CONSERVATIVE PROPERTIES OF AIR MASSES 7 
point. Furthermore, quantities indi- 
rectly obtained by calculation from 
the observations will vary in con- 
stancy. The relative degree of con- 
stancy of a meteorological quantity 
within a moving air mass is defined 
as its conservatism. 
We are now in a position to test 
the meteorological elements and the 
inGirectly calculated quantities with 
respect to their degree of constancy 
(conservatism) as the air mass moves. 
A. TEMPERATURE. 
The temperature of any given par- 
ticle of air within a moving air mass 
is influenced by the following factors: 
1. Conduction and mixing. 
2. Condensation and evaporation. 
38. Expansion and compression 
(adiabatic changes). 
4, Insolation and radiation. 
Temperature, particularly at the 
surface, is so much changed by these 
factors that it cannot be regarded as 
a very representative element by 
which to identify an air mass after 
it has moved away from the source 
region. 
The effect of condensation may be 
eliminated by the use of a quantity 
called equivalent temperature.* This 
is defined as the temperature a parti- 
cle of air would have if it were made 
to rise adiabatically to the top of the 
atmosphere in such a manner that all 
the heat of condensation of the water 
vapor were added to the air and the 
sample of dry air were then brought 
back to its original pressure. Numeri- 
eally this is not much different from 
the temperature the mass of air would 
have if all its moisture were made to 
condense and the heat given off by 
condensation were added to the re- 
maining dry air. Any change in the 
moisture content of the air mass by 
condensation will not affect the equiv- 
alent temperature of a particle, since 
the quantity of moisture subtracted 
by condensation involves a certain loss 
‘or gain of heat which is implied in 
the definition of equivalent tempera- 
ture. 
Evaporation does not greatly change 
the equivalent temperature; but this 
is not an important limitation for our 
practical purpose*. 
Changes in the temperature of a 
particle by means of expansion or 
compression (adiabatic changes) may 
be eliminated by the use of the poten- 
tial temperature. Potential tempera- 
ture is that temperature a parcel of 
air would have if it were brought 
adiabatically to a pressure of 1000 mb. 
If a dry particle were vertically dis- 
placed it would warm or cool at the 
dry adiabatic rate, and therefore its 
actual temperature would differ from 
its potential temperature by about 
1 C deg. per 100 meters from the 
level where the pressure is 1000 mb. 
This holds only in the event that the 
air particle remains unsaturated, for 
as soon as it becomes saturated, latent 
heat of condensation is realized and 
the particle no longer follows the dry 
adiabat, but the saturated adiabat. 
This was pointed out in the first art- 
icle of this series. During ascent, 
therefore, the potential temperature 
of the saturated particle will increase 
by virtue of the latent heat of con- 
densation. 
The most conservative thermal 
quantity is the equivalent-potential 
temperature. This is the tempera- 
ture the chosen air particle would 
have if it were brought adiabatically 
to the top of the atmosphere so that 
along its route all the moisture were 
condensed (and precipitated), the la- 
tent heat of condensation being given 
to the air, and then the remaining dry 
*Footnote on equivalent-potential tempera- 
ture on next page also applies to the equiva- 
lent temperature.—R. G. S. 
