96 CHARACTERISTIC AIR MASS PROPERTIES 
that sufficient forced ascent of a 
vigorous TG current is effected by 
coast line or mountains (southern 
Appalachians) so that convective 
showers are initiated, but this occur- 
rence is restricted for the most part 
to the summer season when insola- 
tional heating of the Tc air mass may 
produce widely scattered heavy con- 
vectional thunderstorms in the east- 
ern United States. 
In the south the prevailing weather 
conditions in the TG air mass are 
essentially those of warmth and high 
humidity. There is likely to be a 
good deal of cloudiness, especially 
during the night and early morning. 
The principal cloud forms are rather 
low stratus or strato-cumulus, which 
the sun frequently dissipates during 
the day. Visibility is likely to be 
rather poor when low clouds prevail 
but becomes good with the dissipa- 
tion of the cloud deck. The cloud 
base is likely to be at only a few 
hundred meters elevation, while the 
upper boundary coincides with the 
temperature inversion which marks 
the upper limit of the extremely moist 
air stratum, usually between 12 and 
16 hundred meters elevation. It is 
seldom that any precipitation is asso- 
ciated with this type of stratus in 
the south, for the inversion above is 
usually sufficient to prevent penetra- 
tive convection and, in its earlier his- 
tory near the source region, the lower 
strata of this air current are not suffi- 
ciently cooled to produce the mist and 
drizzle which are likely to appear as 
it moves northward. Apart from the 
possible existence of a _ turbulent 
stratum near the ground in cases of 
strong air movement, flying condi- 
tions in the TG air mass should be 
definitely smooth in the south. Fre- 
quent low stratus and rather poor 
visibility may prove embarrassing, 
especially in hilly country. 
As the Te current moves northward 
it is continuously cooled in the lower- 
most strata by contact with the in- 
ereasingly cold surface beneath. 
Over dry land this cooling takes place 
rather slowly but over cold water or 
a snow and ice surface it becomes 
very rapid. The natural consequence 
is to produce very rapidly a super- 
saturated stratum at the ground in 
the warm moist air current. If the 
air movement is light, this leads 
quickly to the formation of dense fog 
at the ground (the so-called Tropical- 
air fog’) but since the advance of Tc 
air to high latitudes is normally asso- 
ciated with rather strong air cur- 
rents, mechanical turbulence usually 
maintains a thoroughly mixed stra- 
tum at the ground, topped by a dense 
low stratus cloud deck. As the cool- 
ing continues, rather dense mist or 
fine drizzle is likely to fall from this 
stratus, so that the visibility even 
with rather strong wind may be re- 
duced almost to that of a dense fog, 
with a ceiling of not more than one 
or two hundred meters. This is a 
condition which makes flying practi- 
cally impossible, except over a per- 
fectly flat surface. Long before 
visibility becomes poor at the ground 
the lowering ceiling in the advanc- 
ing Te current makes flying in moun- 
tainous country difficult. For ex- 
ample, even in the lowest passes in 
the Appalachians the ceiling quickly 
closes in, especially on that side of 
the ranges against which the wind is 
blowing. 
The data in Table III for Tc air at 
Boston show clearly the beginning of 
the cooling of the Ta air mass to the 
Nrm condition, as just described. As 
compared with Groesbeck we note the 
marked stability of the first km of 
9See H. C. Willett: Fog and Haze, Their 
Causes, Distribution, and Forecasting, Month- 
ly Weather Review, November, 1928. 
