ISENTROPIC ANALYSIS 
take into account any likely changes. 
Suppose, for example, that an energy 
diagram indicates large positive areas 
and that moist air extends to high 
levels—both indications of thunder- 
storm activity during the day. If a 
tongue of dry air is displacing the 
moist air at upper levels, the prob- 
ability of thunderstorms is greatly 
lessened. Then again, the horizontal 
extent of the source of moisture must 
be considered. A narrow jet of moist 
air will suffer lateral mixing with 
the dry air flanking it on both sides, 
and this dessicating process will act 
against thunderstorm formation. The 
showers, if they occur at all, will then 
be restricted to the very central por- 
tion of the moist tongue (along its 
horizontal axis), where the moisture 
is least affected by the admixture of 
dry air. On the other hand, extensive 
regions or broad tongues of moisture 
may remain comparatively unaltered 
by lateral mixing, and thereby pro- 
vide ideal conditions for continued 
thunderstorm activity. 
Thunderstorms caused, at least in 
part, by radiation from upper levels 
are best forecast by the use of energy 
diagrams in conjunction with the 
isentropic chart. The first step is to 
decide what changes in the tempera- 
ture and moisture distribution are 
likely to take place. The changes in 
the flow pattern of moisture are 
brought about mainly through advec- 
tion and lateral mixing. Wind direc- 
155 
tions and velocities on the isentropic 
chart provide the chief indices of the 
magnitude of both these factors. For 
example, lateral mixing is most fa- 
vored in regions where the horizontal 
wind shear is greatest. After con- 
sidering modifying factors, it is neces- 
sary to determine the layer from 
which the principal radiational loss 
of energy will take place. This layer 
is not difficult to place; it is usually 
the most pronounced dry inversion of 
the sounding. If it appears at low 
levels, in general below about 2 km, 
it will be of little significance in help- 
ing large-scale convective activity, as 
was explained in the discussion of 
the lateral mixing in the dry-inver- 
sion. But the radiational emission 
layer, when at higher levels, becomes 
increasingly important, for the heat 
lost to space at the boundary helps set 
off convection through a thick layer 
of atmosphere. The rate of cooling 
at cloud tops is appreciably greater 
than from unsaturated air under the 
same conditions, and therefore the 
nearness to saturation of the moist 
layer must be considered. 
If the lower layers of the atmos- 
phere are too cold, the tephigram will 
indicate that convective energy aloft 
will be dissipated before it can re- 
ceive supplies of moisture from the 
lower layers. In this case, even though 
the chief emission layer is at high 
levels, thunderstorms are not likely 
to occur. 
§ 8. THE PROCESSES WHICH TEND TO DISRUPT THE CONTINUITY OF 
ISENTROPIC ANALYSIS 
From the standpoint of following 
the same sheet of air from day to day 
the ideal method of representation 
would be one in which non-adiabatic 
as well as adiabatic influences were 
taken into consideration. With such 
a method one could construct charts 
along substantial sheets*—that is, 
sheets which contain the same air 
particles from day to day. By fol- 
lowing these identical sheets and 
describing the motion of elements 
with respect to such sheets, one would 
be using the Lagrangian method of 
*Other writers have used the expression 
“equi-substantial sheet’? instead of “‘substan- 
tial’, when referring to a fluid sheet rather 
than to a surface of a solid; but it does not 
seem necessary to make this fine and cumber- 
some distinction.—Ed. 
