THUNDERSTORMS 
The updraft also continues and often reaches its 
greatest strength in the early mature stage in the upper 
parts of the cloud. Updraft speeds may locally exceed 
80 ft see. The downdraft is usually not as strong as 
the updraft and is most pronounced in the lower part of 
the cloud, although weakening near the ground. The 
descending air is forced to spread laterally at the surface 
of the earth. Thus areas of rain, downdraft, and hori- 
zontal divergence are found together at the ground. 
——— SS SS 
10,000 
Jewne 
3) OOO) —_4 
= Qe OD 
i tag ar eo amen en 
ET Light & Res ACS teal 
ul 
SURFACE DT EUR Oe 
SEEN AEA SEN ENE 
: ie - RAIN 
HORIZONTAL SCALE 2 3 mi 
. » SNOW 
DRAFT VECTOR SCALE 72? FT/SEC + ICE CRYSTALS 
Fie. 4.—Vertical cross section in dissipating stage showing 
vertical motions, inflow, hydrometeors, and temperature dis- 
tribution. 
Temperatures are low in the downdraft, compared 
with the environment, and contrast especially with the 
updraft temperatures. The greatest negative tempera- 
ture anomalies are found in the lower levels. As might 
be expected, there is a close association between strong 
updraft and high temperature and between strong 
downdraft and low temperature. 
In the updraft, mixing of entrained air causes evap- 
oration of some of the liquid water thus removing some 
of the heat gained from condensation. Instead of the 
simple wet-adiabatic rate, the updraft air cools at a 
somewhat greater ‘‘entraining-wet-adiabatic” rate. This 
does not permit the updraft air to become very much 
warmer than the environmental air. The downdraft 
seems to be characterized by reversible wet-adiabatic 
temperature increases in which evaporation counteracts 
to some extent the compressional effects. Since the 
downdraft starts out at a temperature very near that of 
the environment, its wet-adiabatic descent assures that 
it will be colder than the environment which has a lapse 
rate greater than the wet adiabatic. The cold downdraft 
spreads out at the surface as a cold air mass to form the 
683 
well-known pseudo-cold front advancing against the 
warmer surrounding surface air. 
The mature stage represents the most intense period 
of the thunderstorm in all its aspects. At the ground 
heavy rain and strong winds are observed while in the 
clouds the airplanes encounter at this stage the most 
severe turbulence, including in addition to the drafts 
the short, tense accelerations known in aeronautics 
as “susts.”’ Hail, if present, is most often found in this 
stage. From a study of radar echoes it has been con- 
cluded that parts of the cloud containing appreciable 
quantities of water in crystalline form extend to heights 
of 60,000 ft on some occasions. With very strong up- 
drafts it is possible for liquid water to be carried well 
above the freezing level. On the Thunderstorm Project 
in Ohio one case of heavy rain at 26,000 ft, nearly 
10,000 ft above the environmental freezing line, was 
reported. 
Dissipating Stage. When the updraft disappears and 
the downdraft has spread over the entire area of the 
cell, the dissipating stage begins (Fig. 4). Dissipation 
results from the fact that there is now no longer the 
updraft source of condensing water. As the updraft is 
cut off, the mass of water available to accelerate the 
descending air diminishes, so the downdraft also dimin- 
ishes. The entire cell is colder than the environment as 
long as the downdraft and the rain persist. As the down- 
drafts give out, the temperature within the cell rises toa 
value approximately equal to that of the surroundings. 
Then complete dissipation occurs or only stratified 
clouds remain. All surface signs of the thunderstorm and 
its downdraft disappear. 
Thermodynamics of Entraining and the Downdraft 
Computed and observed inflow rates in American 
thunderstorms show that the cumulus cloud which 
develops into a thunderstorm entrains environmental 
air at a rate of approximately 100 per cent per 500 mb 
of ascent, that is, it doubles its mass as it rises through 
a pressure decrease of 500 mb. This is a relatively low 
rate of entrainment.2 With very much higher rates of 
entrainment, the updraft would be theoretically colder 
than the environment, but this is not found to be the 
case except in isolated or transitory conditions. Entrain- 
ing rates greater than 100 per cent per 250 mb would 
prevent the growth of cumulus clouds to heights great 
enough to produce thunderstorms in American maritime 
tropical air. In less favorable air masses the critical 
entraining rate would have to be lower. 
At the present stage of theory and observation of 
entraining, the updraft seems to follow the required 
thermodynamic pattern. The downdraft is a special 
case, however. In Fig. 5 the updraft entraining-wet- 
adiabatic rate in a typical, well-developed growing 
cumulus cloud is represented by line A’B’ and a typical 
environmental lapse rate for American tropical air is 
given by line AB. A saturated parcel displaced down- 
ward from C would follow the wet-adiabatic CD, if no 
2. Consult ‘Cumulus Convection and Entrainment” by 
J. M. Austin, pp. 694-701 in this Compendium. 
