THE WARM FRONT 23 
in temperature would be seen to be- 
come greater and greater. Thus any 
layer of air which was originally 
convectively unstable, when subjected 
to lifting, will acquire a steeper lapse 
rate, eventually reaching a state of 
instability with respect to dry air. 
Inspection of fig. 5 will reveal that 
a decrease in @, with elevation may 
be brought about in different ways. 
For example, it may be due to a 
rapid drop in temperature with ele- 
vation (that is, a small change in po- 
tential temperature), a rapid decrease 
in the moisture content with elevation, 
or of course a combination of both. 
Reference to the relative humidity 
distribution and lapse rate is suffi- 
cient to indicate to the synoptic 
meteorologist whether the convective 
instability is due to the moisture or 
to the temperature distribution. In 
this connection it is well to note that 
convective instability may or may not 
carry with it conditional instability. 
Similarly, conditional instability is 
not necessarily accompanied by con- 
vective instability. 
The idea of convective instability 
may perhaps be made clearer by deal- 
ing with a special case in which the 
base of a stratum is nearly satu- 
rated, while the top is very dry. Lift- 
ing of this layer will lead to satura- 
tion of the base long before the upper 
layer. Thus the lower part of the 
layer cools at the rate for saturated 
air at those temperatures, while the 
upper part of the layer cools at the 
adiabatic rate for dry air. It is 
obvious that the base of the layer is 
becoming warmer and warmer rela- 
tive to the top of the layer, thereby 
increasing the lapse rate. Further- 
more, even after the entire layer 
becomes saturated, the base of the 
layer is receiving more heat of con- 
densation than the top of the layer. 
The mere existence of some layer 
or layers of convective instabiiity 
within an atmospheric sounding does. 
not mean that the energy stored 
therein will be released in the form 
of convection. The meteorologist must 
consider whether’ vertical forces 
brought about by thermal or mechan- 
ical convection will come into play 
sufficient to lift the layer enough to 
convert the potential energy of the 
layer into kinetic energy. In this 
connection reference must be made 
to the synoptic chart. 
If the equivalent-potential tem- 
perature increases with elevation the 
state is one of stability with respect 
to dry or saturated air, and no adia- 
batic process performed upon the 
layer can render it unstable. 
Another important use of the 
Rossby diagram is in distinguishing 
between temperature inversions 
which have developed within the same 
air mass and those which are the 
result of a warm current overrunning 
a cold wedge. In the latter case the 
warm current generally has, level for 
level, a much higher moisture content 
than the cold current. For example, 
let us suppose that we are dealing 
with a wedge of cold air which had 
its source over northern Canada 
while a current of warm moist air 
from the Gulf of Mexico is overrun- 
ning it. A sounding made through 
the cold wedge of air into the warm 
current would then show a rapid 
increase of moisture and potential 
temperature. The rapid increase is 
shown in the Rossby diagram (curve 
EF, fig. 5) where the wedge-like 
curve shows a maximum specific hu- 
midity aloft. In one and the same 
air mass the specific humidity gen- 
erally falls off gradually. This would 
be expected in view of the fact that 
all atmospheric water vapor origin- 
ates at the earth’s surface. On the 
other hand, soundings obtained 
