682 
The life cycle of the thunderstorm cell [7] naturally 
divides itself into three stages determined by the magni- 
tude and direction of the predominating vertical 
motions. These stages are: 
1. The cumulus stage—characterized by an updraft 
throughout the cell. 
2. The mature stage—characterized by the existence 
of both updrafts and downdrafts, at least in the lower 
half of the cell. 
3. The dissipating stage—characterized by weak 
downdrafts throughout. 
Cumulus Stage. Throughout the cell there is an up- 
draft which is strongest at the higher altitudes and 
increases in magnitude toward the end of this stage 
(Fig. 2). Converging air feeds the updraft not only from 
the surface but also from the unsaturated environment 
at all levels penetrated by the cloud. Thus, air is en- 
trained into the cloud and is accommodated by the 
evaporation of some of the liquid water carried in the 
updraft. This entraining continues throughout all of 
the stages. 
SAE TSIEN 
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HORIZONTAL SCALE Lo = Mi. 
1 
DRAFIT WECTOR SCALE LIL Fry SEC 
eee 
* SNOW 
Fie. 2.—Vertical cross section in cumulus stage showing ver- 
tical motions, inflow, hydrometeors, and temperature distribu- 
tion. 
In-cloud temperatures in a strongly developing cell 
are higher than those of the environment at correspond- 
ing altitudes. The greatest differences between cloud 
and environment temperatures are in the regions of 
strongest updrafts, which are in the upper parts of the 
cloud and which are especially well developed at the 
end of the stage. 
Although pilots flying through the clouds in this 
stage report rain or snow, particularly near the end of 
LOCAL CIRCULATIONS 
the stage, these condensation products appear to be 
suspended by the updraft, since no precipitation is 
observed at the ground. As a matter of practice, it is 
found that the end of the cumulus stage and beginning 
of the mature stage can be signalized by the occurrence 
of precipitation at the ground. In the cumulus stage 
there appears to be a considerable concentration of 
hydrometeors at or slightly above the freezing level in 
the form of liquid or solid or a mixture of the two. The 
stronger the updraft the greater is the vertical thickness 
of the transition zone between water and ice. 
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HORIZONTAL SCALE ws MI. * SNOW 
DRAFT VECTOR SCALE 23° FT/SEC <-ICE CRYSTALS 
Fic. 3.—Vertical cross section in mature stage showing ver- 
tical motions, inflow, hydrometeors, and temperature distribu- 
tion. 
Mature Stage. The mature stage begims when rain 
first falls distinctly out of the bottom of the cloud. 
Except under arid conditions, the rain reaches the 
ground. The size and concentration of the drops or ice 
particles have now become so great that they cannot be 
supported by the updraft and they begin to fall relative 
to the earth. The frictional drag exerted by the precipi- 
tation helps to change the updraft into a downdraft 
which, once started, can continue without this frictional 
drive, as will be demonstrated later in the discussion of 
the thermodynamic process. The beginning of the rain 
at the surface and the initial appearance of the down- 
draft are nearly simultaneous. The downdraft appears 
to start in the vicinity of the freezing level, later growing 
in vertical as well as in horizontal extent (Fig. 3). 
