CLIMATIC INFLUENCES
251
of the year's rain, but the heavy clays predominating in this district remain
damp because drainage is slow and evaporation is slight. After April there
is a rise to a secondary maximum in July and August, each with an average
of 13 or 14 days with rain, mainly in summer thundershowers. September
is noticeably dry, but October has the heaviest monthly rainfall, with 11
per cent of the yearly fall, or an average of 2.8" falling on an average of
17 days.
The driest recorded year over this part of Essex was 1921, when only 10.5"
were recorded at nearby Ilford, the wettest 1903, when 40" fell at Buckhurst
Hill and Epping. The mean annual rainfall deviation is 10-11 per cent, a
figure general for East Anglia. The largest fall in one month at Epping
was 7.72" in a September, the smallest 0" in a February. September is the
most unreliable month, having recorded 16 per cent and 320 per cent of its
mean. Summer thundershowers have brought 4.5" of rain in one day
(August 1, 1888) to Upminster in the east, and 2.44" in 50 minutes at
Enfield, on the western margin (July, 1905). Rain (at least .01" per day)
falls on an average of 160-165 days per year.
SNOW AND HAIL
The average number of days with snow falling on the ground below 200'
varies from 15 in the suburban part of the region to 17 in the north-east.
It seems reasonable to assume 18 to 20 days of snow lying at 250' on the
ridges.
Observations at Kew seem to indicate that the yearly average of days with
hail lies between five and six, with a maximum in March. The average
number of days with thunder increases from south to north from 12 to 14.
TEMPERATURE (°F.)
Judged by the annual temperature range, the Epping District is one of
the most extreme in the British Isles, and one of the warmest in summer.
The march of monthly temperatures varies from urban Tottenham, 40.9°
in January to 63.7° in July, to rural Chelmsford, with an average of 39.3°
for January, to 61.6° in July. Extreme monthly averages of 72.2° have
been recorded for July in both Tottenham and Enfield, while a January
average of 33.8° has been known at Chelmsford. Between 1901 and 1940,
90° was exceeded in each of seven months at Chelmsford, and 96° was
recorded on 1st August, 1932. In the same period, less than 5° was
recorded in each of three months at Chelmsford, and the absolute minimum
was 2°. The greatest yearly range is found just outside the district at
Enfield, 23.4°, and is one of the largest in the British Isles; the lowest
at Chelmsford, 22.6°. It seems safe to assume that the average number
of days with frost over the region is about 105, with a maximum in
January.
Evaporation figures are mainly important to the Lea valley reservoirs,
for which a mean annual value of 14.4" is assumed, but in drought years
evaporation has been known to exceed rainfall by as much as 5".
WIND
The most frequent winds over the area are from the S.W., 17 per cent
of the total; next are due W. with 13 per cent, and N.E. and N. with
12 per cent. These directions are possibly emphasised by the marked
N.E./S.W. grain of the land. In spring the frequency of winds from the
N. and N.E. increases slightly. Winds from the E. (9 per cent) are
associated with anticyclonic weather, mainly in winter. Wind velocities
exceeding 25 m.p.h. are rare. The month with most wind is March, with