252 THE ESSEX NATURALIST
a mean wind velocity of about 9 m.p.h.: over half the winds of the year
are between 4 and 12 m.p.h., and the strongest of them occur 60011 after
noon.
SKY CONDITIONS
Mean daily duration of sunshine increases from 38 hours in the industrial
south-west to 4.2 hours in the north-east. These figures represent 33 to 35
per cent of the possible sunshine. Over R.A.F., North Weald, the average
annual number of occasions with cloud below 1,000' is 56 at 1h., and 51
at 13h.
Fog is common; the normal radiation fog is here intensified by London
smoke, carried by south-west winds and canalised by the deep, industrialised
Lea valley. At Enfield the number of days with fog is 34.6 per year, with
a maximum of 8.5 in January. This decreases into Essex to a yearly 30
at Ilford and 25 at Woodford and rural Abridge.
The distinctive features of the climate of the Epping District are its
dryness, its traces of "continentality" in a large temperature range and a
secondary maximum of rainfall in summer and the influence of London, to
be discussed in the next section.
MICROCLIMATOLOOY
Of more practical interest than the average state of weather over the
district are "the climates near ground", or rather, those nuances of climate
which arise from the nature of the earth's cover, be it houses and streets,
crops, forest, grassland or lake.
The microclimates of the Epping District are manifold and virtually
unexplored. Here there are the microclimates of the fringe of a vast city,
those of an ancient forest, those of heavy clay slopes and light gravel ter-
races, those of pasture, croplands and large reservoirs. The apparatus
available was such that only the greatest microclimatic anomalies could be
discerned, and this section will be concerned only with the climatic influence
of the edge of London, of Epping Forest, and frost drainage to the Lea
valley, in the form of an examination of the premises of Dr. Geiger's
authoritative work (1950).
Bilham (1948) calculates that such suburbs as Walthamstow receive only
78 (in winter) to 99 (in summer) per cent of the sunshine of country areas