340 
CHESTER W. NEWTON 
Fig. 1—Ambient and in-cloud winds, convective rainstorm imbedded in 
shearing current; Vp shown by double-shafted arrows 
triggering of new convection by cold air flowing 
outward from beneath a thunderstorm is en- 
hanced if the air brought down in downdrafts has 
initially high horizontal momentum. In the fol- 
lowing, an attempt will be made to analyze the 
broad-scale aspects of this process in terms of the 
physical forces at work. 
Induced pressure field and new cloud growth— 
At a given level in Figure 1, the rainstorm may 
be regarded as an obstacle in motion relative to 
the ambient winds. If Vz is the velocity in the 
undisturbed environment (for example, V, or Vz 
in Fig. 1), and V. the mean in-cloud velocity, 
the basic relative motion is Ve = Ve — V-. 
Hydrodynamic pressure is defined as 
P= Pp = Dr (1) 
being the departure from the hydrostatic pressure 
in the undisturbed environment. Aerodynamics 
experiments [Goldstein, 1938] show that, in the 
case of a circular cylindrical obstacle, a positive 
pressure P = pV,*/2 is induced on the upwind 
side (with respect to Vr), and negative pressures 
shghtly larger on the lateral and downwind sides. 
In Fig. 1, the sign of P is indicated on the 
right flank at upper and lower levels; the signs 
are opposite on the left flank. Evidently one 
effect. of this pressure distribution is to tilt the 
cloud along the direction of the vertical shear. 
Substitution of p from (1) into the equation 
for vertical acceleration gives 
dv (aT _P | ap 
dt di dB P Op 
Here AT is the excess of temperature relative to 
the undisturbed environment. On the right flank, 
the second term (not always negligible) is small 
compared with the third, when averaged through 
the depth of the cloud. 
In general, for ‘triggering’ convective instabil- 
ity, a certain amount of lifting is required to 
reach a state of free convection (positive thermal 
buoyancy). During lifting, the air becomes tem- 
porarily cooler than in its undisturbed state. Eq. 
(2) shows that lifting can be accomplished by the 
hydrodynamic pressure force, if there is a strong 
enough decrease of P with height. 
On the right flank of the rainstorm in Figure 1, 
the induced pressure field favors such lifting. 
Thus, the same pressure field that acts to shear the 
cloud away from the vertical, tends to promote new 
convective cloud growth on the downshear side. 
Radar observations [U. S. Weather Bureau, 
1949; Ligda, 1956] show that nonpropagating 
thunderstorm cells move very nearly in the direc- 
tion of the mean wind in the cloud layer (or the 
700-mb wind). If growth on the right flank pre- 
dominates, this should show up as a tendency for 
rainstorms to deviate toward right of the winds. 
In case studies using hourly rainfall data [Vewton 
and Katz, 1958] this was consistently observed, 
rainstorms moving on the average about 25° 
(10-15 knots vector velocity) to right of the mean 
wind, in situations where the wind veered mark- 
edly with height. 
QUANTITATIVE ESTIMATES 
Relative motions between cloud and environment 
—Because the induced pressure field depends on 
the relative motions, it is essential to establish 
the probable magnitude of Vz to see whether the 
process described above can be quantitatively 
significant. Pressure forces acting horizontally 
across the cloud (Fig. 1) tend to accelerate it, at 
