ISENTROPIC ANALYSIS 
157 
of the isentropic surface of about 
300 m in one day. Nevertheless, it is 
at times necessary to introduce this 
factor to explain changes in moisture 
content or height of the isentropic 
surface which cannot satisfactorily be 
explained by advection or other causes. 
Where the isentropic surface is not 
far from snow-covered mountains, 
cooling by radiation and eddy trans- 
fer of heat must frequently be con- 
sidered. 
Opposite effects are observed when 
there is non-adiabatic heating. Then 
a substantial surface has its poten- 
tial temperature raised so that the 
isentropic surface is lowered, and 
since the specific humidity normally 
decreases with elevation, there ap- 
pears to be an increase in moisture 
within the affected area of the isen- 
tropic chart. This type of non-adi- 
abatic modification is _ significant 
when the isentropic surface is near 
the ground. The influence is greatest 
In continental areas during the sum- 
mer season. It is also important in 
mountainous country during all sea- 
sons. 
Let us consider next the non-adiab- 
atic effects produced by evaporation 
and condensation. Since condensation 
liberates and evaporation consumes 
heat, it becomes important to know 
whether these processes occur above, 
within or below the isentropic sheet 
under discussion. 
If the chosen isentropic surface lies 
above the region where condensation 
occurs, its characteristics will not be 
materially affected. An example is 
afforded by the instability snow show- 
ers of polar continental air masses of 
winter. These flurries are generally 
formed in a shallow layer of air next 
to the earth’s surface—a layer which 
is far below the representative isen- 
tropic surfaces which are chosen so 
as not to intersect the ground even 
in tropical air. In this case isen- 
tropic surfaces remain practically sub- 
stantial surfaces. 
If condensation and precipitation 
set in within the chosen isentropic 
sheet, latent heat is liberated and the 
potential temperature of the substan- 
tial surface is raised. The isentropic 
surface is then found at lower levels, 
and, since the specific humidity nor- 
mally increases downward, the specific 
humidity in the isentropic surface in- 
creases. This increase might errone- 
ously be interpreted as being due to 
advection from a neighboring source 
of moisture. 
When precipitation falls through 
an isentropic sheet which is not sat- 
urated with moisture, evaporation 
will cool the air while the specific 
humidity increases. This lowers the 
potential temperature of the substan- 
tial surface, and raises the isentropic 
surface. If the moisture content de- 
creases with elevation the mixing 
ratio at the chosen isentropic surface 
decreases in proportion to the humid- 
ity gradient. On the other hand, if 
the moisture content increases with 
elevation (as it often does along well 
defined frontal surfaces) the mixing 
ratio will increase. The increase in 
moisture with elevation is usually so 
slight that, though the isentropic sur- 
face rises, little change in the pattern 
of moisture results. 
Probably the most significant proc- 
ess at work in causing isentropic sur- 
faces to depart from substantial sur- 
faces is convection, because vertical 
currents are highly effective in trans- 
porting moisture to higher levels. If 
it were not for the replenishment of 
moisture by convective currents, the 
moist tongues which are not associated 
with upslide motion along frontal sur- 
faces would soon be dissipated through 
lateral mixing with the dry air flank- 
ing them. 
