CLIMATIC INFLUENCES 255
This pollarding gives an unusually thick "crown climate zone" over wide
areas of the forest, a zone of slow temperature change generally some degrees
cooler than either the tree crowns or the forest floor in summer. The micro-
thermal variations shown in Fig. 3 are typical of several vertical traverses
taken in the summer of 1952 and show the cool "blanket" effect of the crown
climate zone.
As a whole, the forest seems to have a considerable effect upon the climate
of the district. The forest ridge appears to receive more rain by virtue of
its cover, so that Epping, amid the forest at 345', has an average annual
rainfall of 29.9", while North Weald, on the same ridge at 250', but now
deforested, has only 24.2". Thus the rainfall gradient from Waltham Abbey
to the forest crest is an increase of .22" for every 10' of altitude, while that
from Waltham Abbey to North Weald is less than .04" for every 10'. This
discrepancy obviously cannot be fully explained by the greater height at
Epping, and North Weald has a similar exposure to the west. A parallel,
and deforested, ridge at Havering atte Bower, six miles south-west, has
only 23.2" at 300'.
The forest is warmer than the surrounding farmlands in winter and
autumn, and cooler in summer; it is generally cooler than the city in summer
and slightly colder in winter. The higher forest is more humid than the
surrounding farmlands, which are themselves about 5 per cent more humid
than the suburbs.
As the forest curves from N./S. to N.E./S.W., its effect as a shelter
belt against the dominant wind is marked. The temperature traverse of
September 5th, 1952 (Fig. 4) shows how Theydon Bois and similar suburbs
to the "lee" of the forest have milder conditions than stations on the opposite
side of the ridge on cold or windy days.
REFERENCES
Bilham, E. G. (1938). The Climate of the British Isles. London.
Geiger, Rudolf (1950). The Climate near the Ground. Trans. Milroy
Stewart. Cambridge, Mass., U.S.A. Harvard University Press.