MICROCLIMATOLOGY 
tausch), but that in more than one-third of the cases 
faster air particles come from below, slower ones from 
above. The latter motions impede austausch and in- 
crease, rather than diminish, the existing gradient. 
Future measurements of A must, therefore, be made 
separately for the two types of vertical austausch mo- 
tion. This makes the measuring technique even more 
difficult. Nevertheless, all heat and water vapor prob- 
lems of the air layer near the ground will not be 
solved unless an exact and, from the practical point 
of view, not excessively difficult determination of the 
quantity A is made possible. 
Contour Microclimates. Another group of microcli- 
mates is caused by topographic formations. The differ- 
ences between sunny and shady locations or between 
valleys and mountain peaks which are known in large- 
scale climatology occur even in minute dimensions. 
An anthill of some 10-cm height, for instance, with 
its conical shape, possesses all the climate typical of 
slope locations, even though only within the air layer 
immediately adjacent to the surface: there is a warm, 
dry, south side (which the ants systematically use for 
breeding) and a shady, cool, moist, north side. Typical 
differences between east and west are also observed; 
in the case of uniformly incident radiation on cloudless 
days these differences are caused by the fact that the 
morning sun serves primarily to dry out the ground, 
while the afternoon sun mainly serves to heat it up. 
Plowed furrows represent mountain chains on a small 
scale; the corresponding microclimate is determined 
by the orientation of these furrows. Kaempfert [17, 
18] has investigated the radiation conditions for this 
case in detail. Variations are occasionally visible in the 
form of differences in the variety of weeds growing on 
the two sides of the furrow. Every large rock repre- 
sents a climatic divide on a small scale. Ullrich and 
Made [34] even found that the air, which is stratified 
in curved layers during the night, reveals a tempera- 
ture stratification, in millimeter dimensions, entirely 
analogous to the temperature distribution in flat val- 
leys. 
A continuous range of intermediate conditions exists 
between these small-scale phenomena and the large- 
scale climatic differences observed by the mountain, 
valley, and slope stations of meteorological observation 
networks. Frequently, however, the laws of micro- 
climatology are different from those applicable to large- 
scale climatology. For instance, in the case of high 
mountains located in a region of predominant west 
winds the west slopes are rich in rain because here the 
water vapor in the rising air is forced to condense. In 
the case of low hills, on the other hand, such thermo- 
dynamic processes have no effect. On the contrary, in 
this latter case the east side is the more humid because 
the field of motion of the wind determines the distribu- 
tion of precipitation. Precipitation is lacking on the 
windward side of obstacles. During snow this can some- 
times be observed directly, but it should not be con- 
fused with the redistribution of already fallen snow by 
the wind. We do not know as yet just where the line 
997 
between “large-scale” and “small-scale” is to be drawn 
in this sense. 
The effect of exposure on the microclimate is also 
determined decisively by the large-scale climate. In 
tropical regions where the sun is almost vertical at 
noon there is little variation between differently ori- 
ented slopes. The variation increases with decreasing 
height of the noon sun, that is, with increasing latitude. 
On the other hand, it is only the direct radiation from 
the sun which produces differences due to the direc- 
tion of exposure, while diffuse radiation from the sky 
affects all slope orientations almost equally. The portion 
of total radiation which is diffused increases with lati- 
tude; differences in exposure are consequently decreased 
again in polar climates. In the far north, where there 
is a deficiency of heat, plants flourish only on southern 
slopes. In subtropical steppes, on the other hand, where 
there is a deficiency of water, it is often found that 
microclimatological conditions are adequate for plant 
growth only on the north side of rocks or hills. 
Mountain and valley winds are known to arise locally 
in mountainous regions. They frequently make the 
microclimate dependent. A microclimate is referred to 
as independent if it can be explained solely on the basis 
of local meteorological conditions; it is dependent if it 
is determined also by extraneous effects in surround- 
ing regions. This distinction is essential to a rapid and 
complete understanding of the various microclimates. 
A field immediately adjacent to a concrete road, or a 
meadow next to a pond, has a different microclimate 
than it would have if situated in surroundings of the 
same type. In any territory it is the wind peculiar to 
the time of day which makes extraneous effects felt 
over wide regions. This is particularly true of night 
winds, which move cold air toward depressions in a slow 
but steady flow persisting during the entire night. Local 
pools of cold air result, an example of which has already 
been offered in Table I. They are decisively affected 
by the ‘‘source regions,” that is, the space from which 
cold air can be supplied. Railroad embankments, hedges, 
and forests are sufficient to stop or deflect these shal- 
low cold air masses; underpasses, excavations, or gul- 
leys may cause them to drain off. This explains the 
fact that forests are located everywhere above the 
vineyards along the Rhine; these forests prevent cold 
air from flowing down into the vineyards durimg the 
frost period in spring. 
A normal decrease of temperature with altitude and 
the accumulation of cold air in valleys are responsible 
for the fact that a “thermal belt” is found to extend 
along a line halfway up most slopes; it is here that 
plants sprout earliest in the spring, that damage due 
to late frost is slightest, and consequently that the 
plant and fruit varieties most sensitive to climate 
thrive. Table III lists measurements made at twenty- 
four observation stations on the east slope of the Arber 
(Bavaria) during spring nights in 1931 and 1932. Here 
the thermal belt is located at about 820 m above sea 
level during radiation nights (continental winds); dur- 
ing west-weather nights (maritime winds) it is situated 
several decameters lower. The last column shows the 
