THUNDERSTORMS 
By HORACE R. BYERS 
University of Chicago 
Introduction 
Thunderstorms are shower clouds or aggregations of 
shower clouds in which electrical discharges can be seen 
as lightning and heard as thunder by a person on the 
ground. Discharges observed on airplanes flying in 
clouds do not constitute conclusive evidence of thunder- 
storms unless lightning is observed illuminating the 
cloud from another source, since most discharges on 
airplanes are believed to be autogenous in origin, that 
is, to result from an intensification of the electric field 
around the airplane brought about by the innumerable 
collisions with precipitation particles.1 Since thunder- 
storms are showers that have reached a stage of 
producing lightning, it has been recognized for centuries 
that they represent an intense form of shower or an 
advanced stage in the development of convection in 
moist air. Modern observations indicate that thunder- 
storms will develop when large accumulations of con- 
densed water, presumably liquid and solid, are being 
carried upward at heights where the ambient tempera- 
ture is less than about —20C. It is only under conditions 
of moderate to high temperature and moisture that 
large accumulations of water can occur in the 
atmosphere below the —20C isotherm. Therefore thun- 
derstorms are seldom observed without the occurrence 
of above-freezing temperatures through a considerable 
portion of the lower atmosphere. They are most likely 
in tropical air masses warmed from below in areas 
favorable for ascending motion, such as over mountains 
or in large-scale synoptic disturbances. 
In summer, thunderstorms in the belt of the westerlies 
form two different distribution patterns easily recog- 
nized on long-range radarscopes. One consists of an 
irregular spacing of individual storms or masses of 
storms, and the other appears as a line of thunder- 
storms, usually running more or less parallel to the 
low-level wind flow. In low latitudes and in the tropics, 
the first type seems to predominate. In middle latitudes 
lines of thunderstorms are favored, as shown by the 
existence in Ohio of thunderstorm lines on thirty-two 
out of fifty-six days on which thunderstorms were 
observed by long-range radar. On only six of these days 
were the lines directly associated with fronts. Figure 1 
is a photograph of the radarseope at Jamestown, Ohio, 
on July 12, 1947, at 3:25 p.m., showing both types of 
distribution. Thunderstorm echoes appear in irregularly 
arranged masses to the north and in a distinct line to 
the south and southeast. 
The tendency for thunderstorms to form in lines, 
even in the absence of fronts, is not well understood. 
Thunderstorms are sometimes observed imbedded 
within a general nimbostratus cloud cover. These are 
1. Consult ‘‘Precipitation Electricity’? by R. Gunn, pp. 
128-135 in this Compendium. 
681 
found in overrunning unstable tropical air in connection 
with warm fronts of extratropical cyclones. They are 
most common in spring. Sometimes thunder and light- 
ning will be observed with the passage of a cold front 
or the center of a depression on a dark, stormy day in 
winter, even when it is snowing, but most commonly 
only with rain. In maritime climates winter thunder- 
storms are not unusual, occurring with fronts and 
depressions or with the characteristic showers of un- 
stable polar-maritime air. 
Fig. 1—Thunderstorm radar echoes at 1436 EST July 16, 
1947 of PPI scope at Jamestown, Ohio. Range circles are for 
each 50 miles; white area in center is local ground return. 
The most thorough study of the nature of individual 
thunderstorms was that accomplished by the large 
U.S. Weather Bureau-Air Force-Navy-N.A.C.A. Thun- 
derstorm Project from 1946 to 1949, reported in the 
government publication The Thunderstorm [43]. Most 
of the factual material given in the following pages is 
taken from that report, but some speculations, leading 
questions, theory, and new results are added. 
Thunderstorm Structure and Circulation 
Numerous investigators have noted that the thunder- 
storm consists of several cells, each having a distinctive 
convective circulation. Originally the cells have been 
separated but later they come together. After they have 
formed a single thundercloud mass the cells can be 
distinguished by the patterns of vertical motion meas- 
ured on airplanes flying through them. The cell bound- 
aries are identified as narrow zones of inactive or 
nonturbulent cloudy air. 
