TROPICAL AIR MASSES—SUMMER 
107 
but in the absence of much moisture 
aloft it is not realized except from 
marked vertical displacements with 
passage of occasional pronounced 
warm or cold fronts.*—R. G.S.]| 
Modification of the Tg and Ta Air 
to Ntm 
As the TG air mass moves north- 
ward from the Gulf of Mexico in 
summer we should not expect as a 
rule any very great changes in its 
properties. During the late spring 
and the summer the North American 
Continent is insolationally warmed 
even at rather high latitudes. Only 
in the region of the Great Lakes and 
off the north Atlantic coast is there 
any possibility of a marked cooling 
of the Te air mass from the surface 
beneath. Apart from these water 
areas the ground is normally warmer, 
especially during the daytime, than is 
the water surface in the source region 
of the air mass, so that the general 
tendency must be towards a warming 
of the air mass from the warmer 
surface beneath as the TG current 
‘moves northward from the Gulf of 
Mexico. 
At greater distances from the 
source region we would expect to 
find a small decrease in the moisture 
content of the TG air mass as the 
result of the precipitation of some of 
the excessive moisture in widely scat- 
tered thundershowers. Especially in 
the west and at upper levels we 
should expect to find the influence 
of the gradual infiltration and over- 
running of dry Ts air from the 
upper levels of the sub-tropical anti- 
cyclones.* But in general, a con- 
dition of oppressive warmth and 
moisture with afternoon thunder- 
storms should characterize MT air 
over the entire eastern and central 
U. S. The temperatures at all levels 
tend to run a little higher than at 
*See discussion of Ts air in summer on 
pp. 94-95 of this booklet; and in the article 
on isentropic analysis by Mr. Namias, as well 
as in the note Showalter at the end of this 
section.—Ed. 
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the Gulf coast stations. This differ- 
ence is, perhaps rather surprisingly, 
a maximum at the 1 and 2 km levels. 
This is to be explained by the early 
morning hour of the majority of the 
ascents, a fact which is indicated by 
