Hints from the Health Department. Leaflet from the archive of the Society of Medical Officers of Health. Credit: Wellcome Collection, London
[Report of the Medical Officer of Health for London County Council]
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It will be noted from fig. 3 that both average and maximum visibility was usually
highest in the quadrant south to west in those months which were not marked by a
generally low visibility in all directions. It is noteworthy that, whereas the mode
for the months January, February, November and December, 1934 was 4 in the
international code (M.O. indication letter H) at County Hall, it was 6 (H) at Kew
observatory, showing the decrease of visibility in London due to the influence of
urban conditions.