700 
the positive area is greater than the negative area, 
that the positive area extends beyond the OC isotherm, 
and that the surface heating is sufficient to mitiate the 
convection. This method is open to serious criticism 
since the cloud air does not follow the path ABPD 
and hence the positive and negative areas have no 
significance as regards the cumulus development. Fur- 
thermore, the negative area is zero when the cloud 
commences to develop. 
A more appropriate application of the parcel-method 
technique is to determine the convective condensation 
level, C in Fig. 8, and to estimate the heating necessary 
to initiate convection, 7z—T4. If this heating is ex- 
pected, then cumulus clouds should develop to the level 
D. This method may be criticized on the same 
theoretical grounds that make the parcel-method an- 
HEIGHT 
TEMPERATURE ——> 
Fra. 8.—The parcel-method analysis of cumulus convection. 
The dashed line is a moist adiabatic and the dotted line is a 
dry adiabatic. 
alysis untenable. Furthermore, observations show that 
this technique overestimates cumulus development. 
Also this method offers no satisfactory explanation for 
the failure of cumulus clouds to develop whenever 
there is dry air in the middle troposphere. : 
Beers [4] has proposed a forecast technique which 
is based upon the slice method. The method treats 
layers of finite thickness and, therefore, 1s an attempt 
to apply the slice-method technique to a deep layer 
of air. This method assumes that the cloud air rises 
adiabatically and hence it is open to question. However 
it may be possible to modify the method so as to allow 
for the mixing. The desirable feature of Beers’ technique 
is the treatment of the cloud growth as the development 
of a circulation cell. 
The concept of the entrainment of outside air suggests 
an alternate procedure for the prediction of cumulus 
convection through heating which may be summarized 
as follows: 
1. The surface dew-point temperature to be expected 
about the time of cloud formation is estimated. This 
LOCAL CIRCULATIONS 
determines the convective condensation level. The heat- 
ing necessary to initiate cumulus growth can be de- 
termined by drawing a dry-adiabatic from the con- 
vective condensation level to the ground. If it is ex- 
pected that this heating will occur during the forecast 
period, some cumulus development may be expected. 
Since these temperature estimates are based on widely 
scattered observations, the forecaster should tend to 
overestimate the dew-pomt and maximum temper- 
atures. This overestimation is justified as there are 
probably locations nearby which are more favorable 
for cumulus development than directly over the isolated 
reporting stations. 
2. If the lapse rate of temperature above the con- 
vective condensation level is less than the moist-adia- 
batic lapse rate, no cumulus development is to be ex- 
pected. If the lapse rate of temperature is in excess of 
the moist-adiabatic rate, the degree of cumulus develop- 
ment should be based upon the relative humidity dis- 
tribution above the convective condensation level. The 
forecaster may be aided by the statistics given by Austin 
{1] and Chalker [7]. 
3. A layer with low relative humidity, or a layer with 
a lapse rate less than the moist-adiabatic rate above the 
convective condensation level, definitely disfayors cu- 
mulus development. 
4. These estimations may be made from radiosonde 
observations taken prior to the forecast period, but here 
consideration must be given to the probable change in 
the lapse rate of temperature and relative humidity 
during the forecast mterval. It is suggested that this 
estimate be based on the trend as illustrated by a 
stability chart which depicts the lapse rate of temper- 
ature from 850 mb to 500 mb and the relative humidity 
within the same layer (see Chalker [7]). 
Suggestions for Research 
The introduction of the concept of entrainment and 
mixing may be considered an advance in the analysis 
of cumulus convection. It recognizes that a cumulus 
cloud does not grow upward as an isolated column of 
saturated air which cools at the moist-adiabatic rate. 
However, many theoretical problems remain to be 
solved. No satisfactory procedure has been offered 
whereby the degree of entrainment and mixing may be 
estimated and recent analyses of entrainment have 
failed to take into consideration the disturbed state of 
the environment. As a consequence of this descending 
environment the cloud is continually in a state of 
change which presents serious theoretical problems if it 
is desired to know the physical properties of the cloud 
at a given time. More theoretical research is clearly 
indicated. 
One feature of cumulus growth which requires more 
consideration is the significance of the surface heating. 
In the past the tendency has been to concentrate at- 
tention on the cloud after it starts to develop and to 
consider that the surface heating is only important in- 
sofar as it mitiates the cloud development. However, 
it appears logical to expect that the degree of the surface 
heating, beyond that necessary to start cloud growth, 
