TROPICAL AIR MASSES—SUMMER 
air in this heated inland Plateau 
which is normally too dry to furnish 
much moisture by evaporation. We 
have found that Pp air in summer 
is characterized by a specific humidity 
on the order of 7 ¢ at the ground, 
decreasing to only slightly over 2 ¢ 
at 3 km elevation. Warm air which 
reaches Ellendale from the northern 
Plateau low pressure region is ob- 
served to have a specifie humidity of 
from 10 to 12 ¢ at 1 km elevation, 
a value which definitely suggests that 
the air is of TM origin. Recent 
studies, notably by Reed, of the upper 
winds in summer in the Southwest, 
indicate that this warm moist air 
moves frequently at the upper levels 
from the TG source region via Mex- 
ico. It is quite possible that it may 
come also from the Ts source region 
occasionally, but apparently it is pre- 
dominantly TGé air. 
The Tropical Gulf, Tropical Atlantic 
and Ts Air Masses 
For the TG and TA air masses the 
normal summer conditions are much 
more favorable than they are for the 
Tp air masses. The combination of 
the tendency toward the development 
of low pressure over the interior of 
North America and the tendency 
toward the development of a well- 
marked center of high pressure over 
the western Atlantic Ocean (Ber- 
muda High) results much of the time 
in summer in a pressure distribution 
which brings MT air northward over 
most of the eastern U. S. and even 
into Canada. Consequently the mT 
air masses in the eastern and central 
U. S. are present a much greater part 
of the time and extend over much 
wider areas in summer than they do 
in winter. They are responsible for 
the oppressive heat with high humi- 
105 
dity which more than anything else 
characterizes our summer weather in 
the eastern and central U. S. The 
map of July 26, 1930, 8 a.m. repre- 
sents almost in ideal type form the 
general condition which leads to the 
widespread prevalence of MT air in 
the U. S. in summer. We notice a 
weak trough of low pressure over the 
northern Plains states, and a broad 
extension of the Bermuda High west- 
ward over the southeastern U. S. The 
resulting general air movement con- 
sists of a light flow from the West 
Indies and Caribbean Sea northwest- 
ward into the Gulf of Mexico, thence 
northward over the southeastern and 
south central U. S., and thence north- 
eastward into the Lake Region and 
towards New England. In other 
words, we observe a slow steady flow 
of Equatorial air which originated in 
the Trade-wind zone over the entire 
eastern and central U. S. As this 
condition is usually very stationary, 
the stream lines on this map as in- 
dicated by the isobars may be taken 
as typical trajectories of the MT air 
masses in summer, the air masses 
to which the following discussion 
applies. 
The general difference between the 
normal winter and the normal sum- 
mer condition as regards the distribu- 
tion and prevalence of the MT air 
masses may be expressed in another 
way by saying that the zone of maxi- 
mum frontal activity between the MT 
and cP air masses, or the sub-Polar 
front, is displaced in summer from 
its normal winter position somewhere 
over the northern Gulf of Mexico, 
northward into the U. S. almost to 
the region of the Great Lakes. 
In general the properties of the 
MT (TG and TA) air masses in sum- 
mer as they leave their source region 
are similar to their properties in 
