TROPICAL AIR MASSES—WINTER 91 
are colder, drier and more stable than 
in Pp air (at Seattle, e.g.) because 
the waters (0°C) are colder and the 
air has spent much less time there- 
over. The upper levels show the 
typical structure of old Pe air, much 
warmed and dried by subsidence, and 
above 3 km the warm-front inversion 
(often due to tropical air) of the 
next approaching disturbance may 
already be present. 
On the coast, therefore, the PA air 
has a surface temperature near freez- 
ing or somewhat below, with a good 
lapse-rate through the first km, more 
or less, the relative humidity rather 
high (w= 2 or 3 g/kg) and increas- 
ing to saturation or nearly so at the 
top of the convective layer, where 
there is a deck of thickening Stcu; 
but Cunb and showers are prevented 
by the marked inversion a short dis- 
tance above the cloud base. Above 
the inversion it is dry, and clear 
unless there are warm-front clouds 
still higher. At Boston, for example, 
the PA air arrives rather suddenly 
following a rapid backing of the wind 
from NW to NE, with thickening low 
Steu clouds scudding in from the 
E; later light flurries or fine mist 
begin, making poor the already fail- 
ing visibility, and the temperature has 
risen perhaps as much as 10°C; the 
front of the maritime air works 
steadily inland, abruptly terminating 
cold weather, till it nestles in the 
lee of the eastern ridges of the Appa- 
lachians. Farther south this type of 
onshore flow is much warmer owing 
to the nearby Gulf Stream, and un- 
stable to higher levels due to its 
longer maritime history; but here 
reports are often insufficient to prove 
Polar origin—it should be called Pa 
or Npa if traced from Pc, otherwise 
Ta or NTA. NPA is PA air warmed 
and unstable beyond typical PA con- 
ditions.—R. G. Stone.| 
The Air Masses. The 
Tc air masses are those which origin- 
ate over the warm waters of the Gulf 
of Mexico and Caribbean Sea, or 
further southeast in the Tropics. 
Searcely to be distinguished from 
them in characteristic properties are 
the Ta air masses from the Sargasso 
Sea region, so that this discussion 
really applies to both. The TP air 
masses are the Pacific counterparts 
of Tc or TA. The transitional forms 
of these masses are usually referred 
to simply as Ntm (transitional Trop- 
ical maritime). There is no attempt 
to make the very difficult distinction 
between NtcG and NTA. 
The Tropical Pacific Air Masses 
The source of the TP air masses is 
that portion of the Pacific Ocean 
lying west and southwest of southern 
California, roughly between latitudes 
25° and 35°N. During the cold season 
this region is one of markedly steady 
weather conditions, as it is normally 
the winter seat of the stationary Paci- 
fic anticyclone. Over the Pacific 
Ocean north of lat. 30°N we find 
normally in winter a pressure gradi- 
ent directed northward towards the 
Gulf of Alaska and the Aleutian 
Island Region, which is the winter seat 
of the north Pacific low pressure area, 
usually referred to as the Aleutian 
Low. From or around the Aleutian 
area move most of the winter frontal 
systems and disturbances which ap- 
proach the north and central Pacific 
coast. When these disturbances fol- 
low an unusually southerly path, or 
develop secondary centers which move 
further south than the usual Pacific 
depression, or develop in themselves 
to an unusual intensity, then the Trop- 
ical maritime air lying in the northern 
portion of the Pacific anticyclone is 
set in motion towards the northeast 
in response to the intensified pressure 
Tropical 
