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in winter tends to the local formation of soot fogs in which water is condensed on
solid nuclei. In summer, when so little solid fuel is used, local conditions have
very little effect on the clarity of the air, hence the good visibility. As to the directions
of greatest and least visibility, these are fairly well defined. From March to
November the average visibility over any arc of 22½° (1/16 of a circle or 2 points of
the compass) was from S.E. to S.W. maxima for individual days and for monthly
averages usually falling in the arc around S.S.W. The minima were usually easterly
mainly around E.N.E. A consideration of the fact that the main industrial quarter
of Greater London is in this direction offers some explanation of the poor visibilitv.