108 
CHARACTERISTIC AIR MASS PROPERTIES 
the stability of the lowest km of the 
air mass at the inland stations, the 
consequence of nocturnal radiational 
cooling. The heating of the upper 
strata of the air mass takes place 
during the afternoon thermal con- 
vection, but during the night the 
temperatures near the ground fall.— 
Excerpts. 
[The highest temperatures aloft 
occur at Ellendale, farthest from the 
Gulf, and indicate the heating effect 
of the continent to perhaps 5 km or 
more. Stations inland all show lower 
w at all levels and, except in the 
southeast, where there is direct in- 
vasion of TA air, lower w at the 
ground, than at Pensacola. North 
from the Texas coast the Ts and Te 
layers become increasingly mixed by 
diurnal convection, which lowers w 
and @E at the surface and increases 
them aloft. Eastward of the western 
plains the influence of Ts air de- 
ereases at the surface as continental 
heating becomes less marked. As in 
winter, it is difficult to distinguish Ta 
and TG air masses. 
The pure or slightly modified Tm 
summer air in the southeastern 
United States is so conditionally un- 
stable and humid to well above 5 km 
that thunderstorm convection is fre- 
quent; this becomes, as we have seen, 
steadily less marked inland to the 
north and especially westward, where 
the dry and stable Tropical Superior 
(Ts and modified TG) air masses 
with their clear skies, good visi- 
Major Frontal and Air 
Bergeron describes nine latitudinal Weather 
Zones of each hemisphere as a function of 
the large scale distribution of Air Masses and 
Fronts*) (see Met. Zts., v. 47, p. 249, figs, 
1, 6, and Ymer, hf. 2-3, 1937, p. 218, fig. 
10) :— 
1. The Stratus zone of the Stable Polar 
Air (or Arctic Air*) in its source region. 
2. The Shower zone of the moist-labile 
Polar Air (or Arctic Air*) in somewhat 
lower latitudes. | 
8. The Dry zone of diverging Polar Air 
(or Aretic Air*) underneath the Polar Front 
surface (or Arctic Front surface*). 
4. The area of continuous precipitation 
from Nimbostratus at the Polar Front (or 
Arctic Front*). 
bility, and large diurnal tempera- 
ture range stand in decided contrast 
to the humid, hazy, showery and 
cumulus-laden TM air:—R. G. S.] 
In summer the modification of the 
typical TG or TA air mass to the 
NtTM condition by cooling from below 
occurs only in rather restricted re- 
gions, for over the continent proper 
such cooling does not take place. It 
occurs only over a cold water surface. 
On the Canadian side of the Great 
Lakes this effect is sometimes notice- 
able, especially in the early summer 
when the Lakes are still cold. At this 
season the MT air from the south may 
arrive on the northern shores of the 
Great Lakes definitely cooled and 
even foggy or with low St clouds. But 
this cooling effect is most important 
over the cold water off the north 
Atlantie coast. In only a day or two 
of slow movement of the warm moist 
MT air mass over this cold ocean sur- 
face, the lower strata are cooled 
almost to the water surface tempera- 
ture, and the cooling effect is carried 
upward, presumably by mechanical 
turbulence, with surprising swiftness. 
The resulting condition is one of 
stable stratification with the rapid 
formation of dense fog of consider- 
able depth. This is the cause of the 
high frequency of summer fog on the 
north Atlantic coast. It is principally 
in this region that the Ntm designa- 
tion is used on the synoptic charts in 
summer.—H«cerpt. 
Mass Zones of the Earth 
5. The Stratus zone (with or without driz- 
zle) in the Stable Tropical Air at the Polar 
Front (or in the Polar Air at the Arctic 
Front*). 
6. The first Cumulus zone of the Tropical 
Air in middle latitudes. 
7. The Dry zone of diverging Tropical Air 
in the high pressure belt of the horse latitudes. 
8. The second Cumulus zone of the Tropical 
Air: the trade-wind zone. 
9. The Shower zone of the moist-labile Trop- 
ical Air in the equatorial region (the Dol- 
drums). 
*When there is also an Arctic Front pole- 
wards of the Polar Front, five more Weather 
Zones are arranged along the former front as 
along the Polar front. 
