110 
AIR MASS ANALYSIS 
it usually is sufficiently modified to 
be called MP. 
Strictly speaking, for air masses 
entering the United States, the nota- 
tion MAK should be reserved for Arctic 
air masses which move directly south- 
ward along the North Pacific coast and 
have only a short trajectory over the 
ocean. The notations MPK and MPw 
are adequate to differentiate between 
maritime polar outbreaks having 
longer trajectories (see figure 1). The 
unusual instability of MAK air, some 
flights indicating that vertical convec- 
tion has obtained to at least 6 km, has 
two important effects on its interaction 
with other air masses in the central 
and eastern part of the United States. 
First, since often it is colder aloft than 
the surrounding air masses, sinking 
from these levels, or subsidence, occurs 
in MA and MP air. Second, and in- 
versely, this same instability in MA 
air is apt to cause an increasing ten- 
dency for vertical divergence and in- 
creasing instability in air masses 
moving into a region occupied or re- 
cently occupied by MA air. In other 
words, the MA gains in stability while 
the surrounding masses lose. 
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$ AW 2% ces Summer 
PL LT fe Winter | 
San T 12 13 
(Ons ne a a eS 10 | 
Fic. 1. Average 6x and Slopes of Character- 
istic Curves for the levels between 1000 and 
1500 meters for American air masses, based 
on U. S. soundings of 1935-6. 
During the winter months MA air 
near the surface is usually warmer 
than the continent and shortly after 
passing the coast line it should be 
labelled as a warm-type air mass ac- 
cording to the Bergeron classification. 
Over snow covered areas MAK air very 
rapidly assumes continental character- 
istics; thus we find the modified form 
of MA air sometimes colder near the 
surface than the original. 
Maritime Tropical Air, MT (TM): 
Since MT air is formed out of air which 
was originally polar, it shows a ten- 
dency for vertical stability with some 
subsiding action in the higher levels. 
It will be noted that although the par- 
tial-potential temperature of the dry 
air increases fairly rapidly with ele- 
vation, the equivalent-potential tem- 
perature usually decreases with eleva- 
tion. This seems to indicate that at 
higher levels this air mass is relatively 
dry. Since MT air usually moves in- 
land with anticyclonic motion it may 
be assumed that the relative dryness 
aloft in MT air can be explained by 
horizontal divergence with subsidence 
in the upper levels of the subtropicai 
anticyclonic cell. There is evidence 
that some of the water vapor at higher 
levels must have been carried upward 
by vertical convection over scattered 
areas in the Gulf and Caribbean, and 
was diffused to the surroundings. The 
evident upward slope of the isentropic 
surfaces to the northward indicates 
that the moisture is carried aloft not 
only by vertical convection but also by 
isentropic mixing. The upward trans- 
port of moist air appears to occur near 
the edges of the subtropical anti- 
cyclones, while the subsiding dry 
tongues originate nearer to _ the 
centers. 
Superior or Subsiding Air, S (TS): 
The notation Ts (Tropical superior) 
was originally applied to air supposed 
to have been derived from the upper 
subsiding portions of the subtropical 
anticyclonic cells. However, of recent 
