenon can be entirely explained by the 
further increase of the diurnal temperature variation 
beyond the pass, or whether it is a purely inertial ex- 
tension of that valley wind which is the more strongly 
developed. Sometimes, as was shown at the Arlberg 
Pass in Tirol [22], a strong upslope wind in one valley 
can, after crossing a pass, augment a valley wind in 
another valley. 
Even in the absence of a pass, the interplay between 
thermal slope winds and typical mountain and valley 
winds sometimes produces considerable anomalies of 
the latter. Veering or backing of the valley wind on 
respective orographic slopes and cross circulation in 
narrow valleys, owing to strong insolation on one slope, 
have been observed. Cyclic wind shifts in the course of 
a day are caused by the fact that the upslope wind 
starts before the valley wind, and the downslope wind 
before the mountain wind. 
The Theory of Mountain and Valley Winds. The 
development of the theory through many decades [15, 
27, 33, 34, 50, 66, 74] finally culminated in the work by 
Wagner [71-73], who made an intensive study of the 
daily pressure and temperature variations in the free 
atmosphere within the valleys of the Alps. These in- 
vestigations led to the result that at a certain altitude 
above the valley, usually at about the height of the. 
surrounding ridges, the otherwise different diurnal pres- 
sure variations are completely equalized. This level of 
equalization was designated by Wagner as the “effective 
ridge altitude.” A further important result of these 
investigations was establishment of the fact that the 
diurnal temperature variations in the valleys up to the 
