998 
frequency with which the thermal belt can be found at 
certain altitudes. The maximum (16 per cent) in the 
valley (660 m) includes the cases of stormy weather 
Taser III. Toermat BELT on THE Hast SLOPE OF THE 
ARBER, BAVARIA 
(Summary of Measurements Made at 24 Observation Stations) 
Temperature minimum 
Alniude above Frequency: ct 
sea level . - A oe Y 
(m) oeat ci tinentaidl le wihimacieine (%) 
breeze (°C) breeze (°C) 
860 8.2 6.5 10 
820. 9.0 7.0 45 
780 8.2 7.4 17 
740 6.8 7.0 6 
700 5.3 6.5 6 
660 4.1 6.1 16 
when no thermal belts were formed and consequently, 
in view of the normal temperature gradient, the foot 
of the valley was warmest. Otherwise, however, the 
highest night temperatures were found regularly at 
altitudes of about 820 m. Such facts are of the greatest 
importance in farming and fruit growing. 
Microclimates in the Vegetation Layer. An intimate 
mutual dependence exists between a plant and the 
microclimate within the stand of vegetation. As soon 
as a young sprout has pierced the ground and de- 
veloped its first leaves, it is no longer passively sub- 
jected to climatic conditions, but helps to form them. 
The surface is uniformly roughened. The air layer 
near the ground becomes more quiet, plant organs 
absorb radiation from the sun, casting a proportionate 
amount of shadow on the ground. Incident heat radia- 
tion is distributed vertically, extremes of surface tem- 
perature are decreased. In order to live, the plant must 
evaporate water which it obtains, if necessary, from 
layers deep below the ground. Humidity is thereby 
increased and then maintained nearly constant in the 
layer of stagnant air near the ground. 
Microclimate is affected to an even greater extent 
by an entire society of plants. It is well known that 
plant societies grow more vigorously as soon as single 
plants have become large enough to touch the leaves 
of their neighbors. This is primarily due to the effect 
of the change in microclimate which also exerts a 
favorable influence on the nature of the soil. Research 
has been initiated during the past few years to investi- 
gate the dependence of these conditions on the type of 
plant, the density of planting, and the cultivation of the 
soul. This will offer the farmer, gardener, and horticul- 
turist numerous artificial methods by which the growth 
of plant societies and their yield can be improved. 
Tn high, close-knit plant societies, most clearly in the 
case of forests, new conditions arise because of the fact 
that an independent microclimate develops in an en- 
closed air space, separated from the free atmosphere. 
The ground surface has become a secondary boundary. 
Transfer of heat and water vapor takes place almost 
exclusively at the surface of the plant society. The 
interior climate is characterized by the fact that all 
meteorological quantities change very slowly, air hu- 
CLIMATOLOGY 
midity is high, the air is extremely quiet, and tempera- 
ture conditions are moderate. This microclimate can 
often be understood most readily if one conceives of 
the space occupied by the plant society as being part 
of the ground, and then works with a heat conductivity 
and air exchange which are larger than within the 
ground, but very small compared to the conditions in 
the free atmosphere. 
Now every plant society produces microclimates at 
its periphery which are widely different in nature, 
depending on the direction of exposure. To a first ap- 
proximation, these can be compared with the corre- 
sponding microclimates near vertical walls. However, 
the analogy is not entirely accurate, in view of the 
fact that air exchange takes place between the interior 
climate of the plant society and the peripheral climates. 
In forestry these peripheral climates have been exploited 
for about a century to favor young forest growth. 
Young forests are raised in the climatic protection of 
old forests. The effect of peripheral climate, however, 
is not always favorable. At a southern periphery it 
may become too dry and hot, whereas a northern edge 
may be too shady; at an eastern periphery plants may 
be ‘deprived of dew too soon. Frequently the danger of 
late frost 1s increased because of the forced quiescence 
of air near forests, particularly where plants have been 
stimulated to early sprouting, as at southern peripher- 
ies. These microclimatological conditions, which are 
known and exploited in practice, have hardly been 
investigated scientifically so far. Work in this field 
seems very promising. In particular, questions concern- 
ing the extent to which experience gained in one forest 
range can be applied to another region cannot be de- 
cided without a scientific basis. 
APPLICATIONS AND FUTURE PROBLEMS 
OF MICROCLIMATOLOGY 
Agriculture and Microclimatology. No branch of eco- 
nomics is associated with microclimatology as inti- 
mately and in as many ways as agriculture and the 
related fields of horticulture, viticulture, and fruit- 
growing. Preceding sections have shown that the loca- 
tion environment of all cultivated plants depends upon 
the microclimate and the way in which it is affected by 
topography, surroundings, soil treatment, and the type 
of plant. An agriculture designed for maximum yield 
must therefore be based on a knowledge of and an ex- 
pedient control of microclimatological effects. Without 
microclimatology it is not possible to deal effectively 
with the three major problems which face farmers all 
over the world. These are (1) wind protection, (2) 
frost protection, and (8) artificial watering. 
The problem of wind protection has been compre- 
hensively treated in the international literature. How- 
ever, the present state of our research is very unsatis- 
factory because many occasional observations, but few 
systematic experiments, have been made. Distinction 
must be made between the changes of the wind field, 
which are caused by the measures taken for wind pro- 
tection, and the effect of these changes on the micro- 
climate and the harvest yield. 
