106 
CHARACTERISTIC AIR MASS PROPERTIES 
winter in the same region. The ocean 
surface temperatures during the 
warmest season average over the 
entire Gulf and Caribbean Sea region 
close to 28° or 29°C. In the Carib- 
bean Sea this is only 3°C warmer 
than the temperature during the cold- 
est season, a difference which in- 
creases probably to nearly 10°C on 
the immediate Gulf coast. Thus we 
should expect the mT air mass in 
summer to leave the source region 
somewhat warmer and somewhat 
moister than it does in winter, but 
quite similar to its winter condition 
in its general vertical structure. We 
should also expect to find that as the 
mT air mass passes inland from the 
source region the general tendency of 
the effect of the continent will be 
towards a raising of the air mass 
temperature, instead of towards a 
lowering, as it is in winter. 
Pensacola, Fla., doubtless gives the 
best indication of the source proper- 
ties of TG air in summer. We note 
a surface temperature in the TG air 
which is almost identical with the 
water surface temperature of the 
source region. Above the surface we 
find a moderate lapse-rate, about 0.6 
of the dry adiabatic rate, which in- 
dicates a condition of thermal sta- 
bility in the air mass. Afternoon 
ascents would doubtless indicate a 
steeper lapse-rate near the ground. 
The relative humidity is surprisingly 
high, in view of the warmth of the 
air mass, so that the specific humi- 
dity exceeds slightly the large value 
of 20 g. Probably this value is as 
great as would be found as an 
average in any maritime location in 
Equatorial regions. Not only is w 
extremely high at the ground at Pen- 
sacola, but the values found up to 
at least the 3 km level indicate very 
high relative humidities at the pre- 
vailing temperatures. At the same 
time the values of w observed at Pen- 
sacola at all levels in the TG air are 
higher than those found at any other 
station. In spite of the high moist- 
ure content found at the 3 km level, 
the excessive amount of water vapor 
present in this air mass at low levels 
gives rise to a condition of marked 
potential instability. Up to the 3 km 
level a decrease of 18° in @£ is 
noted, while the high value of w at 
this level would indicate that the 
decrease in @§E should continue for 
at least 2 km further. This condition 
implies that all convective or me- 
chanical turbulence up to at least 5 
km elevation must effect an upward 
transport of latent heat, and the high 
values of the relative humidity in- 
dicate that very little vertical dis- 
placement is necessary in order to 
initiate active convection which 
should extend well beyond this level. 
This same marked potential instabi- 
lity which we observe in the TG air 
masses coming from the Gulf of 
Mexico and the Caribbean Sea in 
summer, and which we shall find 
presently to be characteristic of all 
maritime Equatorial air masses, is the 
source of the great amounts of 
energy consumed in the genesis and 
maintenance of the Tropical hurri- 
cane. When we consider the added 
effect of insolational heating of the 
TG air mass near the ground during 
the daytime as it moves inland from 
the Gulf coast, it is obvious why Cu 
clouds which develop during the 
afternoon into Cunb and heavy local 
thundershowers are so characteristic 
of this air mass.—H«acerpts. 
[In Texas the invasion of summer 
TG is typically shallow, with dry Ts 
air aloft above 1 or 2 km, even less 
sometimes, which makes thunder- 
storms in TG much less likely than 
farther east; the decrease in 6, aloft 
indicates much potential instability 
