684 
environmental air is entrained into it. With entraining 
it would warm at some other, less rapid rate, such as 
CE. If the parcel is dragged downward beyond D or E, 
it will become colder than the environment and sink. 
The frictional drag of the mass of liquid water provides 
the means whereby a parcel in a thunderstorm can thus 
be forced below point D or EH, whence it continues as the 
thunderstorm downdraft. With a large quantity of 
liquid water available for evaporation, saturation can 
be maintained in spite of the creasing temperatures 
during descent and the parcels will reach the ground, 
arriving there with a temperature several degrees lower 
than the surface environmental wet-bulb temperature. 
PRESSURE 
TEMPERATURE 
__ Fie. 5.—Diagram of temperature vs. logarithm of pressure, 
illustrating thermodynamic process in downdraft. 
Thunderstorm Weather near the Surface 
Rainfall. The rainfall pattern follows closely the 
arrangement of the cells and reflects to a considerable 
extent their stages of development. In some regions, 
such as the New Mexico area studied by Workman and 
co-workers [48], single, isolated thunderstorm cells are 
common. In more humid areas such as the eastern and 
southern United States, a single-celled thunderstorm is 
comparatively rare, and when it does occur, it is gener- 
ally weak and not representative of the average thun- 
derstorm. Usually a group of three or more cells join 
together to form a thunderstorm and each cell manifests 
itself in the rainfall pattern. Along with the downdraft 
and area of horizontal divergence, the rain from a newly 
developed cell first covers a very small area and then 
gradually spreads. However, the cold air of the down- 
draft is able to spread laterally from the cell while the 
rain falls directly to the ground, so that an expanding 
outer area of cold air without rain develops. In the 
dissipating stage this cold-air area continues to expand 
while the rain area contracts. 
LOCAL CIRCULATIONS 
If the rainfall is considered with respect to the 
moving cell, it is found that the duration of moderate 
or heavy rain from a single cell may vary from a few 
minutes in the case of a weak, short-lived cell to almost 
an hour in a large, active one. At a fixed point on the 
ground the duration of the rain depends upon such 
factors as the number, size, and longevity of the cells 
passing over the point, the position of the point with — 
respect to each passing cell, and the rate of translation 
of the cell. In the eastern and southern United States 
the average duration of thunderstorm rain at a given 
station is about twenty-five minutes, although it is 
highly variable from case to case. 
The most intense rain occurs under the core of the 
cell within two or three minutes after the first measur- 
able rain from that cell reaches the ground and the 
rain usually remains heavy for a period of five to fifteen 
minutes.-The rainfall rate then decreases, but much 
more slowly than it first increased. Around the edges 
of the cell, lesser rainfall rates occur. 
Wind Field. Early in the cumulus stage there is a 
gentle inward turning of the surface wind, forming an 
area of weak lateral convergence under the updraft. As 
the cell grows and a downdraft develops, the surface 
winds become strong and gusty as they flow outward 
from the downdraft region. The outward-flowing cold 
air underruns the warmer air which it displaces and a 
discontinuity in the wind and temperature fields is 
established. The discontinuity moves outward, pushed 
by the downdraft, resulting in strong horizontal diver- 
gence. Divergence values as high as 8 X 107% sec or 
about 1000 times the values observed in intense cy- 
clones, have been evaluated over a small area of the 
surface micronetworks of the Thunderstorm Project. 
The outflow is radial in the slowest-moving storms 
but mm most cases the wind field is asymmetrical with 
considerably more movement on the downwind side. 
The prevailing air movement of the lower layers nul- 
lifies the radial flow on the upwind side. With respect 
to the moving cell the outflow may still be radial in 
character, although not with respect to the ground. 
Thus the wind discontinuity in most cases is easily 
detected only in the forward portions of the storm, 
where it appears as a micro-cold front. 
The cold dome of outflowing downdraft air has a 
form illustrated in Fig. 6. In this sketch the thunder- 
storm cell is considered to be in the mature stage and 
is moving from left to right. The cold air is represented 
as having spread out considerably farther on the down- 
wind side of the cell than on the upwind side, as would 
be expected in a moving system. 
In 15 to 20 min after the outflowing starts, the dis- 
continuity zone has traveled about five or six miles 
from the cell center. The surface winds near the dis- 
continuity are still strong and gusty but, well within 
the cold-air dome, the surface wind speeds have de- 
creased so that the strongest winds are no longer under- 
neath the cell itself. A continued settling of the outflow 
air, transporting momentum downward, causes the wind 
speed to increase as one approaches the discontinuity 
zone from within the cold air. 
