670 
low, but relatively steep, slope to the otherwise warm 
coast of the Adriatic Sea. 
The outstanding characteristic of the bora is its 
extraordinary violence, which often causes heavy dam- 
age. The cause of these stormlike fall winds is the 
liberation of potential energy when the cold air begins 
to drop to the sea. There are calms between violent 
gusts of 50-60 m sec ~ called reffoli, and barometric 
fluctuations of 4 mm Hg are not rare in these cases. 
The bora does not extend far over the sea. However, it 
induces a heavy sea and atomizes the wave crests to 
such an extent that a cloud of mist (fwmarea) is 
formed over the ocean. The bora of the Adriatic Sea 
has a pronounced diurnal period of intensity and fre- 
quency of occurrence. The maximum intensity occurs 
between 0700 and 0800, the minimum around 2400. It 
occurs most frequently about 0600 to 0700, most rarely 
about 1400. 
We may distinguish between cyclonic and anticy- 
clonic bora, according to the origin. The cyclonic bora 
is dependent on the existence of a low-pressure area 
over southern Adria, with an especially warm sirocco 
blowing from the south over the front portion of this 
low and aloft over the bora. Consequently, the sky is 
usually cloudy, and there is precipitation. The bora 
then blows more steadily and covers the entire Adriatic 
Sea. For an anticyclonic bora to develop, a strong high- 
pressure area must exist over central Hurope with an 
extension of high pressure over Dalmatia which need 
not be opposed by a cyclone with closed isobars in 
the south. In that case the bora exhibits a violent 
character, but does not extend far out to sea (about 10 
nautical miles). The sky remains clear, with the excep- 
tion of a foehn wall over the mountains. 
Under favorable conditions, well-developed local 
winds of bora character occur in many areas in the 
world. These conditions are (1) a short topographic 
drop, and (2) a sharp climatic division between a cold 
plateau and a warm plain so that, under suitable pres- 
sure conditions, cascading of the deeply chilled con- 
tinental air can take place. Especially well known are 
the bora winds of Novorossiisk on the northern Cau- 
casian shore of the Black Sea [1, 3] and those of Novaya 
Zemlya [75]. 
A. Defant [16] has dealt theoretically with the ques- 
tion of the drainage of cold air along a slope. His method 
of attack, in which he bases a somewhat schematic 
model on the equations of motion and continuity in a 
doubly stratified atmosphere, allows a very good esti- 
mate of the influences of gravity and pressure gradient 
on this drainage of cold air masses. It is interesting to 
note that these two influences are of almost the same 
order; in other words, not only gravity, but also the 
pressure gradient, has an influence on the drainage of 
the cold air. If the steady current is disturbed, it may 
remain stable up to a critical slope angle, in which case 
small initial disturbances propagate as waves with a 
definite period and eventually fade out under the in- 
fluence of friction. If the critical angle is exceeded, 
small initial disturbances grow exponentially with time, 
LOCAL CIRCULATIONS 
and the current assumes an unstable and turbulent 
character. A ground slope of 1:100 seems to be the 
critical value, fully confirmed by observation. An ex- 
ample of a stable current is the orderly, nocturnal 
outflow of the mountain wind through slightly inclined 
valleys; one of unstable flow is the extremely turbulent 
bora in Dalmatia. 
The Mistral and Jet-Effect Winds. The bora-like 
local wind of Provence and the French coast of the 
Mediterranean up to Perpignan [5, 29, 51] is called the 
mistral. It is a combination of a bora and a wind that 
is increased through the so-called jet effect. The mistral 
is due to the drainage of cold air initiated by a high- 
pressure ridge which frequently extends from the Azores 
to France, and by a permanent low-pressure area over 
the warm Gulf of the Lion. Its intensity is increased 
and its duration prolonged through the constriction of 
the geographic gate between the Pyrenees and the 
western Alps. If fresh polar or arctic air enters the 
Mediterranean basin in the rear of a cyclone that 
moves off to the east, the bora and the jet effect build 
up the mistral to a destructive intensity. It should be 
noted that it is not the shallow cold surface air of the 
plateau of central France, but the deep break-through 
of polar air into the western Mediterranean basin that 
favors the development of the mistral. Similar fall 
winds and jet-effect winds are caused on the Pacific 
coast of North America by the break-through of polar 
continental air; they are known as northers [41]. 
The jet effect, that is, a purely local increase in wind 
intensity because of certain orographic configurations, 
is observed in many localities. The convergence of 
streamlines in a constricted path necessitates a sub- 
stantial increase in wind speed. The pressure gradients 
responsible for the jet-effect wind extend only over 
very short horizontal distances and can be detected 
only by special investigations. A very detailed study 
of such conditions in the Vienna basin was made by 
Margules [56]. In that area, the air masses that stream 
through the gate between the Kahlenberg and the 
Bisamberg during a west wind display all the phe- 
nomena of the jet effect, and the corresponding pressure 
disturbances are clearly revealed by barograms. 
SURVEY OF DESIRABLE STUDIES OF 
LOCAL WINDS 
Land and Sea Breezes. Further intensive investiga- 
tion of the vertical structure of land and sea breezes 
on especially suitable coasts by means of continuous 
aerological measurements over land and water at vari- 
ous distances from the shore would be most desirable. 
As regards the theoretical aspects, the problem appears 
adequately solved. 
Mountain and Valley Winds. Accurate aerological 
cross sections along favorably situated slopes up to the 
ridge are still needed. Likewise, systematic upper-air 
soundings should be carried out in a particularly favor- 
able valley location from its deepest recesses out into 
the plains. Special attention should be given here to 
the interrelation between the slope wind and the valley 
