METEOROLOGICAL ASPECTS OF AIRCRAFT ICING 
By WILLIAM LEWIS 
U. S. Weather Bureau, Washington, D. C. 
Introduction 
The formation of ice on airplanes has long been 
recognized as a serious problem affecting the regularity 
and safety of aircraft operations. With the improve- 
ment of instrument-flying techniques and navigational 
aids and the resulting increase in regularity of opera- 
tions in inclement weather, the problems associated 
with icing conditions have become of prime impor- 
tance to airline operators, airplane manufacturers, and 
meteorologists. 
Until recently, very little quantitative data was avail- 
able on the actual physical characteristics of icing 
conditions. Most of the available mformation on the 
meteorology of icmg was based either on studies of 
pilots’ reports [3, 11, 16, 17] or on theoretical inferences 
regarding cloud composition. In particular, the ideas 
of Bergeron on the composition of precipitating strati- 
form clouds in a warm-front situation have been used 
as a basis for a model of icing zones in a warm-front 
cloud system which has been presented in several text- 
books on aeronautical meteorology (e.g., [5]). A some- 
what different model, later proposed by Findeisen [6], 
contained a more accurate representation of the typ- 
ical composition of precipitating stratiform clouds. The 
importance of orographic effects m the geographical 
distribution of icing conditions was emphasized by Riley 
[16]. 
With the development of thermal methods of ice 
protection, the need for quantitative information on 
liquid-water concentration, cloud-drop diameter, and 
temperature in icing conditions was recognized and 
projects were undertaken to obtain the necessary data. 
The first of these was the Mount Washington Project, 
conducted by the Mount Washington Observatory, 
Harvard University, the U.S. Air Force, and the U. 8. 
Weather Bureau. This project has resulted in the col- 
lection of a large amount of data on the composition 
of clouds at the summit of Mount Washington, N. H., 
at an elevation of approximately 6300 ft [7]. 
Flight measurements of liquid-water concentration 
and drop diameter in clouds have been made by the 
National Advisory Committee for Aeronautics [13-15], 
the Air Materiel Command of the U. 8S. Air Force [4], 
and other groups. As a result of these researches the 
probable range and relative frequency of occurrence 
of various values of the significant variables have been 
tentatively established and values have been chosen 
to be used as a basis for the design of thermal ice- 
prevention equipment [9]. This work has also led to a 
more complete understanding of the composition of 
clouds in general and the type and severity of icing 
conditions to be expected in various weather situations. 
The Physical Characteristics of Icing Conditions 
For most practical purposes, the physical charac- 
teristics of icing conditions may be regarded as deter- 
mined by the values of three basic parameters: liquid- 
water concentration w, drop diameter d, and tempera- 
ture 7. The necessary and sufficient condition for ice 
formation upon a small object moving through the air 
with a velocity V is that w be greater than zero and 7’ 
be below freezing by an amount equal to or greater 
than AT, where AT is the wet-air kinetic temperature 
rise at the velocity V. Although the value of AT’ is 
influenced to a slight extent by several factors, iclud- 
ing the shape and thermal conductivity of the moving 
object and the value of w, it is given with sufficient 
accuracy for most practical purposes by the following 
expression : 
Vata 
AT = 0.8{—-]} —, 1 
lal = oy 
where ym and ya are the saturated- and dry-adiabatic 
lapse rates, V is in miles per hour, and AT’ is in centi- 
grade degrees. The rate at which water is intercepted 
by an object, expressed in grams per second per square 
centimeter of projected area, is given by 
[= EwV X 10°, (2) 
where w is in grams per cubic meter and V is in centi- 
meters per second. This equation defines the quantity 
E, called the collection efficiency, the value of which 
depends upon V, d, and the size and shape of the ob- 
ject. In general, the collection efficiency is greatest for 
small objects, high air speeds, and large drops. 
The variation of the kinetic temperature rise with 
air speed, and the dependence of collection efficiency 
upon drop size, air speed, and size and shape of the 
object, furnish an explanation of the importance of 
temperature and drop size in determining the severity 
of icing for particular components of an airplane. In 
the case of wings, the sizes and curvatures of the wing 
sections commonly used on modern airplanes, and the 
speeds at which these airplanes operate, are such that 
the values of collection efficiency vary from zero or 
near zero for the smallest cloud drops to 70 to 80 per 
cent for large cloud drops and virtually 100 per cent 
for freezing drizzle or freezing rain. The drop size is 
therefore nearly as important as the liquid-water con- 
centration in determining the severity of wing icing. 
In the case of propellers, the small cross sections and 
high blade speeds result in very high values of collec- 
tion efficiency for almost the entire range of cloud drop 
sizes; hence, drop size is relatively unimportant for 
propeller icing. Because of the large values of the 
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