Chapter 2 
METEOROLOGICAL EQUIPMENT FOR SHORT WAVE 
METEOROLOGICAL EQUIPMENT FOR 
PROPAGATION STUDIES# 
Outline of Problem 
| Aisin GAINED during the recent years with 
radar, especially microwave radar, and with ex- 
perimental microwave communication equipment has 
shown that the electromagnetic radiation field pro- 
duced by a transmitter is subject to large variations 
depending on the weather. These variations are caused 
by refraction and so are related to variations of dielec- 
tric constant in the atmosphere. Pressure, temperature, 
and. humidity determine the dielectric constant (re- 
fractive index). 
It has been found that above a certain height vari- 
able with season and geographical location but rarely 
exceeding 1,500 m above ground, atmospheric refrac- 
tion is reasonably constant. In the lower levels and 
especially in the lowest hundred meters of the atmos- 
sphere, temperature and moisture conditions strongly 
affect the radiation field and thereby influence the 
operation of radar and other short and‘ microwave 
equipment. In order to evaluate this effect in quanti- 
tative terms, the temperature and moisture distribu- 
tion in the lowest layers must be determined with as 
high a degree of accuracy as is compatible with speed, 
edse of operation, and other practical limitations. 
A number of methods have been tried during the 
recent years which range from measurements with 
ordinary radiosoride equipment to the use of a psy- 
chrometer on the steps of a fire ladder. Two facts have 
appeared rather clearly: First, hairs are not suitable 
for moisture measurements of this type on account of 
their great sluggishness (except perhaps for station- 
ary use on towers), since the time of adaptation of a 
hair to appreciable changes in humidity is of the 
order of 3 to 5 minutes. Secondly, it has been found 
that ordinary radiosondes are not usually appropriate 
because the readings obtained from them normally are 
taken about 100 m apart in vertical distance and for 
this particular problem a more detailed knowledge of 
the temperature and.moisture distribution is necessary. 
With a clock-driven radiosonde this can be remedied 
by loading the sonde down by means of a ballast 
(water or sand) which slows down the ascent of the 
instrument in the lower levels. If the ballast is made 
to run out gradually, the full lift of the balloon may 
be restored at any given level. This method cannot be 
applied to the U.S. Weather Bureau radiosonde in 
aBy W. M. Elsasser, Columbia University Wave Propaga- 
tion Group. 
226 
which temperature and moisture data are sent out by 
a mechanism in which electric contacts are closed by 
a pressure cell at predetermined levels (see, however, 
pages 230-231). 
On the whole it has been found more advisable to 
develop new or improved instruments or to adapt spe- 
eial instruments for a low-level sounding technique 
rather than to rely on the existing facilities for aero- 
logical measurements. The methods developed so far 
involve the use of planes and dirigibles as well as cap- 
tive balloons and kites. For the lowest strata, specially 
built towers and ship installations have come into use. 
Wet and Dry Bulb Methods 
The use of humidity data for radio propagation 
problems involves new features in instrumental tech- 
nique because the main effects of strong refraction are 
found under approximately calm weather conditions. 
Therefore, when wet and dry bulb methods for humid- 
ity measurements are used, particular care must be 
taken to insure satisfactory aeration of the wet bulb. 
As a rule, an air speed of about 3 m per second (about 
6.5 mph) is considered adequate ventilation for the 
wet bulb. In a plane, dirigible, or kite the necessary 
aeration is automatically provided. But on a tower 
or when carried by a captive balloon, artificial aeration 
will frequently be necessary. It has been claimed,? 
however, that if a wet bulb electrical resistor is used in 
conjunction with a captive balloon adequate aeration 
can be provided by giving the balloon cable a few 
violent jerks of about 5-ft amplitude. 
An ordinary sling psychrometer held out of the 
window of a flying plane and aerated by the slip stream 
has been found to give fairly reliable results, provided 
the wet bulb is kept properly moistened. This method 
has been used with good success for preliminary re- 
search work. It may be presumed that the use of a 
rather slow-flying plane is essential, in order to keep 
the dynamic temperature correction small and also in 
order to insure a not too excessive rate of evaporation. 
Thermocouples, thermopiles, and temperature-sen- 
sitive resistors frequently are used as temperature 
Tesponsive elements in place of actual wet and dry 
bulb thermometers. They are incorporated in spe- 
cially designed electrical bridge circuits in which the 
temperatures are read on either indicating or record- 
ing meters. 
bData, courtesy of U.S. Navy Radio and Sound Laboratory, 
unpublished. 
