1198 
kinetic temperature rise associated with the high speeds 
of propeller blades, the free-air temperature is an im- 
portant factor in determining the extent of ice forma- 
tion. For propellers of the size and speed ordinarily 
used on transport airplanes, ice formations which occur 
at free-air temperatures of —5C or higher do not ex- 
tend far from the root of the blade and have only a 
very slight effect on propeller efficiency. With decreas- 
ing air temperature, the extent of ice formation gradu- 
ally increases, reaching almost to the blade tips at 
—15C. At temperatures below —15C, serious propeller 
icing may occur even with low values of liquid-water 
concentration and small drops. 
In the case of windshields, the configurations most 
commonly used, such as the familiar V-type, are or- 
dinarily affected by icmg conditions with average-sized 
drops; however, in the special case of a windshield 
which is flush with the fuselage contours, the collection 
efficiency may be so low that icing occurs only in the 
presence of exceptionally large drops. 
The shape of the ice formations is influenced greatly 
by the rate of freezing. Under conditions of low tem- 
perature, low water concentration and small drops, the 
water freezes as it strikes without forming a liquid film, 
CLOUDS, FOG, AND AIRCRAFT ICING 
mits, which are usually more severe than those en- 
countered in the free air, conditions defined by it as 
“moderate” or “heavy” are substantially more severe 
than conditions described in the same terms by pilots. 
In fact, experience with pilots’ reports suggests that 
the conditions defined as ‘‘trace,” “light,” and “moder- 
ate’ according to the Weather Bureau scale would or- 
dinarily be reported by pilots as “light,” “moderate,” 
and “heavy,” respectively. For this reason some mis- 
understanding has resulted from the application of this 
scale to icing conditions encountered in flight. Accord- 
ingly, it is suggested that the scale be modified for use 
as a basis for quantitative definitions of icmg conditions 
encountered in flight by assigning different descriptive 
terms to the four degrees of icing intensity defined by 
the scale. Table I shows the scale now used by the 
Weather Bureau for reporting icing conditions at moun- 
tain stations and the scale suggested for flight use. The 
definitions are given in terms of the rate of accretion 
on a cylinder three inches in diameter; corresponding 
values of liquid-water concentration for four values of 
drop diameter are also included. 
Measurement of Liquid-Water Concentration and 
Cloud-Drop Diameter. Although several methods have 
TasxeE I. Ictnc-INTENSITY SCALES 
Rate of ice accretion on uignnaatien concentration (ein) fouicentaingyaliies Weather Bureau scale of icing Sreaesinil gonile of Seine Artionck 
cyl. eae aa mph P gnicusity (on mcun iain 88 for flight reper ty 
10 p 5p 20 n 30h 
0.00- 1.00 0.00-0.22 0.00-0.10 0.00-0.07 0.00-0.05 trace of ice light icing 
1.01- 6.00 0. 23-1 .32 0.11-0.60 0.08-0.42 0.06-0.30 light icing moderate icing 
6.01-12.00 1.33-2.64 0.61-1.20 0.43-0.84 0.31-0.60 moderate icing heavy icing 
>12.00 >2.64 >1.20 >0.84 >0.60 heavy icing very severe icing 
thus producing rather narrow, smooth, pointed ice 
accretions. On the other hand, temperatures near freez- 
ing, and larger values of water concentration and drop 
size, result in the formation of a film of liquid water 
which spreads as it freezes, giving rise to irregular 
ice formations with flat or concave surfaces facing 
the air stream. These two types of ice are generally 
called rime and glaze, respectively. 
The Definition of Icing Intensity. The preceding dis- 
cussion shows that the intensity of icing is dependent 
on liquid-water concentration, cloud-drop diameter, and 
temperature. Since the relative importance of these 
factors is different for different airplane components, 
no single function of these quantities can be used to 
define a scale of icing intensity that will apply equally 
to all components of all aircraft. The only scale now in 
use which defines light, moderate, and heavy icing in 
quantitative terms is that used by the U. 8. Weather 
Bureau in reporting the intensity of icing observed 
at Mount Washington. This scale is based on the rate 
at which ice would form on a cylinder three inches in 
diameter moving at a speed of 200 mph, which repre- 
sents a reasonable compromise among the various air- 
craft components. Because of the fact that this scale 
was designed to describe conditions on mountain sum- 
been employed for measuring liquid-water concentra- 
tion and drop diameter in clouds [10], nearly all of the 
reliable observations now available have been made by 
the rotating-multicylinder method devised by Arenberg 
[1]. As used in flight, this method employs a series of 
rotating cylindrical collectors (usually four) of different 
diameters which are mounted coaxially and exposed 
with the axis at right angles to the air stream for a 
measured time interval. The weights of ice collected on 
the cylinders are used to determine the liquid-water 
concentration and the average drop diameter. The 
calculations are based on values of the collection 
efficiency of cylinders calculated by Langmuir and 
Blodgett [12]. This method yields reliable and fairly 
accurate values of liquid-water concentration. The 
determination of drop sizeisreliable and reasonably accu- 
rate forvalues of drop diameter below 20 p, but becomes 
increasingly inaccurate and unreliable for larger drops. 
Values of drop diameters around 30 uw are subject to 
considerable error and values over 40 p are highly un- 
reliable. 
The value of drop diameter obtained in this way is 
called the mean-effective diameter and is believed to 
represent approximately the volume median size; half 
of the water is contained in larger drops, half in smaller 
