AIRCRAFT METEOROLOGICAL INSTRUMENTS 
[15], but a practical instrument has not resulted. 
Another approach is to allow the drops to enter a box 
of quiet air (carried with the airplane) and observe 
in some way the different rates of deceleration. 
A raindrop-size meter which has received much at- 
tention to date operates on the attenuation of a small- 
area light beam by individual drops passing through 
the beam [84]. This method avoids some of the uncer- 
tainties of those which require catching the drops on 
any sort of collector. For example, the drops are un- 
affected by the instrument as they are measured, its 
“collection efficiency”’ can be 100 per cent for all sizes, 
and its calibration is only a second-order function of 
air speed. Attenuation of the light beam causes a 
phototube to emit an electrical pulse for each drop. 
The size of the pulse is a measure of the size of the 
drop, so that sorting and counting circuits can be used 
to give a nearly continuous measure of drop-size dis- 
tribution. Calibration can be accomplished with ac- 
curately sized solid spheres such as glass beads. The 
method, however, has a poor signal-to-noise ratio, and is 
much too complex and expensive at present for more 
than research use. 
Snowflake and Ice-Crystal Size. Observation of snow- 
flake and ice-crystal size and type is obviously of paral- 
lel importance to the measurement of rain and cloud 
particles. It is also obvious that it is a more complex 
assignment, due to the intricate and varied shapes of 
snowflakes. An ingenious plastic replica technique has 
been developed [87] which can be used at the ground 
or in the air. For aircraft observations, a decelerating 
tube [88] is mounted forward from the nose of the air- 
craft to catch the samples with minimum damage by 
allowing them to decelerate with respect to the air- 
craft in a cushion of still air. Large crystals and agelom- 
erations usually break apart. Plastic replicas are then 
made and can be examined at any later time. However, 
the method is discontinuous and requires the operator’s 
full attention. A count of the number of particles 
caught in a given time is an indication of the number 
per cubic meter originally in the air. A better count 
might be made by one of the raindrop-counting de- 
vices, which, for snow, would give little idea of the 
size but might still count the number per cubic meter. 
Other Research Instruments. Many other measure- 
ments now made for research purposes may soon be- 
come of more general importance. For example, a nu- 
clei count, particularly of nuclei of crystallization, may 
become a significant observation [42]. Such an instru- 
ment would require a representative sample of air, 
which, in this case, should be easy to obtain on an air- 
craft since particles as small as nuclei follow the air 
stream closely. 
The Thunderstorm Project [5] made extensive use 
of vertical gust and draft measurements. Accurate 
measure of these quantities is extremely difficult be- 
cause of their very nature, and because the character- 
istics of the aircraft and the manner in which it is 
flown enter the measurements. Fortunately, for many 
purposes only a general indication of turbulence and 
vertical drafts is required. To obtain such information, 
1229 
the aircraft should be trimmed for level flight and 
flown with as little control as possible. Vertical accelera- 
tions and air-speed fluctuations are then a rough meas- 
ure of turbulence, and altitude changes are a rough 
measure of vertical drafts. Standard, relatively simple 
instruments are available for recording accelerations, 
air speed, and altitude; but conversion of the measure- 
ments to meteorological parameters is difficult [9]. 
Changes of the angles of pitch and yaw, that is, relative 
direction of the air stream past the aircraft, may also 
indicate turbulence, and instrumentation for measur- 
ing these angles also exists [48]. 
A project investigating atmospheric electricity has 
developed electric field strength meters [17, 53] for 
airborne use, and instruments for measuring and re- 
cording the electric charges on individual precipitation 
particles. These instruments appear to be entirely satis- 
factory, and could be engineered for routine use. 
Certain other research instruments have already been 
described. For example, instruments for measuring light 
transmission and hence visibility were discussed as 
means of determining liquid-water content. As visibil- 
ity meters, they would naturally be restricted to low 
visibilities (less than 1500 m) due to short path lengths 
[28, 35]. Velocity-of-sound instruments were first men- 
tioned as thermometers. They also have application 
as hygrometers, anemometers, and possibly other pur- 
poses [1]. 
A rather specialized but interesting instrument has 
been suggested [86] which would measure directly the 
refractive index of air at microwave radio frequencies. 
The index is a function of temperature and humidity. 
Some meteorological quantities at a distance from 
the aircraft can be measured, others observed. Condi- 
tions below are measured by ‘‘dropsonde,” a radiosonde 
released from the aircraft to parachute to the surface. 
When precipitation, rain or snow, is present within a 
hundred miles, or perhaps more, its location can be 
accurately determined by airborne radar and its in- 
tensity estimated [46]. Active research proceeds in this 
field, so that the usefulness of radar may be expected to 
increase in the near future. It is already possible to 
detect clouds over short ranges, permitting the altitude 
and thickness of cloud layers above and below flight 
level to be observed. The freezing level can often be 
located by radar, turbulence within storms indicated, 
and, as mentioned earlier, some indication of liquid- 
water content given. The effectiveness of hurricane 
reconnaissance is tremendously increased by airborne 
radar. Because of the continuing development of 
weather radar, and because of its navigational use to 
aircraft flying in bad weather, the meteorological appli- 
cations of airborne radar should receive serious study. 
Further space is not devoted to radar here because of 
detailed discussion elsewhere.” 
SUMMARY 
It is apparent from the foregoing discussion, first, 
that meteorological measurements from aircraft are of 
increasing importance; second, that techniques, how- 
ever complex, have been devised for a wide variety of 
