124 
J. HOPKINSON—CLIMATE OE HERTFORDSHIRE. 
Throughout all the chalk area the rivers have cut their way 
through the superficial deposits, so that whatever may he the 
nature of the soil upon the hills, that on their flanks, as far down 
as the alluvial plains, is dry. In the valley of the Lea such 
a plain extends downwards from Hertford, and in the valley of 
the Colne from about a mile above Watford, where for a short 
distance it is lower than the present bed of the river. Both these 
plains are frequently flooded, thus increasing the moisture of the 
air above them, and even in dry weather, early in the morning 
or late in the evening, a thin stratum of ground-fog may sometimes 
be seen resting upon them while the air above the higher ground is 
perfectly clear. Over the greater part of the county, also, there 
are dry valleys in the Chalk down which rivers flowed when its 
plane of saturation was at a higher level than it is now, or at 
a more remote period when the London Clay extended farther 
over the Chalk; they are cut into the Chalk, thus affording 
a permanent record of the former existence of many more 
streams than we now possess,—streams which, where they have 
flowed over clayey lands, have by their erosive action extended 
the area of bare chalk, and have thus contributed to the present 
dryness of the soil. 
On the whole, therefore, the division of the county, along the 
line indicated above, into two very unequal portions, a large 
north-western area with a dry soil and atmosphere, and a small 
south-eastern area with a comparatively moist soil and atmosphere, 
is a sufficiently accurate climatic division. 
Under the title of “ Climatological Observations taken in Hert¬ 
fordshire,” and subsequently, when other observations were added, 
of “ Meteorological Observations taken in Hertfordshire,” there 
have been published annually in our ‘ Transactions ’ some of the 
results of observations taken at five meteorological stations in the 
county during the twelve years 1887 to 1898. It is believed that 
this period is a sufficiently long one, and that the stations are 
sufficiently numerous and widely-distributed, for the results of 
these observations to be of value in enabling a knowledge of the 
climate of the county to be arrived at, except for one element 
of climate, and that is rainfall. 
To arrive at the average rainfall over an area of 630 square 
miles, and to form an idea of the extreme falls, five stations are 
inadequate, and the period of twelve years is much too short. 
Although, therefore, the rainfall is tabulated from these observations, 
and the mean monthly fall for the 60 years 1840 to 1899 is given, 
reference should be made for full information on this subject to 
the “Reports on the Rainfall in Hertfordshire” which have 
appeared annually in our ‘Transactions’ for the last twenty-five 
years, and also to the author’s papers on “ Half-a-Century’s Rainfall 
in Hertfordshire,” * and on “Hertfordshire Rainfall, Percolation, 
and Evaporation.” f 
* ‘Trans. Herts Hat. Hist. Soc.,’ vol. vi, pp. 81-88. 
f lb., vol. ix, pp. 33-72. 
