ISENTROPIC ANALYSIS 
Comparing this pattern with the 
normal pressure field observed at the 
surface it will be seen that while the 
flow of air in the surface layers over 
eastern United States is uniformly 
from the south and southwest, there 
are regions where the current system 
is reversed aloft. Thus, although the 
lowest layers of air are generally 
characterized by considerable homo- 
geneity, there are sections where this 
moist and warm air, normally of 
Tropical Maritime origin, is overrun 
by dryer air coming from the north. 
Moving southward, the dry air sub- 
sides, so that by the time it reaches 
the core of the anticyclonic eddy it 
has become warmer and drier than 
air anywhere in its vicinity. This 
type of air mass (TS, or S) is discussed 
in Prof. Willett’s chapter on air mass 
properties and in the appendix thereto 
by Mr. Showalter. 
Soundings of type 1 may generally 
be explained on the above basis. The 
stable transition zones between the 
moist and overlying dry air are main- 
tained in part by continued subsi- 
dence. These stable zones exist for 
long stretches of time during the sum- 
mer season, and, when they are es- 
pecially tenacious, periods of drought 
result. They are frequently destroyed, 
however, after the advection of a 
moist air current aloft. Fig. 12 shows 
some typical examples when dry sta- 
ble layers (marked by arrows) were 
destroyed through advection of moist 
air aloft. Experience shows that 
soundings of type 1 will be trans- 
formed into type 3 through advection 
of moist tongues aloft. Through 
turbulent redistribution of heat and 
moisture, and through radiative ex- 
change of heat, soundings of type 3 
may be transformed into type 2. 
Summer showers and_ thunder- 
showers occur frequently with type 3, 
while they are rarely observed with 
153 
type 1. Part of the reason for this 
‘is that the stable layer of type 1 
effectively damps out any impulses 
from below, while the same impulses 
have a better chance of penetrating 
the less stable transition zone of type 
3. But if we consider isentropic mix- 
ing and Parr’s principle (that this 
kind of mixing is more pronounced 
the greater the stability), it becomes 
clear that in type 1 ascending currents 
of moist air from below are quickly 
robbed of their moisture as they enter 
the stable zone, while in type 3 this ef- 
fect is not so pronounced. In the case of 
type 2 the lateral mixing is less pro- 
nounced than in 1 or 3 and, moreover, 
the mixing in this case does not de- 
plete the moisture of the rising cur- 
rent appreciably. Therefore, the con- 
densation levels of type 1 are raised 
to high levels, and latent heat of con- 
densation is not made available for 
the growth of Cu clouds and showers. 
Moreover, the increased lateral mix- 
ing with type 1 reduces the total up- 
ward momentum of impulses. Cumu- 
lus growth is, however, more likely in 
type 3 and most likely in type 2. The 
few thunderstorms observed in con- 
nection with soundings of type 1 are 
likely to be high-level thundershowers, 
and the precipitation from them 
rarely reaches the ground in appre- 
ciable amounts, since it is evaporated 
into dry air below the cloud base. 
In connection with type 3 it has 
been pointed out that frequently the 
lapse rate is so stable in the lower 
layers, even at the time of maximum 
temperature, that upward impulses 
are soon damped out. Convective 
thundershowers which occur with 
such stratifications normally occur at 
night-time, especially in the early 
morning hours. The thunderstorms 
observed over midwestern United 
States in summer appear to be mostly 
of this type. for the diurnal fre- 
