THUNDERSTORMS 
cell development is in the area between two existing 
cells whose edges are three or less miles apart. A three- 
mile band downwind is next in importance, then the 
lateral edges, and least frequently the upwind or rear 
side. 
In some cases the time interval between the begin- 
ning of the outflow and the appearance of the new cell 
on the radarscope is too short to permit explanation of 
the new development as a result of the underrunning 
cold air or similar time-consuming process. There are 
cases, as indicated by the radar echoes, in which one or 
a cluster of new cells comes mto existence almost 
simultaneously with the initial or parent cell, which 
suggests that a preferred region of convergence and 
ascent favors the development. 
Cloud Structure in Relation to Thunderstorm 
Electricity 
Atmospheric electricity and its phenomenal aberra- 
tions in thunderstorms are treated in other articles in 
this Compendium.!:* A few observations might be made 
at this pomt relating the meteorological thunderstorm 
structure and dynamics to the electrical structure and 
electrodynamics. 
In recent years abundant information has been pre- 
sented indicating that thunderstorm electrification is 
somehow associated with the ice phase in the cloud or, 
more probably, with a heterogeneous mixture of liquid 
and solid condensates. For example, recent studies by 
Workman and Reynolds [48] and by the U. S. Thunder- 
storm Project [43] show that lightning does not occur 
until the visible cloud top has ascended to where the 
temperature is less than about —28C or the top of the 
echo seen on a 3-cm height-finding radar at a range of 
twenty miles has surpassed the isotherm of —20C. 
A number of studies indicate that the upper positive- 
charge center is located where the temperature is near 
—20C and the lower negative center is at a temperature 
of 0 to —10C (roughly). Between these two centers is 
the lightning hearth region, that is, the region where the 
first lightning develops. In these temperature ranges 
mixtures of liquid and solid precipitation particles are 
commonly observed in growing and established thun- 
derstorms. 
Workman and Reynolds [49] discovered an effect, 
earlier indicated indirectly by Dinger and Gunn [11], 
involving the freezing of water, which offers a clue as 
to the processes which might produce electrification in 
the freezing parts of thunderstorms. They found that 
water containing contaminants which are likely to be 
found in the atmosphere could produce, as it freezes, 
potentials of more than 150 volts across the ice-water 
interface. Extremely dilute solutions of the order of 
10 to 10 normal, constituting very pure water by 
industrial standards, produce the desired effect. Calcium 
hydrocarbonate, CaH(CO;)s, formed from calcium car- 
bonate in the presence of carbon dioxide, both found in 
the atmosphere, is observed to transfer negative ions 
3. Consult ‘‘Universal Aspects of Atmospheric Electricity” 
by O. H. Gish, pp. 101-119. 
687 
across the interface to the ice, leaving the water positive 
and the ice negatively charged. Ammonium compounds 
and acids or substances with high pH produce the 
opposite polarity, that is, with the water negative and 
the ice positive. Workman suggests that in the cloud 
some of the water freezing around an ice pellet is torn 
off and carried upward as small droplets in the cloud 
while the frozen core, being larger and heavier, falls. 
If the cloud particles contain a contaminant, such as 
calcium hydrocarbonate, which produces positive water 
and negative ice in the freezing process, the carrying 
upward of the positively charged portion and the de- 
scent of the negative ice would produce the observed 
polarity of thunderstorms at these levels. 
Observations by Weickmann [44] and Kuettner [19] 
on the Zugspitze in the Bavarian Alps, where the 
observatory was at or near the freezing level in most 
thunderstorms, indicate that a predominant form of 
solid precipitation other than ordinary snow is a graupel 
made up of an agglomeration of tiny supercooled drop- 
lets to form rime clusters. An ice core with a film of 
liquid around it, as might be expected in a hailstone, 
would be required for Workman’s hypothesis. Further 
observations of the exact nature of precipitation par- 
ticles in the freezing regions of thunderstorms are 
needed in order to clarify the discussion. The precise 
recognition of these precipitation forms is next to im- 
possible from an airplane. 
Kuettner also found that the region of greatest light- 
ning activity coincided with the downdraft and heavy 
rain core. Furthermore in this center of activity there 
was frequently a small center of positive charge em- 
bedded within the main lower negative charge region 
and centered on the 0C isotherm. This corresponds to 
some extent with the picture given by Simpson and 
Robinson [82] of an isolated lower positive-charge 
center. Observations of what appeared to have been a 
lower positive charge in an area of heavy rain and down- 
draft were obtained in a series of traverses through the 
lower part of a thunderstorm on one of the Thunder- 
storm Project flights. This is another phenomenon that 
needs further investigation. 
In seeking to explain the electrification of thunder- 
storms, most scientists have looked to aspects of the 
updraft. However, the Thunderstorm Project data indi- 
cate that the greatest lightning activity isin the region 
of downdraft and heaviest rain, as noted by Kuettner. 
It is not entirely logical to conclude from this, however, 
that the charge-generating mechanism occurs in the 
downdraft, because the boundary separating updraft 
from downdraft in the mature stage is not vertical and 
a region of updraft may be found over a downdraft. 
On the Thunderstorm Project a study was made 
using recording point collectors located on the ground 
and a height-finding radar scanning the thunderclouds 
from a point about twenty miles away. The following 
conclusions concerning lightning were indicated by the 
data: 
1. The cloud tops (echo tops) reach a height where 
the environmental temperature is around —20C before 
the first ightning occurs. 
