This section is from the book "The Principles And Practice Of Modern House-Construction", by G. Lister Sutcliffe. Also available from Amazon: How Your House Works: A Visual Guide to Understanding & Maintaining Your Home.
"Climate is the genera] resultant of temperature, atmospheric pressure, humidity, rainfall, and other less-known factors, modified by soil, aspect, and various local conditions."
Temperature is primarily dependent upon latitude, t»ut is lot-ally and periodi-cally modified by the direction and character of the ocean-currents and prevailing winds, whether warm or cold; thus, Labrador and Newfoundland are in the same latitudes as the British Isles and France, but while the western shores of the latter countries are washed by the warm Gulf Stream from the West Indies, the former are chilled by the icy currents flowing southwards from the eastern coasts of Greenland and Baffin's Land, and by the westerly gales from the Pacific which, though warm and moist when they heat on the wooded hills of British Columbia, have been changed in their passage across the bleak wilds of Rupert's Land. These general characters of eastern and western shores may also be observed in our own islands, in the contrast between the dry. bracing climate of the east coast from Norfolk to Aberdeen, and the mild, humid climate and heavy rainfall of Ireland, Wales, Devon, and Cornwall. And the unimportant part played by latitude alone is shown in the fact that so far north as the Shetlands snow rarely lies many days together, though the extreme humidity of the air and the absence of dry heat preclude the maturation of cereals, oats excepted. Yet there are. even within narrow limits, modifying influences, as in the valley of the Shin in Sutherland, where crops ripen in the drier air which fail to do so in Argyle, 2° of latitude further south.
By Edward F. Willoughby, M.D.(Lond.) Diplomats In State Medicine Of London University, And In Public Health Of Cambridge University Author Of "Public Health And Demography "Health Officer's Pocket Book", Etc.
Mean temperatures, unless given for every month in the year and with the maxima and minima or range of the monthly oscillations, afford little useful information as to the true character of a climate. No part of Great Britain is remote enough from the sea to produce what is called a continental climate, that is, one in which hot, dry summers alternate with winters of severe cold and frost, continued until the air becomes clear and dry. Our climate is essentially insular, and in choosing a residence regard should be had to the prevailing winds, rainfall, humidity, the amount of sunshine, and the local features of elevation, aspect, and soil. In mild and humid districts, as Devon and Argyle, small tracts of moderate rainfall may be found, and in colder and drier regions, as Yorkshire and the N.E. of Scotland, there are spots or areas sheltered by hills from the biting east and north-east winds.
Air is capable of holding in an invisible and impalpable form a quantity of water-vapour varying with the temperature but increasing more rapidly as this rises. Even when the temperature is far below freezing the air is not absolutely dry, holding about 1 grain of water in the cubic foot. The air is said to be saturated when it contains as much water as, at that particular temperature, it can hold without depositing any as dew; and vice versd, given a certain amount of vapour, the temperature at which it would begin to fall is called the dew-point. The proportions for saturation are 2 grains at 30° F., 3 grains at 40°, 4 grains at 50°, 6 grains at 60°, 8 grains at 70°, 11 grains at 80°, and 15 grains at 90°. The percentage of saturation represents the degree of humidity; thus, 4 grains in the cubic foot will be saturation at a temperature of 50° F., 66 per cent of humidity at 60 F., and only 50 per cent at 70° F. In the three instances just cited the air will be felt damp, agreeable, and painfully dry respectively, although the actual amount of moisture remains the same.
As the temperature rises, more vapour is absorbed by the air from the surface of land and water, to fall as dew or appear as mist should the temperature sink below the point at which the additional vapour can be held (as may be seen on every clear summer evening and night), and to l)e taken up again invisibly with the returning warmth of day; indeed, there is absolutely more moisture in the air on a hot "dry" day in August than in a cold "damp" day in November; and more in a warmed and occupied room than there is in the open air, especially in winter, the vapour in this case being derived mainly from the breath of the occupants and the combustion of gas.
Table XLI. gives the weight (in grains) of vapour in a cubic foot of saturated air at temperatures from 14° to 88° F., and the diagram, Fig. 716, exhibits the same information graphically. From these it may be seen that at 80° F. the air is saturated by 11 grains, at 60° F. by 6 grains, and at 40° F. by 3 grains; and it is clear that if the humidity be 75 per cent at 60° F., dew will fall when the temperature sinks to 53° F., but if the humidity at 60° be only 50 per cent dew cannot fall until a temperature of 41° is reached. One can thus see how the young leaves and flowere of fruit-trees are more liable to suffer from light frost in clear nights after warm sunny days, than in the cold, dull weather often accompanying dry N.E. winds; for with a day temperature of 60° and 75 per cent of humidity the lightest frost will cause a heavy deposit of hoar-frost, while
Fig. 710. - Weight (in grains) of water-vapour in a cubic foot of saturated air from 16o to 90o F with a temperature of 40° and humidity of 50 per cent none will fall unless the night temperature sink to 24°.