MICROCLIMATOLOGY 
cold air parcels. These fluctuations are visible in the 
form of scintillations which can be observed at noon 
above heated roads or railroad embankments. If, in 
making this observation, one moves his eye downward 
until a layer directly above the ground is observed, 
one will be surprised to note a considerable increase 
in scintillation. The high lapse rate, moreover, gives 
rise to reflections by air strata, also known as road 
mirages which also occur only in the microclimate near 
the ground. 
From the great decrease in temperature with altitude 
at noon (incoming-radiation type) and the great in- 
crease during the night (outgoing-radiation type) it 
follows that daytime temperature fluctuations near the 
ground are considerable. If a meteorological station 
located 2 m above ground observes a daytime fluctua- 
tion of 10C, the value at 1 m increases to about 15C, 
at 10 em to about 20C, and at the ground to a value 
of 30C or even 40C. This variation can occasionally be 
observed in the erosion of vertical rocks or buildings. 
Effect of the Type of Ground. Since microclimate is 
controlled by the ground, it follows that the type and 
condition of the soil must have a considerable effect 
on the microclimate near the ground. Surface proper- 
ties determine, first of all, how much radiation is ab- 
sorbed or reflected. Dark areas (peat bogs) absorb 
more, light areas less; moist ground absorbs more than 
dry ground. Surface properties are usually different 
for the absorption of short-wave radiation from sun 
and sky during the day than they are for the emission 
of longer wave lengths during the night. The thermal 
conductivity of the surface controls the distribution 
of absorbed heat between the ground and the air. If 
the thermal conductivity is large, the ground absorbs 
much heat during the day and then conducts much 
heat to the radiating ground surface during the night. 
Temperature conditions in the air layer near the ground 
are consequently moderate. Ground of poor conductiv- 
ity transmits much heat to the air during the day and 
‘removes heat from the air during the night. In such 
cases harmful air temperatures are attained at noon, and 
at night the danger of frost is correspondingly great. 
Thermal conductivity depends on the condition and 
composition of the soil, and on its structure. The lat- 
ter is determined by the treatment which the soil has 
been given. A table of various soil constants (which 
can serve merely as a qualitative mdication in view of 
the great variety of soil conditions) will be found in 
the textbook on microclimatology by Geiger [11]. 
The microclimatologist can understand the climate 
near the ground only if he is familiar with the climate 
prevailing in the ground itself. The farmer, however, 
by his practice of cultivating and fertilizing the soil, 
changes its condition and thus affects the climate near 
the ground—the climatic environment of the plants he 
cultivates. Even changes in the ground’s surface and 
its properties often have a great effect, for instance 
covering the ground with evergreen twigs, straw, etc. 
(mulching) and covering peat bogs with sand. The 
greatest obstacle to the investigation of this ground 
climate is the difficulty of measuring in a simple man- 
995 
ner the water content and the water balance of the 
ground. Nevertheless, at the present time, investiga- 
tions that raise hope for progress in the near future 
are being conducted in various countries. If soil vol- 
umes of more than one cubic meter are sunk into the 
ground flush with the surface, and if the weight of this 
soil can be measured with sufficient accuracy, the water 
loss (evaporation) or the water gain (dew and frost 
formation) of the soil can be determined from the pre- 
cipitation and the seepage. In 1930-1937, J. Bartels 
and J. Schubert [8, 10] obtained a good series of meas- 
urements of this kind m Eberswalde. However, the 
instrumental equipment is very expensive because of 
the extensive installation work and the balances that 
must have a high accuracy (about 0.1 ke) under great 
loads (about 1500 kg). For this reason, employment of 
this method at many locations can hardly be expected. 
At present, C. W. Thornthwaite is undertaking promis- 
ing experiments at Seabrook, N. J., on the evapotrans- 
piration of natural and cultivated soil surfaces. In 
India, Ramdas [25] found a method for measuring the 
water content of the soil without changing (digging) 
the soil itself; this method consists in electrically heat- 
ing a soil thermometer (mercury) and measuring the 
subsequent rate of temperature change. As soon as prog- 
ress has been made in these instrumental problems, it 
will be possible to study the influence of the type and 
state of the soil more adequately than has been the 
case thus far. 
The air layers near water and snow are of special 
interest for the further systematic development of mi- 
croclimatology. The uniformity of the water surface 
and of the freshly fallen snow cover facilitates the 
elimination of all advective imfluences. Because of its 
small heat conductivity, the snow cover also eliminates 
the influence of the soil. Investigations by Bruch [4] 
and Roll [26] of the temperature and wind field over 
water surfaces have greatly promoted knowledge of 
the air layer near a water surface. Moreover, these 
researches have furnished new information regarding 
the process of wave formation, the change of an air 
mass in moving from land to sea, and the usefulness of 
the temperature of the water surface reported by ships 
in the synoptic weather service. 
Moisture Conditions. The measurement of the at- 
mospheric humidity near the ground is nowadays 
attempted in three different ways: (1) by hair hygrome- 
ters in which the orientation of the hair and the con- 
struction of the instrument are adapted to the horizontal 
stratification of the humidity (Diem [6]), (2) by psy- 
chrometers with very fine thermoelements whose radia- 
tional error can be neglected (Franssila [9]), (8) by 
aspirating air from the desired height into a modern 
dew-point hygrometer (Thornthwaite [81, 33]). The 
first two methods are distinguished by their simplicity, 
the third method by its high accuracy. 
Unfortunately, extensive series of measurements of 
the water-vapor distribution near the ground are as 
yet unavailable. As with temperature, this distribution 
is quite different from the conditions prevailing at a 
height of 2 m. The temperature at ground level may 
