104 
PA source region, are very quickly 
cooled over the cold water to such 
an extent that dense fog is immedi- 
ately formed. Partly because of their 
greater initial warmth and moisture, 
and perhaps partly also because of 
less wind and mechanical turbulence, 
the dense fog appears almost immedi- 
ately at the surface and grows deeper 
with prolonged cooling. It is this 
condition which gives to this region 
its reputation for spring and sum- 
mer fogginess. These air masses of 
Tropical origin, cooled in the PA 
source region, are designated on the 
M.1I.T. maps as NtmM. The symbol 
NPA may appear on the summer 
weather maps for PA air which has 
moved far south over warmer water, 
or been brought to a considerable 
distance inland.—EH«cerpts. 
The Tropical Pacific Air Masses 
During the warm season the Tropi- 
cal Pacific air masses play a negli- 
gible role on the Pacific coast. It 
was remarked in the discussion of the 
Pp air masses in summer that the 
pressure distribution at this season 
along the entire middle and north 
Pacific coast of North America is such 
that there is found a persistent on- 
shore gradient, and a correspondingly 
persistent movement of air at least 
to a considerable elevation from the 
north and northwest. This pressure 
gradient may temporarily disappear 
to such an extent that the air move- 
ment for a time is almost stagnant, 
the winds becoming light variable. 
At such times the aerological ascents 
at Seattle indicate the presence of 
higher temperatures at all levels, and 
usually of somewhat higher specific 
humidity at the ground, than occur 
in the Pr current when it is well 
developed. But at upper levels one 
usually finds a dryness indicative 
either of subsidence or of a light 
CHARACTERISTIC AIR MASS PROPERTIES 
continental outflow of air with some 
féhn effect from the mountains 
inland. During the summer of 1930 
there was not found a single instance 
either of marked southerly air move- 
ment or of a moisture distribution 
indicative of TP air at Seattle at the 
ground. Probably this particular 
summer was quite typical of the 
average summer in this respect, so 
that it may be concluded that real 
Tp air does not appear on the Pacific 
coast of the U. S. in summer, at 
least at low levels. This can cer- 
tainly be said of Seattle. At San 
Diego the situation is much more 
uncertain, as the condition there is 
normally one of comparative stagna- 
tion. Probably it would be somewhat 
warmer and somewhat moister in 
summer than in winter, correspond- 
ing to the higher temperature of the 
ocean surface in the source region. 
On the California coast this air mass, 
if it appears at all, should be defi- 
nitely cool at low levels and foggy, 
because of the extreme local coldness 
of the water along the coast. 
It seems quite probable, however, 
that in the Plateau and Rocky Moun- 
tain regions the TM air is of real 
importance in summer. Tempera- 
tures in the Plateau region are rather 
high at this season, and the pressure 
normally rather low. Under these 
conditions rather steady southerly 
winds frequently are observed in this 
region for rather prolonged periods, 
with the result that the moisture thus 
brought inland establishes a specific 
humidity high enough so that con- 
siderable cloudiness and widespread 
thundershowers develop throughout 
the low pressure trough. If the air 
thus brought into the Plateau region 
had its source in the comparatively 
cool and dry PP current prevailing 
along the coast, an extremely low 
relative humidity would prevail in the 
