94 CHARACTERISTIC AIR MASS PROPERTIES 
dicate a large degree of turbulent 
mixing up to a considerable elevation 
in the Tp current. This marked tur- 
bulence probably is a consequence of 
the high wind velocity which must 
characterize any winter air current 
of Tropical origin reaching Seattle, 
and of the ruggedness of the north 
Pacific coast. But in its general char- 
acteristics the air mass is found to be 
at Seattle as it was at San Diego, 
warm, moist, and relatively stable. 
As at San Diego, we note that @E in- 
creases slightly with elevation, a fact 
which indicates the absolute convec- 
tive stability of the mass. Consequent- 
ly as at San Diego the air mass is 
characterized by stratiform clouds, 
and by the restriction of any con- 
siderable amount of precipitation to 
the frontal zones. The characteristic 
curves on the Rossby diagram for Tp 
air at these two stations are closely 
parallel and their approximate coin- 
cidence with the @E£ isotherms is 
striking. A comparison of these curves 
with the characteristic curves of the 
Te and TA air masses will call to at- 
tention at once the comparative insta- 
bility of the latter air masses, which 
is indicated by their more nearly hori- 
zontal course on the diagram. 
The Tg and Ts Air Masses 
As would be expected from the 
uniform warmth of the waters in the 
TG source region, marked warmth 
and high moisture content at lower 
levels is characteristic of these air 
masses. This is shown strikingly in 
Table III by the high values of T, w 
and Oe at the surface and the 1 km 
level in the TG air, values which are 
much higher than are found in any 
other American air mass in winter. 
But at Groesbeck we find at the 2 
and 3 km levels a surprisingly large 
decrease in the moisture content of 
the air mass. This decrease of moist- 
ure is best interpreted as due to the 
presence of another air mass aloft 
which is designated as S, or Ts (Trop. 
superior), air. 
This designation is used for that 
warm, dry air mass which is found 
to be present a great deal of the 
time over the southern U. S. at inter- 
mediate and upper levels and fre- 
quently over the central U. S. at 
somewhat higher levels, at all seasons 
of the year. It is the warmest air 
mass observed, averaging a few de- 
grees warmer than TG at intermediate 
levels but, owing to the steep lapse- 
rate frequently observed in it at 
higher levels, above about 5 km, it is 
likely to average slightly colder than 
Te. It has a relative humidity less 
than 40%, which is often observed 
to fall below 20% or even 10% in 
the upper strata. Normal values (not 
computed means) of the temperature 
and specific humidity of this air mass 
at the 2 km level, at San Antonio, 
Texas, and Omaha, Nebraska, for 
January and July, might be given as 
follows: 
January July 
Ts at 2km T qd 1 q 
San Antonio 10°C 3g POO) Isyae 
Omaha H°G we 22 Ce 
At higher levels q frequently falls 
to half a gram or less, and even in 
summer to less than a gram. 
The hot dry weather which occurs 
in the western Plains states in sum- 
mer and in the Southwest even in 
winter, is always associated with the 
presence ee this air mass at inter- 
mediate levels and frequently, especi- 
ally in summer, even at the ground. 
It occurs frequently above a shallow 
stratum of moist Tropical maritime 
air at the surface, but on summer 
afternoons this moist stratum is 
likely to be lost by convective and 
turbulent mixing into the dry Ts 
strata above. 
It is the first air mass designation 
