994 
In some of these cases one deals with a single weather 
process, that is, a meteorological event. Thus there 
exists also a science of micrometeorology. Thornthwaite 
[31] ascribed the difference between these disciplines 
to the research methodology used, stating that 
Micrometeorology deals with the physics of a layer and 
aims at discovering physical principles, whereas microclima- 
tology is concerned with the geographical distribution, both 
areally and vertically, of the various properties of the air, 
and seeks patterns of geographical distribution. 
It has so far been customary (and will probably also 
prove expedient in the future) not to distinguish be- 
tween two separate disciplines, but to assemble all 
small-scale investigations under the collective term mz- ' 
croclimatology. For just as modern climatology with its 
method of frequency statistics accepts the synoptic 
weather processes in the domain of its studies, so micro- 
climatology will also include small-scale single weather 
processes, that is, micrometeorological events. 
THE PRESENT STATE OF RESEARCH 
Microclimate as the Climate near the Ground. We 
shall first consider microclimate as the special climate 
prevailing in a layer of air about two meters in height 
adjacent to the surface of the ground. In this layer, 
friction between the air and the earth’s surface plays a 
decisive role. In the lowest few decimeters, wind velocity 
is very low; mixing of the air due to turbulence is 
therefore also slight. However, the greatest portion of 
radiation from the sun and ground is absorbed at the 
surface of the ground. Heat radiation during the night 
also takes place at the surface. Precipitation is absorbed 
by the ground. The ground furnishes water vapor as 
well as carbon dioxide and other substances to the 
atmosphere. Exchange of heat and water thus takes 
place in the boundary layer between ground and at- 
mosphere. The layer near the ground is the first inter- 
mediary between ground and atmosphere, and it is 
precisely here that the vertical transport of all proper- 
ties is greatly impeded by wind resistance along the 
ground. It is therefore in this layer that we encounter 
the greatest differences in the smallest space. 
Temperature Conditions. The slight air movement, 
the strong radiation reflected by the ground surface, 
and the large vertical temperature gradients make it 
difficult to obtain exact measurements of the tempera- 
ture of the air layer near the ground. The problem of 
the measuring technique is not as yet satisfactorily 
solved. 
The use of thermometers which are artificially venti- 
lated and protected from radiation is possible only at 
heights above roughly one meter. The first exemplary 
experimental arrangement of this kind was applied to 
microclimatology by Flower [7] in England. Closer to 
the surface, ventilation destroys the natural tempera- 
ture stratification that is to be measured. Moreover, 
no suitable protection from radiation by the sun, the 
sky, and the earth’s surface has yet been found. In most 
cases, the unavoidable radiational error increases sys- 
tematically with the approach of the thermometers 
CLIMATOLOGY 
to the ground surface. Thereby, excessively large gradi- 
ents are simulated when the insolation is strong. In 
addition, the protective device also shades the ground 
and thus affects the measuring field. 
At present, Albrecht’s method of temperature meas- 
urement [1], involving a single platinum wire as an 
electrical resistance, is probably the best. It is well 
known that the radiational error of a wire exposed to 
sunshine decreases with decreasmg diameter. If wires 
of only 0.015-mm diameter are used, the error becomes 
negligibly small even in calm air, a fact that now can 
also be proven theoretically. Such a wire is stretched 
horizontally at the desired height of measurement. Its 
length serves to average the temperature of many 
eddies at the selected height of measurement. This is 
an advantage considering the extraordinary tempera- 
ture fluctuations near the ground. This arrangement 
has proved satisfactory even for temperature measure- 
ments in the Gobi Desert. 
The most important characteristic of the air tem- 
perature near the ground is the unusually large vertical 
temperature gradient, especially by day. Table II refers 
TasBLE II. YEARLY PERCENTAGE FREQUENCIES OF LApsE 
Rates oF Various MaGniTuDES IN KARACHI 
Lapse rate 
(°F/100 m) 57 F433 bad 3.) 
Layer 4-56 ft | 0.1 0.4 2.0 8.5 13.0 138.0 12.0 7.1 
(%) 
Layer 56-156 |— — — — 0.6 
ft (%) 
2.5 24.1 23.7 
Lapse rate ee =e a. 
(*F/100 m) USE Sean! 
Layer 4-56 ft 
(%) 
Layer 56-156 
ft (%) 
6.6 5.3 7.6 6.7 6.2 6.4 3.6 1.1 0.3 
16.7 7.9 7.3 7.2 6.3 2.7 0.7 — — 
to measurements by Mal, Desai, and Sirear [21]; it lists 
the frequency distribution of temperature gradients 
observed at the Karachi airport by means of venti- 
lated electric-resistance thermometers; values are con- 
verted to degrees Fahrenheit per 100 m for the purpose 
of comparison. The measurements were not actually 
made in the layer near the ground. They serve to illus- 
trate, however, how variation in the prevailing gradient 
increases as the ground is approached. According to 
measurements made by Best in England, by means of 
ventilated thermoelements, the gradient in the layer 
between 1.2-m and 0.3-m altitude at 12 noon, even 
in the middle of June, amounts to 139F per 100 m, 
and as much as 1230F per 100 m in a layer between 
0.300 and 0.025 m, if conversion to a 100-m altitude 
is made. 
These high gradients are accompanied by a number 
of other phenomena which are equally characteristic. 
Rapidly recording electric thermometers reveal extra- 
ordinarily great temperature fluctuations which never 
subside and are particularly strong durmg periods when 
the incident radiation is large. They are caused by the 
rising of hot air parcels in proximity to descending 
