690 
of a warm anticyclone and the other two were in rather 
flat, uniform pressure fields. These lines were usually 
short compared with the prefrontal types and seemed 
to be less well defined. 
Remarks on Thunderstorm Forecasting 
It is not practical in this article to discuss all of the 
important points related to the forecasting of thunder- 
storms. The problem is intimately connected with the 
general subject of forecasting treated elsewhere in this 
Compendium. 
Historically, thunderstorm forecasting has been of 
unique interest because it seemed to be the one atmos- 
pheric disturbance that could be treated quantitatively 
from a thermodynamic consideration of the buoyancy 
forces’ in an atmosphere in labile equilibrium. A series 
of rival thermodynamic diagrams was developed by 
different meteorologists, largely stimulated by the pos- 
sibilities of forecasting convection through thermo- 
dynamic analysis of upper-air soundings. Refinements 
of the parcel method and finally the introduction of the_ 
slice method and variations of it were aimed at this 
problem.”* 
Some meteorologists drew attention away from the 
analysis of individual soundings to studies of upper-air 
charts which they believed would hold the key to 
thunderstorm forecasting. Namias [21, 22] introduced 
the isentropic chart as a means of forecasting thunder- 
storms, showing that the occurrence of summer thun- 
derstorms depended on the presence of a moist tongue 
on isentropic surfaces in the vicinity of a potential 
temperature of 315K, usually found at an altitude 
around 3 to 4 km in the eastern United States 
in summer. The moist tongues, with dry ‘“‘tongues” or 
areas between them, occur over tropical air masses that 
are more or less homogeneous horizontally in the low 
levels. Thus the upper-air humidities appear to be 
critical. Namias’ work implied a mechanism similar to 
what is now known as entrainment in order to account 
for the arrested growth of convection in a dry upper- 
air environment. 
The picture seems to be somewhat as follows. While 
on the lower isentropic surfaces there seem to be only 
slight gradients of water-vapor content within the trop- 
ical air masses, there is great variability of moisture on 
the surfaces around 315K. These variations can be 
tracked from one isentropic chart to the next in the form 
of moving moist and dry tongues. Analyses of separate 
soundings in terms of stability may lead to error because 
a dry tongue may replace a moist tongue or vice versa 
in the forecast period at a station. 
If a strong dynamic cause for ascent exists in the 
tropical air mass, water vapor will be carried aloft to 
isentropic surfaces where it existed only in small quanti- 
ties before. Thus, in addition to a chart of the moist 
tongues existing aloft some study of the possibilities 
of strong low-level convergence with accompanying 
ascent should be included in the forecast preparations. 
4. Consult ‘“Thermodynamics of Clouds” by F. Mller, pp. 
199-206 in this Compendium 
LOCAL CIRCULATIONS 
There is evidence to indicate that the two main sum- 
mer moist tongues of the United States are caused by 
convergence and ascent over (1) the Mexican Plateau 
[45] and (2) the Florida Peninsula [10]. Some meteor- 
ologists conclude that thermal instability may be a 
necessary condition for thunderstorms but is not a 
sufficient one. A dynamic process, inducing ascent, is 
also considered to be required. If remote convergence 
and ascent which supplies the extensive moist tongues 
is included in this dynamic process, the conclusion 
probably is correct. Direct studies of the wind field dis- 
close local areas of low-level convergence if their size 
is in the proper relationship to the density of the 
observation network. 
The Thunderstorm and the Airplane 
Besides the usual difficulties of flight in clouds, the 
thunderstorm presents the additional, unique hazards 
of lightning, hail, and heavy turbulence. Flight records 
show that turbulence is the most predominant danger 
in thunderstorms and may be the principal cause of 
thunderstorm accidents. The difficulties of maintaining 
proper flight attitudes or air speeds within highly tur- 
bulent clouds may lead to loss of control or structural 
damage. It is believed that hail damage is second in 
importance as a hazard, and that lightning is third. 
Since hail and lightning are covered in other articles 
in this Compendium, thunderstorm turbulence effects 
will be emphasized here. 
It is convenient to recognize two classes of turbulence 
—eusts and drafts, corresponding to two fairly dis- 
tinct types of response experienced on an airplane and 
measured by two different techniques. In a draft, the 
airplane is displaced in altitude in one direction over 
several seconds of time because of the mean upward 
or downward motions of the air. Gusts subject the 
airplane to a series of sharp accelerations without a 
systematic change in altitude. These accelerations are 
caused by abrupt changes in velocity of the drafts and 
by small vortices or whirling masses of air. The larger 
gusts are invariably associated with strong drafts. If 
the airplane is flown at constant power setting, attitude, 
and air speed, the draft velocities can be measured by 
rates of vertical displacement shown on a recording 
altimeter. The gusts can be obtained from an acceler- 
ometer trace. The techniques of both measurements 
have been developed to a high degree of reliability by 
the Gust Loads Section of the National Advisory Com- 
mittee for Aeronautics and are described m N.A.C.A. 
publications [12]. 
Tn order to provide a gust measure that is applicable 
to studies of the gust loads imposed on airplanes, aero- 
nautical engineers determine the “effective gust ve- 
locity.”” In simplest terms, this may be defined as the 
vertical component of the actual velocity of a ‘‘sharp- 
edged gust”? that would produce the acceleration in 
question on a particular airplane flown in level flight 
at a known air speed and a given air density. The 
effective gust formula is of the form 
a 
= Kime itisechn 
U Sloe 
