TROPICAL AIR MASSES—WINTER 95 
to be used at M.I. T. which obviously 
does not apply to a mass of surface 
origin. For this reason it was clearly 
identified and designated as a separate 
air mass only after rather much experi- 
ence in the use of aerological data in 
preparing atmospheric cross sections. 
Formerly it was treated simply as a 
dry upper stratum characteristically 
present with the Tropical maritime 
masses, and not as a distinct air mass. 
However, it has been found to be of 
much wider distribution than the 
Tropical maritime masses, occurring 
above polar as well as Tropical mari- 
time masses, and obviously requiring 
separate treatment as a distinct air 
mass. 
In winter at least, the TS air mass 
seems to be of tropical origin, as in- 
dicated by its warmth, and with an 
extended past history during which 
it has remained aloft, as indicated by 
its dryness. Consequently the logical 
source region to attribute this air 
mass to is the upper portion of the 
sub-tropical belt of high pressure.* 
We know that slow sinking of pole- 
ward moving air must be occurring 
in this region. This sinking is especi- 
ally concentrated in the great anti- 
cyclonic cells in the sub-tropical high 
pressure belt, and according to the 
scheme of J. Bjerknes, especially on 
the northern and eastern sides of 
these anticyclonic cellular units. 
Hence we should expect to find this 
warm dry air predominating where 
we have prevailing west or west- 
northwest winds at intermediate or 
upper levels in south temperate lati- 
tudes. We should expect to find this 
current at higher levels further north, 
owing to the increasing prevalence 
and depth of the polar air masses at 
*See my “‘Dise. and Illustr. of Problems. . . . 
of Cross-Sections,’ M.I.7T., Papers in Phys. 
Ocean and Met., Vol. 4, No. 2, 19385; and G. 
Emmons: Atmospheric Structure over So. U.S. 
Dec. 30-31, 1927, M.I.T. Met’l Course Prof. 
Notes, No. 9, 19385 (rev’d Bull. Am. Met. Soe., 
Aug.-Sept. 1936, pp. 268-271). 
higher altitudes. This condition pre- 
vails a great deal of the time in the 
southern U. S., accounting for the 
prevalence of the Ts air there. Much 
of this air probably comes to us from 
the eastern regions of the Pacific 
anticyclone, after crossing the Rocky 
mountains.” Probably fohn effects on 
the eastern slopes of the Rockies help 
to bring the Ts air down to the sur- 
face, and to give it the particularly 
high temperatures observed in the 
Southwest. However, the appearance 
of this type of air will be by no means 
restricted to this region, but should 
be noted throughout the southern 
temperate latitudes of the continent. 
Recent studies in isentropic analysis 
by Mr. J. Namias show quite def- 
initely how in summer many of the 
so-called Ts currents are apparently 
Pc air masses that have swung around 
anticyclonically from the eastern Ca- 
nadian source region and _ subsided. 
Thus the heated continent and the 
seasonal shift of the Westerlies bring 
the Ts source region well north in 
summer. (See art. X.) 
In spite of the marked potential 
instability of the TG air masses in 
the south, their thermal stability is 
such that convective precipitation 
oecurs rarely within the air mass in 
winter. But from the warm front 
at which the overrunning warm cur- 
rent is TG air, the precipitation is 
often convectively irregular and ex- 
tremely heavy. With the high rela- 
tive humidity observed in the lowest 
km of the Teé current, comparatively 
little forced ascent of the warm cur- 
rent is necessary before the potential 
instability present becomes available 
for thermal convection. In fact it 
occasionally happens even in winter 
8The TJs designation has supplanted the 
Tc (tropical continental) designation on 
the North American weather maps, for there 
is no really extensive Tec source region on this 
continent. The Wea. Bur. uses ‘‘S’” for Ts. 
