22 AIR MASS ANALYSIS 
ility of the Polar Canadian air. The 
TG curve shows the warm and moist 
character of the tropical maritime air. 
The Rossby diagram is helpful in 
the treatment of stability. Thermo- 
dynamic diagrams, for the most part, 
deal with the stability or instability 
of a particle of air with respect to 
its surroundings. This is of import- 
ance in penetrative convection, such 
as in cumulus cloud and thunderstorm 
formation. However, in the more im- 
portant types of convection, that is, 
in the case when a layer of warm 
air of large horizontal extent is 
forced over an underlying cold air 
wedge or a mountain range, the 
classical energy diagrams fail to give 
a measure of the potential energy 
available in the layer. It is here that 
the Rossby diagram is invaluable. 
It is true, however, that in regard to 
particle stability the classical dia- 
grams are more useful than the 
Rossby diagram, but even in this 
type of stability it is not difficult to 
apply the equivalent-potential tem- 
perature diagram of Rossby. This 
may be done with the aid of the lines 
of equal potential temperature (the 
horizontal lines) and the elevations 
of the points which may be conveni- 
ently indicated by figures beside the 
individual points of the sounding. 
Thus if there is no increase in po- 
tential temperature through a layer, 
the lapse rate is equal to the dry 
adiabat, or 1 C deg. per 100 m. If 
there is an increase in potential tem- 
perature of 10 deg. in 1000 m the 
lapse rate is isothermal. In general, 
the greater the increase in poten- 
tial temperature with elevation the 
greater the stability. If the potential 
temperature decreases with elevation, 
the lapse rate is superadiabatic. 
The second type of stability, that 
within a layer which is being dis- 
placed vertically, is best treated by 
the Rossby diagram. From the slope 
of the characteristic curve relative 
to the slope of the lines of constant 
6, one can determine whether the 
layer in question is convectively un- 
stable (sometimes called potentially 
unstable). This condition is defined 
as one in which the equivalent-poten- 
tial temperature decreases with ele- 
vation, and is indicated on the Rossby 
diagram by a line which possesses a 
slope between those of the lines of 
constant 6, and constant §. In other 
words the line representing convective 
instability is one which lies between 
the potential and the equivalent-po- 
tential isotherms. The importance of 
this particular distribution of tem- 
perature and moisture lies in the fact 
that lifting of the entire layer brings 
about a more unstable condition, in 
fact, if the layer is lifted sufficiently 
it will eventually become unstable 
with respect to dry (unsaturated) 
air. The following illustration will 
serve to show this increasing in- 
stability. Suppose we have the layer 
CD, which is by definition convectively 
unstable, having the equivalent-poten- 
tial temperature 820°A at the base 
of the layer and 315°A at the top. 
If the layer is now lifted pseudo- 
adiabatically to the top of the atmos- 
phere, the base of the layer will have 
an equivalent-potential temperature 
greater than that at the top of the 
layer by 5 C deg., the additional heat 
being supplied to the bottom of the 
layer by the extra heat of condensa- 
tion given to it by the greater amount 
of water vapor at the base compared 
with the top of the layer. This de- 
crease of potential temperature, of 
course, denotes instability with re- 
spect to dry air. If the temperatures 
at the top and base of the layer were 
taken at successive intervals in the 
ascent of the stratum the difference 
