The nearness of large surfaces of water in summer tends to increase the rainfall, when water is colder than its surroundings, while in winter it has the opposite effect. The neighbourhood of the sea is for the west of England and islands adjacent, a cause of increased rainfall.
The influence of winds on the rainfall has been already considered. In Great Britain south-west winds more especially increase the rainfall. In their course they have travelled over the Gulf Stream and the general equatorial current, and have thus received warmth and moisture. The condensation of their moisture liberates a large amount of latent heat, thus raising the temperature of this country. In summer, however, south-west winds are cool and moist, as the Atlantic is not so hot as the continents of Asia and Europe over which other winds have travelled.
In England the average rainfall is about 33 inches, in Scotland 46, and in Ireland 38 inches. In the east of Great Britain, the rainfall is from twenty to twenty-eight inches. On the west coasts of Scotland and Ireland it is from 60 to 80 inches; and in some parts of Cumberland may be about 150 inches per annum. The annual rainfall varies greatly from the average for a number of years. In this country it has been estimated that the maximum annual rainfall exceeds by one-third, and the minimum annual rainfall is less by one-third than the average rainfall of a series of years.
The number of rainy days by no means corresponds with the amount of rainfall. There are fewest rainy days at the equator, where the rainfall is greatest. The rain diminishes the relative humidity of the air, and purifies it from dust.
[CHAPTER XXXV.]
METEOROLOGICAL OBSERVATIONS.
The Royal Meteorological Society recognises stations for the making and recording of observations of three kinds: (1) Second Order Stations, at which observations are taken twice daily at 9 a.m. and 9 p.m.; (2) Climatological Stations, at which the observations are taken once daily, at 9 a.m.; (3) Stations at which one or more elements only, e.g. rainfall, are observed. All instruments used should have been previously verified at Kew Observatory, so that the corrections for index error may be known.
The Barometer used should be of a standard kind. Five chief kinds of barometer are in use, only the last two of which are sufficiently accurate for scientific purposes.
1. The Dial or wheel barometer consists of a bent tube A B, the open end of which supports an ivory float B. This, as it rises and falls with the mercury, by means of the rack C turns a wheel, in the axle of which a needle is fixed. The needle turns in one direction, or the other as the mercury rises or falls (Fig. 45); the dial is divided by comparing it with a standard barometer. As the ordinary variations of the barometer are from 28 to 31 inches, the circumference of the wheel is made exactly 1½ inches, and thus the float B will rise or fall 1½ inches for a rise or fall of 3 inches in the barometer.
