1226 
cooling cycle. It is short, however, a matter of 1 or 2 
seconds. This instrument is complex and requires some 
cooling agent, such as dry ice, for the dew-point sur- 
face, but these problems are not fundamental difficul- 
ties and have been overcome for certain special studies. 
Dew-point recorders of this type have been success- 
fully flown in both airplanes [14] and sounding bal- 
loons [3]. 
Another interesting device is the spectral hygrome- 
ter [13, 19, 36] operating on absorption of radiation by 
water vapor. Several absorption bands occur between 
the visible spectrum and a wave length of 4 wu. Most 
of these bands are too far into the infrared for good 
sensitivity with ordinary photocells so that an instru- 
ment with satisfactory sensitivity has not yet been 
developed, but there are newly developed photocon- 
ductive cells [6, 20, 43] now available which might 
make such an instrument more useful. Stronger and 
very broad-band absorption occurs at and around 6.5 
uw where thermal detectors must be used. Here again 
there are newly developed infrared detectors [20, 55] 
which might make the instrument practical. Possibly 
an optical instrument of this type could be arranged 
to measure attenuation of its light beam due to liquid 
and solid particles when present in the air, as well as 
absorption due to water vapor, thus serving a double 
function. This would require comparison of attenua- 
tion over three paths: one restricted to a water-vapor- 
absorption band, another where gaseous absorption 
was a minimum, and the third a standard reference 
path. 
It was pointed out when mentioning the sonic ther- 
mometer that the speed of sound is a function of 
humidity as well as temperature, so that the imstru- 
ment can be used as a hygrometer if the temperature 
is accurately known. At low humidities, however, its 
moisture dependence is very small. At present the 
dew-point hygrometer leads the field in accuracy at 
low humidities. 
MEASUREMENT OF PRESSURE, ALTI- 
TUDE, AND WIND VELOCITY 
These three measurements are grouped together be- 
cause they are made with standard aircraft imstru- 
ments available on any long-range aircraft [47]. Meas- 
urement of barometric pressuré is the simplest of all 
instrumental problems. A standard precision aircraft 
altimeter, when attached to a properly designed static 
fitting, is accurate to 1.5 mb (50 ft) [29]. It is most 
important, however, that the static pressure fitting be 
carefully imstalled and calibrated on each individual 
aircraft. 
Of course, for the pressure reading to have signifi- 
cance as such, the altitude must be known, and accurate 
measurement of altitude is relatively difficult. For- 
tunately, constant pressure surfaces have little slope 
in the atmosphere, so that it is sufficient to fly along 
such a surface by pressure altimeter, making periodic 
measurement of true altitude. Knowledge of the baro- 
métric pressure at the surface, and the density of the 
air at all levels from the surface to flight level, permits 
METEOROLOGICAL INSTRUMENTS 
accurate calculation of true altitude. This information 
could be obtamed by parachute radiosonde, but with 
poor accuracy.! However, altitude above terrain can be 
accurately measured (=E50 ft) by a radio altimeter [47]. 
Such an altimeter should be carried by any reconnais- 
sance aircraft for navigational purposes as well as for 
meteorological purposes. Consequently, the measure- 
ment of altitude and pressure offers no problem over 
ocean surfaces or flat terrain of known altitude. Over 
mountainous terrain, certain established check points 
might extend the usefulness of the radio altimeter. On 
a regular route or airway, radio responders or beacons 
could be used. 
Wind velocity and direction at flight level are deter- 
mined by drift meters or other navigational aids. These 
methods are well established and are a routine part of 
aircraft navigation [47]. The accuracy is generally in- 
ferior to the best balloon-tracking techniques, but satis- 
factory for many purposes. 
MEASUREMENT OF LIQUID-WATER 
CONTENT 
Liquid-water content M is the equivalent aloft of 
precipitation rate at the ground, and just as important 
a parameter. Its most common values are below 1 ¢ 
m~, but values in excess of 5 g m= have been ob- 
served. M should be measured to an accuracy which 
will match the accuracy of the humidity measurement 
q, so that the contribution of liquid water to the total 
water content will have significance. This requires 
about 10 per cent accuracy, which is not difficult to 
attain under good conditions. In practice, the observa- 
tion of water contents below 0.1 g mis most difficult, 
and in that range plus or minus 0.02 g m~ is a com- 
mon error. Even at —30C saturated air holds 0.34 g 
m-* of water vapor, so that JJ measured to plus or 
minus 0.02 would be close to the required accuracy 
limits. 
Most M-measuring instruments present a trapping 
surface or opening of some kind to air flowing past the 
aircraft and measure the water caught per unit time. 
From the true air speed and the area of the surface, 
one can calculate the volume swept per unit time and 
hence M, if one knows the efficiency of collection. 
No surface is 100 per cent efficient for all drop sizes. 
The smaller drops are deflected around the surface by 
the air stream. Because the problem of collection effi- 
ciency, in its various phases, is of first importance in 
studies of aircraft icing, it has received much atten- 
tion [81]. The first curve in Fig. 2 gives the collection 
efficiency of a sphere 1 cm in diameter as a function 
of drop size. It shows that a collector must be at least 
that small to catch a representative sample of cloud 
drops. This results in a secondary problem because the 
low rates of collection are difficult to measure. Also a 
small collector which is efficient for cloud drops is in- 
adequate for light rain because the wide spacing of the 
drops may cause a serious sampling error. A satisfac- 
tory solution may be a long narrow collecting surface. 
It is sometimes convenient to use two collectors of dif- 
ferent sizes as a means of separating the M due to rain 
