AIRCRAFT METEOROLOGICAL INSTRUMENTS 
tions, would introduce only secondary effects and should 
pose no insurmountable problem. High accuracy is 
possible with very short time lags, and there would be 
no radiation effects. The difficulties seem to be those of 
a relatively cumbersome installation, the usual but in 
this case not very difficult deicing problem, interference 
from sound produced by the aircraft, and an over-all 
instrument that is very complex compared to a com- 
mon thermometer. At its present stage of development 
it stands as an interesting and valuable instrument for 
research purposes, but it is too complicated for routine 
use. However, its potentialities are very great. 
A second very different principle would use radiant 
energy. Many radiant-energy thermometers have, of 
course, been developed, but the author knows of no 
published work on one for the measurement of free- 
air temperature from aircraft. However, an instrument 
has been devised for measuring the temperature dif- 
ference between the air at flight level and any clouds 
within the line of sight of the instrument [54]. If the 
energy received by such a device could be confined to 
wave lengths strongly absorbed and radiated by the 
atmosphere (carbon dioxide and water-vapor bands), 
an accurate free-air temperature device might result. 
On paper such an instrument appears barely possible 
with some of the new infrared radiation devices de- 
veloped over the past ten years [20, 55]. It would be less 
accurate at low temperatures where energy levels and 
humidities are also low. In dense cloud, rain, and snow, 
it would tend to indicate the temperature of the par- 
ticles instead of the free-air temperatures. 
Tt seems unnecessary to discuss here the relative 
advantages of thermocouples, resistance thermometers, 
bimetallic strips, and other types of temperature-meas- 
uring devices. The choice is primarily one of the sim- 
plicity of auxiliary equipment and time of response. 
The latter requirement is largely one of the mass and 
shape of the element. The lowest time lags are usually 
obtained by using small thermocouples or resistance 
elements and electronic amplifiers. With such devices, 
time lags can be reduced to a tenth of a second or less. 
It becomes important then to consider what time 
lags are desirable. We will assume an aircraft travelling 
at 100 m sec. Temperature discontinuities are of two 
types. Those produced by instability and turbulence 
will cover a wide scale range. The shorter the time lag 
the finer the observed temperature detail. For most 
purposes we are interested only in the larger convective 
motions with dimensions of several hundred meters, 
so that a time lag of one second is adequately short. 
Temperature discontinuities produced by horizontal 
stratification may amount to several degrees within a 
few meters but lie nearly parallel to the direction of 
motion of the aircraft. However, interesting wave phe- 
nomena are often observed in such surfaces and may 
be of importance. Such wave structure is sometimes 
made visible by clouds, and, if this be a criterion, is 
seldom less than 100 or 200 meters in wave length. 
In such cases, to obtain at least a rough measure of the 
sharpness of the inversion, the time lag should be of the 
order of a few tenths of a second. Most aircraft ther- 
1225 
mometers in use today have lags of several seconds, 
so there is much room for improvement here. 
MEASUREMENT OF HUMIDITY 
There is a great variety of methods of measuring 
humidity, both at the ground and aloft. Most aircraft 
humidity instruments are beset with the same problems 
as aircraft thermometers, and in addition must be ac- 
curate at very low values. Wet bulbs, electrolytic con- 
ductivity elements, hair hygrometer types, and all their 
counterparts become very sluggish and inaccurate at 
low temperatures and humidities. Some sluggishness 
is inherent in these types due to the necessity for moist- 
ure transfer between the measuring element and the 
air. This moisture transfer requires ventilation, and 
again, as in the case of temperature measurement, 
dynamic heating effects cause errors. The “standard” 
aircraft instrument at present in use in the United 
States is the military ML-313/AM, which is simply 
wet- and dry-bulb mercury thermometers in a special 
housing [29, 47]. Its use below OC is awkward, slow, 
and of questionable accuracy. It has the high time lag 
characteristic of mercury-in-glass thermometers. In the 
presence of liquid water its dry-bulb reading has no 
significance. 
Much development work has gone into improvement 
of electrolytic and hair hygrometers (using the term 
hair hygrometer in its broadest sense). It seems reason- 
able to expect that an electrically conducting material 
may some day be developed which, as an electric 
hygrometer, will have low lag and constant calibration. 
The use of carbon shows promise.! However, the liquid- 
water problem must be kept in mind. A drenching in 
liquid water changes the calibration of some elements, 
requires a long drying-out period for others, and cer- 
tainly results in a false reading for most elements when 
flying through rain in unsaturated air. 
As in the case of temperature measurement, the most 
accurate instruments under all flight conditions are the 
most complicated. One of these automatically measure 
and records the dew point [14, 45]. The dew point ob- 
viously is unaffected by transient changes in tempera- 
ture and pressure, so that the sampling problem is rela- 
tively simple. When liquid water is present, it will be 
difficult but perhaps not impossible to obtain a repre: 
sentative sample. The problem is to avoid evaporation 
or condensation while separating the liquid water from 
the moist air. Separation of the larger drops is relatively 
easy, and fortunately more important. When many 
small cloud drops are present, nearly saturated air 
can be assumed. With this instrument there may be 
uncertainty between dew point and frost point due to 
supercooling. Except for this factor, which can perhaps 
be eliminated, it is accurate to about 1C and is useful 
down to dew points of —40C or below. Although this 
instrument also requires exchange of moisture between 
the air and the sensing element, it is only a surface 
phenomenon. Most of the lag occurs in the heating and 
1. See ‘“‘Instruments and Techniques for Meteorological 
Measurements” by M. Ference, Jr., pp. 1207-1222 in this 
Compendium. 
