THUNDERSTORMS 
The most interesting of all surface weather features 
associated with the thunderstorm is the discontinuity 
zone, which everywhere marks the limit of the cold 
downdraft air. At a station passed by the cell core in 
the early mature stage, there is a sharp increase in the 
wind speed, occasionally to destructive force, and a 
marked reduction in temperature, occurring with the 
passage of the discontinuity zone. The sharp increase 
in wind has been termed the “‘first gust,” since it often 
appears as the first major gust of a period of strong, 
gusty winds. After the cold air has spread outward 
from the cell core in the middle and late mature stage, 
the wind speeds near the boundary of this air decrease. 
It is often found that the discontinuity zone is displaced 
faster than the normal component of the winds im- 
mediately behind it. The rapid downflow of cold air 
toward the ground along the boundary zone thrusts 
the discontinuity surface forward with a momentum 
transported from above. 
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—> 
DIRECTION OF 
MOVEMENT 
18 20 22 24 26 28 30 
Da a 6 ui) LA 14 16 
HORIZONTAL DISTANCE (THOUSANDS OF FT.) 
HEIGHT ABOVE TERRAIN (THOUSANDS OF FT.) 
ney Oo ES Gr) Se) () 
Fie. 6—Schematic cross section showing formation of cold 
dome from downdraft. Stippling represents falling rain. Cold-air 
boundary outlined below the cloud. 
With the discontinuity, the wind shows clockwise 
shifts in most cases. This is especially true in American 
tropical air currents in middle latitudes where the 
winds usually are from southwest or south and shift to 
west or northwest at the discontinuity. 
Temperature. The “first gust” and the ‘‘temperature 
break,” z.e., the point on the thermogram where the 
temperature suddenly starts its drop, are two of the 
most pronounced features observed at the surface, and 
they occur essentially together. At the time of formation 
of summer afternoon thunderstorms in the United 
States the temperature is usually above 85F, often in 
the 90’s. As a result of the rain and the downdraft, the 
temperature may reach a value as low as 65F without 
change in air mass. As the thunderstorm activity dies 
down after sunset, the temperature usually has recov- 
ered to an intermediate value representing a mixture of 
strongly cooled and less cooled or uncooled portions of 
the air mass. 
The area affected by the cooling is many times greater 
than that over which rain falls, but the temperature 
685 
change is, of course, most marked in the rain core 
(downdraft center). Cooling may be detected as much 
as fifteen to twenty miles downstream. Near the center 
of a mature cell the temperature reaches a minimum 
ten to fifteen minutes after the temperature break; 
farther from the cell the temperature drop is much 
slower. The amount of the temperature decrease ob- 
served in any given storm varies inversely with the 
distance of the observation point from a cell core. The 
temperature discontinuity is sharp and well defined 
under the core but is less pronounced farther away. 
Since the downdraft is only a few miles in diameter, 
the area over which the first and most rapid temperature 
fall occurs is relatively small. As a result, strong hori- 
zontal temperature gradients are created after the 
downdraft first reaches the ground. Gradients exceed- 
ing 20F per mile have been observed. As the storm 
ages, the cold air spreads out and the magnitude of the 
horizontal temperature gradient decreases. Regardless 
of the spread of the cold air, the area of minimum 
temperature remains in the general location where the 
cold downdraft made its first appearance at the surface, 
except in cases where a new cell with its own downdraft 
and rain core develops over another part of the cold 
dome. 
Pressure. Karly in the cumulus stage a fall in surface 
pressure almost invariably occurs. This fall is observed 
before the radar echo forms, and it is recorded over an 
area several times the maximum horizontal extent of 
the echo. When the radar echo appears, the pressure 
trace levels off in the region directly underneath it, but 
continues to fall, and frequently at a more rapid rate, in 
the surrounding areas. The pressure drops in the 
cumulus stage are usually small in magnitude—less 
than 0.02 in. (0.67 mb) below the diurnal trend—and 
take place over a period of 5 to 15 min. Following the 
fall, the pressure trace remains steady for as long as 
30 min. 
The pressure falls appear to be caused by the com- 
bined effects of vertically accelerated air motions, the 
expansion of the air due to the release of the latent heat 
of condensation, and the failure of the convergence near 
the surface to compensate fully the expansion or diver- 
gence aloft. Wind patterns in the vicinity of thunder- 
storms showed velocity convergence in the developing 
stages with divergence above 20,000 ft, suggesting a 
mass balance. 
In the mature stage, two features of the pressure 
trace—the “dome” and the “nose’’—are recognized. 
The dome is registered at all stations to which the cold 
outflow air penetrates. The pressure nose, the abrupt, 
sensational rise that some meteorologists regard as 
typical of the thunderstorm, really occurs only at sta- 
tions that happen to be passed by the main rain and 
downdraft just after they have first reached the earth 
in the beginning of the mature stage. It is superimposed 
upon or may mark the start of the pressure dome. 
The displacement of the warmer air by the cold 
outflowing air from the downdraft results in the pres- 
sure rise, initiating the pressure dome. A study of 206 
thunderstorm pressure records from the surface micro- 
