CONSERVATIVE PROPERTIES OF AIR MASSES 11 
midity at the surface offers a worth- 
while aid to the identification of indi- 
vidual air masses as well as in the 
upper air. In winter the specific hu- 
midity at stations on either side of a 
well marked front may differ by ten 
grams per kilogram of air. Although 
it is true that with large specific hu- 
midity contrasts at a frontal zone one 
generally finds good temperature con- 
trasts, there are many cases in which 
the temperature immediately after a 
front passage changes but slightly 
' with the wind shift, yet the specific 
humidity changes considerably. This 
is particularly true in summer in con- 
nection with air masses of continen- 
tal characteristics which displace 
maritime air masses. As an example 
of this type of front a case may be 
cited in which a Tropical maritime air 
mass over New England was dis- 
placed by an older transitional air 
mass originally of Polar Pacific ori- 
gin. At Boston the specific humidity 
within the tropical air mass averaged 
about fifteen grams per kilogram. 
With a shift of wind from south-south- 
west to southwest the specific humidi- 
ty fell fairly rapidly to ten grams per 
kilogram, even though the tempera- 
ture rose a few degrees Fahrenheit. 
It cannot be doubted that the addition 
of the specific humidity to the data on 
our daily weather maps would facili- 
tate the analysis and might be the 
deciding factor in the correct place- 
ment of a front when other indica- 
tions are not pronounced. 
The numerical value of the specific 
humidity is readily calculated by use 
of the approximate formula 
g = 622 e/p 
where q represents the specific hu- 
midity expressed in grams per kilo- 
gram; e, the existent vapor pressure 
(obtained through the relative hu- 
midity and temperature observa- 
tions); and p, the total atmospheric 
pressure. The units for e and p may 
be chosen arbitrarily (providing both 
are expressed in the same unit) since 
they constitute a ratio. 
D. CONDENSATION FORMS. 
The type of cloud formation is 
largely a result of the vertical dis- 
tribution of temperature (the lapse 
rate) and moisture. Since both these 
quantities are fairly conservative, it 
follows that certain condensation 
forms are more or less characteristic 
of each type of air mass. Care must 
be exercised in differentiating be- 
tween clouds formed within an air 
mass and those formed by the inter- 
action at the front between. two dif- 
ferent air masses. In addition, local 
condensation forms, such as ground 
fogs, must be eliminated. 
EK. ‘VISIBILITY. 
The visibility in the lower layers of 
the atmosphere is generally an indi- 
cation of the lapse rate therein. If 
the lapse rate is stable, then smoke 
and dust tend to remain close to the 
surface; if the lapse rate is steep 
then vertical motion is easily possible 
and the foreign matter diluted by be- 
ing mixed throughout a layer of con- 
siderable thickness, thereby increas- 
ing the visibility in the surface layers. 
In cold masses which are moving 
over a much warmer surface a steep 
lapse rate is soon established and vis- 
ibilities become good. On the other 
hand, when a warm current moves 
over a much colder surface the marked 
stability keeps the foreign matter 
concentrated in the lower layers mak- 
ing the visibility poor. As an index 
of the air mass present, visibility 
must be used with considerable care, 
since there are numerous factors other 
than turbulence affecting visibility. 
F. WIND DIRECTION AND VELOCITY. 
Wind direction and velocity are in 
themselves not very conservative ele- 
ments. Polar air masses are fre- 
