1146 
techniques as applied to meteorological problems, in- 
cluding those posed by atmospheric pollution, is avail- 
able.t 
POLLUTION BY MULTIPLE SOURCES 
The problems of pollution by multiple sources include 
those of single sources and a large number of others as 
well. The interplay of meteorological and other factors 
is extremely complex in the most important instance, 
that of a city, and only a start has been made in the 
study of such factors and their various roles. The main 
conclusions of the few studies of city pollution which 
have been made are set forth below. 
Effect of Wind. The role of the main air currents of 
the atmosphere’s circulations in transporting pollution 
is an obvious one. The effects of large-scale features of 
the general circulation and of local winds on pollution 
in the Los Angeles area have been described in detail 
by Beer and Leopold [7]. Of the latter type, both land 
and sea breezes and mountain and valley winds play 
a part in alleviating or contributing to the nuisance in 
the Los Angeles region. More detailed studies of the 
effects of wind on city pollution have been made, such 
as that at Leicester, England [24]. There the average 
effect of wind in shifting the center of surface pollution 
was surprisingly small. Pollution was naturally greater 
downwind from the center of the city, but the points of 
maximum surface concentration of smoke and sulfur 
dioxide never moved more than half a mile from the 
city’s center as a result of wind; as the wind speed 
increased, the nearly circular contour lines of equal 
average pollution at the surface moved downwind by 
distances up to one mile, without any change in their 
radii. These results suggest that the upward diffusion of 
pollution by turbulent mixing is a major factor in 
limitmg surface pollution, a conclusion confirmed by 
Fig. 4, which shows the effect of winds of various speeds 
on ultraviolet daylight in Leicester during the winter. 
The figures in the circles give the loss of ultraviolet 
light, expressed in logarithmic units, as a percentage of 
that received in the country districts around. Since the 
loss of ultraviolet light will be an approximate measure 
of the smoke above that portion of the city, the dia- 
gram indicates that the maximum of total smoke in 
winter may be found as far as two miles downwind 
from the city’s center. The surface maximum was never 
more than half a mile downwind. A comparison between 
daily mean smoke and daily mean wind speed at Leices- 
ter was also made. In general the pollution decreases 
with increasing wind speed, the decrease being pro- 
gressively more marked with increasing stability, and 
more pronounced in winter than in summer. 
Less elaborate studies of city pollution have been 
made, such as that of Davidson [17] for New York City. 
The main meteorological conclusion of this investiga- 
tion is that dust concentrations at the surface vary 
inversely as the square root of the wind speed. Other 
studies suggest that surface smoke concentration varies 
inversely as the wind speed [31, 47]. 
1. Consult ‘“Model Techniques in Meteorological Research” 
by H. Rouse, pp. 1249-1254 in this Compendium. 
ATMOSPHERIC POLLUTION 
Effect of Atmospheric Stability. The immediate effect 
of stability on surface city pollution depends on whether 
the stable layer is at the surface or aloft. At Leicester 
the connection with surface stability was studied. It was 
found that daily mean smoke increases with stability, 
the increase being small with winds of 10 mph but 
becoming progressively more pronounced with decreas- 
ing winds. The increase was more marked in winter 
than in summer. There is evidence that diurnal varia- 
tions of stability, and hence of turbulence, influence the 
pollution from multiple sources. An investigation of 
pollution in fourteen of the largest cities in the United 
States [42] showed a low level of contamination in the 
early morning, followed by a peak in the forenoon, 
which occurs earlier in summer than in winter, at a 
5-10 M.P.H. 
WINDS 
MILES 
—=> 
DIRECTION OF WIND 
Fic. 4.—The effect of wind speed on ultraviolet daylight 
losses during winter weekdays in Leicester. Logarithmic units: 
one unit represents a loss, due to smoke, of 9.4 per cent of the 
possible ultraviolet daylight. (Reproduced by permission of H. 
M. Stationery Office.) 
time ranging from about 6:30 to 8 a.m.; this maximum 
is followed by a rapid fall to a minimum about 2 P.M. 
when there is another rise to a peak about 8 p.m. The 
afternoon minimum is as low or lower than the early 
morning one. Similar results have been found in studies 
of pollution in British cities. These observations suggest 
strongly that the pollution which causes the morning 
maximum is carried downward from aloft by increased 
turbulence associated with increased lapse rates caused 
by solar heating, in much the same manner as described 
in the preceding section for elevated single sources [88]. 
The seasonal shift in the time of the morning maximum 
is clearly related to the seasonal variations in the solar 
heating. 
The stability of air aloft has a considerable influence 
on summer pollution in the Los Angeles area, as pointed 
out by Beer and Leopold [7], and by Magill [54]. During 
that season the air at higher levels is relatively warm as 
