1228 
some quantity related to MW. Such instruments should 
be more widely used. 
SPECIAL RESEARCH INSTRUMENTS 
Obviously, there are meteorological quantities other 
than temperature, humidity, pressure, wind velocity, 
and liquid-water content which are important for par- 
ticular studies, and in some cases for general meteor- 
ological use. Some research programs have required 
very extensive aircraft instrumentation, and many in- 
genious instruments have been devised [10]. Figure 3 
MILES FROM M.I.T ALONG AZIMUTH OF 134 DEGREES 
40 38 36 34 32 20) 23} 2G) re 
52 RADAR ECHO INTENSITY 
g METER 
TEMPERATURE- 
LIQUID WATER CONTENT 
FE ALTOSTRATUS, UPPER 
us BOUNDARY UNKNOWN 
u 20 
co} 
a I8F 
= 16 
3 14 
= ie 
= (© 
Zz 8 
5 6 
? 4p 
ee 
a o r - 
teh 12:53 12:54 12:55 12:56 12:57 12:58 12:59 13:00 13:01 13:02 
fs TIME | 
a 
Fic. 3—An atmospheric cross section and plot of instru- 
thental measurements and observations, most of which were 
obtained from a B-17. At the top is a plot of radar echo in- 
tensity (DBM = decibels below one milliwatt) from the storm; 
next are recorded vertical accelerations; then temperature 
records, liquid water contributed by cloud and by rain, and a 
vertical cross section along the flight path constructed from 
radar information as well as direct observations. The flight 
was at a level where snow was melting to rain, creating a 
stratified radar echo as shown by the heavy slant hatching. 
(Courtesy of Signal Corps Weather Radar Research at M.I.T.) 
presents in chart form some of the measurements ob- 
tained with equipment designed for measurements as- 
sociated with the uses of radar in meteorology [7]. 
Aireraft icing studies have also required many special- 
ized instruments. Only a few can be mentioned here. 
Cloud and Raindrop Size. A small cloud drop may 
be only 5 » in diameter, a large raindrop 5000 pu. So 
wide a range of sizes requires quite different tech- 
niques of measurement. In both cloud and rain there 
always exists a range of sizes, so that a size distribu- 
METEOROLOGICAL INSTRUMENTS 
tion is what should be measured. To obtain a repre- 
sentative result, an instrument must sample or meas- 
ure upwards of a thousand drops. 
Cloud drops have been measured at mountain sta- 
tions by catching samples in oil or on greased slides 
and measuring them with microscope techniques [23, 
25]. Some success with these techniques in airplanes 
has been reported [8], but a simpler airborne method 
is to expose a sooted slide [40, 52] to the air stream 
for a brief known interval and measure afterwards the 
splash traces remaining. As in the case of liquid-water 
collectors, the sampling device must be kept small. 
The rotating cylinder method [39], which makes use of 
selective collection efficiencies, has been mentioned as a 
liquid-water measuring instrument. All these methods 
are seriously limited by being essentially discontinuous 
and far from automatic. 
The measurement of associated optical phenomena 
[26, 56] shows much promise, as 1t provides a means of 
sampling a large number of drops simultaneously, and 
yields a size distribution instead of individual size 
measurements. The corona has been used extensively 
[30], and the rainbow [83] and forward-scattering [24] 
studied as size measuring methods. Transmission meas- 
urements [28, 35] and back-scattering for either light 
or radar energy? might also be used if the water content 
were known, but such measurements would determine 
only an effective drop size, not a drop-size distribution. 
In designing such equipment, it is important to remem- 
ber that cloud density and general illumination may 
change rapidly, making it difficult, for example, to 
locate the angle of a corona or rainbow by means of 
angular scanning. 
Direct photography of the droplets has been at- 
tempted [11], but so few drops are found in focus per 
photograph that a representative sample is not feasible 
within the short time available. Electrical methods are 
interesting. One, similar to measuring naturally charged 
particles [16], calls for inducing and measuring the 
maximum charge on individual drops as they pass 
through the instrument. Another measures the elec- 
trical effect of drops impinging on a charged probe 
[18]. 
Most of these instruments for airborne use have been 
developed in connection with research programs in 
cloud physics and aircraft icing, and have resulted in a 
general knowledge of drop-size distributions for dif- 
ferent types of clouds. For more exacting studies, an 
accurate, short-time-constant instrument with auto- 
matic recording is needed. 
No satisfactory instrument exists for measuring rain- 
drop size from aircraft. Because the mass of raindrops 
is more than a thousand times greater than that of 
cloud drops, most methods which involve physically 
catching and measuring individual drops are imprac- 
tical. Soot-coated screens have been tried in place of 
the sooted slides or solid surfaces used for cloud drops 
[2]. To obtain a satisfactory sample on one screen 
would require large unwieldy sizes. Some study has 
been made of techniques of measurmg momentum 
transfer from individual drops to small collecting plates 
