92 CHARACTERISTIC AIR MASS PROPERTIES 
gradient toward the north or north- 
west. Occasionally an extension of 
the anticyclone northeastward towards 
the California coast occurs, causing 
the establishment of a pressure gra- 
dient which brings a general flow of 
Tp air northward along the entire 
length of the middle Pacific coast even 
beyond Seattle. Thus in advance of 
the southern portion of the cold front 
or the occluded front which marks 
the southward advance of fresh Pp 
air behind the depression approaching 
from the northwest, we may find a 
broad open warm sector of TP air 
moving northeastward or even north- 
ward along the coast with rather high 
wind velocities. Frequently this warm 
sector is occluded before the warm 
air reaches the coast, especially when 
the coastal region is covered with cold 
continental air. Under these condi- 
tions California gets its heaviest win- 
ter rain, caused by condensation both 
within the Tp air aloft, and also 
within the following unstable Pp air 
as it overruns the cold continental 
air (warm-front-type occlusion). The 
occasional winter development of a 
marked depression inland as far as 
the Great Basin or the southern Pla- 
teau region of Nevada and Utah, ac- 
companied by widespread heavy pre- 
cipitation, is apparently associated 
with marked overrunning at high 
levels by Tm (either Tp or TG) air 
from the south. On the other hand, 
the Tp air frequently sweeps north- 
ward along the Pacific coast with 
little or no resistance by colder air 
masses, consequently without appre- 
ciable overrunning at the warm front 
and with little or no rain before the 
approach of the cold front. A simi- 
lar development may take place also 
along the Gulf or Atlantic coasts, but 
in general the greater warmth and 
moisture content of the TG and TA 
air masses make the occurrence of 
moderate to heavy convective showers 
in the warm current much more 
probable. 
The properties of the TP air mass 
may be anticipated to some extent 
from the nature of the source region. 
The ocean surface in this region is 
rather cool for the latitude, ranging 
in temperature during the coldest 
season from about 14°C at latitude 
385°N to about 19°C at latitude 25°N. 
The weather condition is prevailingly 
anticyclonic, which suggests the proba- 
bility that more or less subsidence is 
taking place aloft. Thus we should 
expect to find that the air masses 
which come from this region are only 
moderately warm, or even cool for 
Tropical maritime air, at least near 
the ground. We should expect to find 
these air masses also relatively stable, 
perhaps with indications of a subsi- 
dence inversion which should be 
marked by an abrupt decrease of the 
w. w should be moderately high near 
the ground, but definitely not high for 
a Tropical maritime air mass. Finally, 
we should expect to observe an ab- 
scence of condensation forms in this 
air until it has moved some distance 
northward from its source and has 
been appreciably cooled from below. 
However, the data for San Diego in 
Table V probably give a fairly good 
picture of the Tp properties in this 
region. The temperatures we notice 
are rather high at all levels, though 
not markedly so considering the prox- 
imity of the maritime Tropical source 
and the southerly latitude. The sur- 
face temperature is about 2C° higher 
than the water temperature off the 
coast at San Diego, a fact which in- 
dicates a moderate recent movement 
of the air from a warmer region. We 
notice also the marked stability of the 
lowest km of the air mass, as well as 
the large decrease in w, both facts 
which indicate either a low subsidence 
