626 
trough aloft, it will probably not change into a dynamic 
anticyclone until the trough fills in response to a change 
in the wave pattern; this may involve consideration of 
forces acting over the entire belt of westerlies. When 
the upper trough fills, leaving the polar anticyclone well 
to the south of the westerlies, surface conditions become 
important insofar as they maintain or strengthen the 
shielding layer which, as explaimed previously, will 
permit the accumulation of the subsiding air aloft, 
leading to anticyclogenesis. 
There remain problems not explained by Rossby’s 
theory. For example, a deep combined polar and dy- 
namic anticyclone at the source region might be ex- 
pected to move more rapidly equatorward than a shal- 
low polar anticyclone possessing its “thermal” cyclone 
aloft. This is not in accord with synoptic experience 
and, in fact, the deep anticyclone may remain stationary 
for such long periods that it appears on a mean monthly 
map. Such a situation occurred in February 1947 over 
northern Greenland, as described in the mean by 
Namias [20]. In this case the deep anticyclonic vortex 
appeared to be bounded on the south by cyclonic 
vortices. Applying Rossby’s theory we might interpret 
the stagnancy of the vortex pattern as an equilibrium 
between forces tending to drive the anticyclone vortex 
southward and the cyclonic vortices northward. 
The ultimate fate of anticyclones which survive as 
individual entities in their progress to south and east 
seems to be absorption in and strengthening of the sub- 
tropic anticyclones. The subtropic anticyclones them- 
selves are examples of dynamically caused anticyclones ° 
located south of the westerlies over areas where both 
shielding layers (trade inversion) and low, surface fric- 
tional outflow favor location of the anticyclones [37]. 
ANTICYCLONES AND WEATHER 
Too often, in the minds of the meteorologist interested 
in weather forecasting, the anticyclone is considered 
merely as something that occupies the space on the 
weather maps between one front and the next. To be 
sure there is always the problem of forecasting the 
minimum temperature and the formation and dissipa- 
tion of stratus and fog, but on the whole, anticyclonic 
weather is a period of relaxation for the forecaster. 
This apparent lack of association of spectacular weather 
with anticyclones probably accounts for the far greater 
emphasis on study of cyclones. However, under certain 
conditions the anticyclone does exert a strong, though 
perhaps subtle, control on weather phenomena, and 
it is the purpose of this section to describe a few exam- 
ples. 
Summer Showers in the United States. The anti- 
cyclonic control of summer shower distribution in the 
United States has been demonstrated by Reed [25] and 
Wexler and Namias [388]. In the warm season the 
westerlies are displaced northward near the United 
States-Canadian border thus allowing frictionally 
driven anticyclonic eddies to cover much of the United 
States. These eddies have preferential average positions 
—one center located over the central Great Plains 
and the other off the southeastern Atlantic Coast [37]. 
MECHANICS OF PRESSURE SYSTEMS 
These preferential locations mdicate that something 
more than purely dynamic effects of the westerlies are 
involved in the maintenance of these eddies. The ‘“‘an- 
choring” of the summer anticyclonic eddies in the 
United States is believed to be the result of the very 
strong horizontal solenoidal field created by the cold 
air above the eastern Pacific Ocean and the heated air 
above the elevated plateau region of the western United 
States, acting in such a manner as to form a pronounced 
trough in the westerlies located in a mean position off 
the West Coast. This thermally produced trough creates 
a “resonance” trough in the Mississippi Valley, the 
exact position changing according to the varying 
strength of the westerlies and the position of the West 
Coast trough. Other troughs are found farther down- 
stream, but in the absence of a long series of aerological 
observations their average locations must be deduced 
from other evidence. Between the troughs of the west- 
erlies are the anticyclonic cells composed of two main 
currents spiraling clockwise—one a dry current from 
the north and the other a moist current from the south. 
Summer showers occur more frequently under the deep 
moist tongues. The preferential average locations of the 
positions of the anticyclonic eddies and their moist and 
dry tongues have profound effects on the summer cli- 
mate in the United States. For example, the dry tongue 
curving around the West Coast trough accounts for 
the dry summers in southern California and western 
Arizona, the moist tongue curving anticyclonically from 
the Gulf of Mexico around the western anticyclone 
accounts for the summer maximum in shower activity 
in eastern Arizona, New Mexico, and portions of Colo- 
rado. The dry tongue of this same eddy curving anti- 
cyclonically down the Mississippi Valley into the West 
Gulf and eastern Texas accounts for the fact that this 
region has less than 50 per cent of the summer precipita- 
tion enjoyed by the eastern Gulf states which are under 
the influence of the moist tongue of the eastern anti- 
cyclone. Similar anticyclonic control of summer shower 
activity must exist in other regions south of the wester- 
lies, and perhaps enough data now exist to permit 
location of the mean position of the centers of the 
eddies for the rest of the Northern Hemisphere. The 
eddy positions and sizes vary from year to year; this 
has a pronounced effect on the summer rainfall dis- 
tribution. 
The “Indian Summer’’ Anticyclones in the Eastern 
United States. During the autumn, and especially in 
October, stagnant anticyclones appear frequently over 
the eastern United States and account for calm, mild, 
hazy days and cool nights. The haze results from the 
inability of the atmosphere to disperse combustion 
products and other surface material. The large thermal 
stability of the atmosphere and the strong solar heating 
result in large ranges of diurnal temperature; insola- 
tion, however, in many cases does not succeed in 
“burning-off”” entirely the inversion created by sub- 
sidence within the anticyclone. In some extreme cases 
where pockets of cold air form in hilly country or river 
valleys, not even the shallow nocturnal inversion may 
be completely destroyed by solar heating. This is par- 
