Isothermal lines (lines of equal mean temperature) around the world bend up and down, the bendings being determined by the relative position of continents and oceans. New York has the same mean temperature as London, though New York is as far south of London as Madrid. This fact illustrates the fallacy in judging of the climate of a locality by the annual mean temperature. Means, it has been well said, are general truths but particular fallacies. One should know the extremes of temperature, and the extremes for each month of the year, as well as the amount and distribution of the rainfall, and the amount of sunshine, before judging of a local climate.
Winds are due to differences in atmospheric pressure caused by changes in temperature and moisture. Inasmuch as the temperature and degree of moisture of air vary with the prevailing winds, their consideration becomes very important. Winds bring with them the temperature of the air they have traversed: thus, in England, south winds are warm, while north winds are cold. Winds coming over an ocean cause less variation in temperature than those which have passed over an extensive tract of country. Thus, moist ocean winds are accompanied by a mild winter and cool summer, while dry continental winds cause the reverse conditions. The amount of moisture capable of being carried by a current of air increases with its temperature; therefore, equatorial winds become moister as they proceed, while north winds become drier. The south-west winds, in the British Isles, being both oceanic and equatorial, are very moist, while the north-east winds, being both northerly and continental, are peculiarly dry and parching.
Owing to the atmospheric pressure diminishing from the south of Europe northwards to Iceland, south-west winds are the most prevalent in Great Britain; and as this diminution of atmospheric pressure is greatest in the winter months, south-west winds are most common at this season. The result is that the temperature of these islands is higher than that due to mere latitude, and the temperature on the west coast is fairly uniform from Shetland to Wales.
Mountain ranges have an important bearing in determining the character of the prevailing winds. If the range is perpendicular to the direction of the winds, the latter lose the greater part of their moisture, and the places to leeward being exposed more completely to solar and terrestrial radiation (from comparative absence of aqueous vapour), winter becomes colder and summer hotter. The difference between the climates of the west and east parts of Great Britain is largely due to this cause. In Ireland, the mountains are not grouped in ranges running north and south, but in isolated masses, and the difference in climate between the east and west coasts is consequently less marked.
The prevailing winds have a great influence on the rainfall. (1) Thus if the wind has traversed a considerable extent of ocean, the rainfall is moderately large. (2) If a wind reaches into a colder region, its saturation point is lowered, and the rainfall is greatly increased; and if a range of mountains lies across its path, the rainfall on the side facing the wind is greatly increased, but diminished on the opposite side of the range. (3) If a wind after reaching land proceeds into lower latitudes or warmer regions, the rainfall is small, or absent. This accounts for the rainless summers of California, North Africa, and South Europe.
The Barometric Pressure varies daily, being at its maximum at about 9 a.m. and 9 p.m. The average range in the tropics amounts to 0·1 inch, but in this country does not usually exceed 0·02 inches. During the year the minimum barometric pressure usually occurs about the end of October, while the maximum is usually at the end of May or early in June. The ordinary variations in barometric pressure with changes of weather have little apparent effect on health; but more extreme changes produce marked effect. In mountain-climbing faintness and nausea may be caused at great altitudes. At the opposite extreme, in pier-driving and laying the foundations of bridges, men have to work in air-chambers at a pressure of from three to four atmospheres. Then what is known as “caisson disease” may be produced. The usual symptoms are discomfort or pain in the ears, giddiness, bleeding at the nose, vomiting, or even temporary paralysis. In such occupations it is most important that on leaving the air-chambers the atmospheric pressure should be gradually lowered.
The use of the barometer as a weather indicator is based on the fact that moist air is lighter than dry air. Hence, if the air is moist and rain imminent, the barometer falls rapidly. The maximum daily range in this country is rarely greater than 3 inches. Weather observations can be based on records kept at one spot. Their value is greatly enhanced, when such observations are compared with others distributed over a wide area. The wider the area from which such observations are collated, the more accurate the deductions that can be secured. If observations of places at which the barometrical pressure is identical be recorded on a map, we have a synoptic map, and the lines of equal barometrical pressure connecting these points are called isobars. The modern development of meteorology, enabling forecasts of weather to be made with approximate accuracy, is based chiefly on telegraphic communication of information, enabling isobars to be constructed.
It is found that isobars arrange themselves into seven chief forms (1) Cyclones. (2) Secondary cyclones. (3) V-shaped depressions. (4) Anti-cyclones. (5) Wedge-shaped isobars. (6) Cols. (7) Straight isobars.
Each of these varieties is shown in Fig. 43, which embraces the conditions in Europe, the eastern part of the United States, and over the North Atlantic on a certain day.
The closeness of the isobars, i.e. the rapidity of changes in atmospheric pressure determines the barometric gradient. The steeper this gradient, the greater the velocity of the wind in any given place. The distance between two isobars is equal to a change of a tenth of an inch in the mercury in the barometer. The direction of the wind in a given place is from the higher to the lower isobars. This is expressed in Buys Ballot’s law, which states that in the northern hemisphere, if you stand with your back to the wind, the lowest pressure is to your left and in front.