TROPICAL AIR MASSES—-WINTER 
inversion or a low turbulence inver- 
sion. The rather surprising frequency 
of low St or Stcu clouds in the TP air 
mass at San Diego seems to'favor the 
second possibility. If the air mass was 
originally characterized by marked 
stability and a rapid decrease of w 
with elevation at low levels, then the 
decrease of w at an inversion pro- 
duced by turbulence might be as large 
93 
as the difference observed at San 
Diego between the value at the ground 
and that at I km, otherwise the de- 
crease would not be so large. Above 
1 km we note a stable lapse rate, but 
one which increases somewhat with 
elevation. The decrease of moisture 
noted in the first km continues at 
higher elevations, but especially mark- 
edly at the 3 km level. 
TABLE V 
TROPICAL PAcIFIC AIR—-WINTER (FEBRUARY 1930) 
ELEVATION SAn DIEGO 
ft w RH 65 
2G g % OP 
Surface Ife TL GSS BUS 
1 G45) Gail 315 
2 UL) HF 319 
3 AL BAT Bis — 
ELLENDALE 
SEATTLE (Tp aloft) 
T w 6E T w GE 
Gi °A 2G g oA 
ae Wb ROS (Rs Bs Beil) 
0) er OS (SRS BYES 287) 
7 4b} BOS | (ql By BS) 
2S) Bo Be iy Bis S07 
In case the water surface in the 
source region is rather cool for the 
latitude, as is true for the source 
region of the Tp air masses, then the 
air mass does not become even con- 
ditionally unstable. Thus we note that 
at San Diego in the TP air mass there 
is a slight increase of gE with eleva- 
tion, and the same thing is found to 
be true for this air mass at Seattle. 
This fact explains why the TP air flow 
along the Pacific coast gives quantita- 
tive precipitation only at an active 
front. It is too stable for the coast 
line to be effective in forcing much 
ascent of the warm current. And even 
at an active warm front the amount of 
precipitation from TP air is consider- 
ably less than the amount at an active 
warm front where the more unstable 
Te or TA air is overrunning. 
At Seattle (TABLE V) the general 
properties of the TP air mass are 
found to be similar to those observed 
at San Diego. The air mass has be- 
come cooler throughout, but the differ- 
ence is not very great in view of the 
15° difference in the latitude of the 
two stations. The surface iempera- 
ture at Seattle in the TP air is ap- 
proximately 5C° warmer than the low- 
est average ocean surface tempera- 
ture which is found during the winter 
off the coast from this station. The 
relative warmth of the TP air indi- 
cates the rapidity with which this air 
mass has moved from the south. At 
San Diego, where the air mass move- 
ment is much more sluggish, the dif- 
ference was found to be only 2C°. But 
in spite of the warmth of the TP air 
at Seattle relative to the ocean sur- 
face, a fair lapse rate from the ground 
up is observed in this air mass. The 
decrease with elevation both of T and 
of q is much more uniform than it is 
at San Diego. In particular has the 
marked stratification of temperature 
and moisture, which was noted in the 
first km of this air mass at San Diego, 
disappeared at Seattle. This is a 
change which would scarcely have 
been expected, considering the increas- 
ing relative coolness of the underlying 
water surface with the progress of the 
air mass northward. It seems to in- 
