Encyclopedia of Environmental Science and Engineering, Volume I and II

AIR POLLUTION METEOROLOGY 67

direction on the other, is that the wind direction has different

effects at various sites, depending on the location of the

sources; the other factors have similar effects generally.

EFFECT OF AIR POLLUTION ON LOCAL

AND REGIONAL WEATHER

Visibility

The most obvious effect of air pollution is to reduce vis-

ibility. This effect has been studied frequently by comparing

visibility in different parts of a city, or the visibility in a city

with visibility in the country. For a summary of many such

investigations, see Peterson, 1969.

To give some examples: Around London and

Manchester, afternoon visibility less than 6^14 miles occurs on

more than 200 days; in Cornwall in SW England, the number

is less than 100. In central London, there are 940 hours a year

with visibilities less than^58 mile; in SE England, only 494.

In many cities, visibilities have recently improved prob-

ably due to control of particle emissions; however, as men-

tioned before, some of this change may be due to changes in

large-scale weather patterns.

Although decreased visibility is usually associated

with industrial or automobile pollution, considerable atten-

tion has been paid recently to decreased visibilities due to

the “contamination” of the atmosphere by water droplets

by industry. This problem arises because many processes

generate excess heat; if this is added to streams and lakes,

undesirable effects ensue; hence, progressively more and

more heat is used to evaporate water which is then emit-

ted into the atmosphere, and later condenses to form water

plumes.

There are many unpublished studies estimating the

effect of cooling towers on visibility. This varies greatly

with meteorological conditions, but is particularly serious in

winter, then the air is nearly saturated and little additional

vapor is required to produce liquid drops. Under those con-

ditions, water plumes from industries produce clouds and

fog which may reach over a hundred miles from the sources.

Automobile accidents have been blamed on such fogs, par-

ticularly when the particles freeze and make roads slippery,

adding to the visibility hazard.

Sunshine Intensity

“Turbidity” is an indicator of the reduction of light due to

haze, smoke and other particles. Turbidity is now being

monitored at many places in the world. It is quite clear that

it is larger over cities than over the country; it has been sug-

gested that the average decrease of sunshine over cities is

15 to 20% due to pollution. The effect is even larger if only

ultraviolet light is considered.

Control of smoke emission in cities such as London has

caused a very noticeable increase of sunshine intensity: for

example the hours of “Bright sunshine” increased by 50%

after control measures had become effective. Again, for a

summary of some of these studies, the reader is referred to

Peterson, 1969.

Precipitation Amount

There have now been several studies suggesting that pre-

cipitation is increased downstream of industrial centers.

The investigations are of a statistical nature, and it is not

known whether the effects are due to increased convection

(increased heat), increased condensation nuclei or increased

water vapor. Further, the reliability of the statistics has been

questioned.

For example, Changnon (1969) found a large precipita-

tion anomaly at La Porte (Indiana) just downwind of large

industrial complexes of Northwestern Indiana. But change in

observational techniques of rainfall and other uncertainties

have thrown doubt on the results. Hobbs et al. (1970) have

compared rainfall distribution in Western Washington before

and after the construction of industries and found an increase

by 30% or so; but some of this increase may have been due

to “normal” climatic change. For a summary of these and

other studies see Robinson (1970). It becomes quite clear

from this summary that more, careful investigations of this

type are needed before effects of air pollution on precipita-

tion patterns can be definitely proven.

A large study (Metromex) found strong enhancement of

precipitation downwind of St Louis. But this may be due to

the St Louis heat sources rather than to pollution.

Acid Rain

There is no question that acid rain is produced by atmospheric

pollution. The acidity of rainfall is large only when the wind

direction suggests industrial or urban sources. Most impor-

tant is sulphuric acid, produced by power plants or smelt-

ers, the effluent from which contains SO 2. Also important is

nitric acid, which is formed mostly from nitrogen oxides in

car exhausts. Acid rain has done important damage to lakes

and forests; but there is controversy how to deal with the

problem. For example, the relation between acidity and SO 2

may be nonlinear, so that substantial reduction of SO 2 may

not effect acid rain significantly.

GLOBAL EFFECTS OF AIR POLLUTION

Natural Climatic Changes

We will assess the effect of some atmospheric pollutants as

to their ability to change the earth’s climate. In doing so,

we are hampered by the fact that the present climate is pro-

duced by a multitude of interacting factors; if one factor is

changed, others will too, and a complex chain reaction will

ensue. These reactions can be studied by complex math-

ematical models of the atmosphere, which so far have been

quite successful in describing the existing climate. But, as

yet these models contain assumptions which make it impos-

sible at this time to assess accurately the effects of changes

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