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Summary Report of the Weather in Canada, April, 1919
Magnetic Disturbances, April-May, 1919
Earthquake Records at Toronto and Victoria
A Study of Certain Nebulæ for Evidences of Polarization Effects
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BY SIR FREDERIC STUPART.
PROBLEM which still remains unsolved in Meteorology is
why corresponding seasons in different years differ so much in character. In a previous paper* entitled "Is the Climate Changing" the various conditions in nature which lead to the distinctive features of the climatic zone were given as follows: 1. Distance from the equator.
2. Geographical position in relation to land and sea.
4. The prevailing winds, the outcome of a general atmospheric circulation.
It was pointed out that of the factors enumerated, distance from the equator, distance from the sea, and altitude were, in short periods of history, constants, and that unless deforestation could be shown to have had an appreciable effect, we are thrown back to the assumption that climatic variations through comparatively short periods of history must result from changes in atmospheric circulation, which changes must be further investigated.
*This JOURNAL, 17, 197, 1917.
If to variation in the atmospheric circulation are due the pulsations in climate which we know have occurred down through the centuries and in our own time, how much more certain is it that the variations from year to year are attributable to a somewhat similar cause?
The question to be considered then is-apart from the constant factors which produce the distinctive characteristics of the climatic zones-what are the more direct varying conditions which lead to abnormalities and is it possible to account for these changing conditions?
The past two winters, that of 1918 and that of 1919, were in strong contrast; the former was phenomenally cold and the latter phenomenally mild. Why the difference?
It will be in order to describe what may perhaps be termed the normal distribution of atmospheric pressure, and then show in what respect the distribution of atmospheric pressure in these two years has differed from normal and led to unusual air movements.
Observations on the surface of the earth and of the atmospheric currents, as indicated by clouds and the ballon-sonde, indicate a primary distribution of atmospheric pressure and a general air circulation about as follows:
A belt of low pressure around the globe in the equatorial regions; both northward and southward the pressure increases towards a belt of high pressure between 25° and 30° north and south latitude. and the outcome of this distribution is the northeast and southeast trade winds of the tropics. As the latitude increases beyond the high pressure belts, the pressure gradually diminishes and the barometric gradient of the middle and higher latitudes produces the prevalent westerly winds. If the globe were all land or all water this distribution of atmospheric pressure, and hence the winds, would probably be almost constant and simply fluctuate somewhat in latitude with the annual change in the sun's declination.
But the earth is part land and part water and the very different absorptive and radiating properties of land and water lead to a secondary atmospheric distribution which changes with the seasons and is impressed on the primary system. In the colder seasons over the continents, the high pressure of the primary belts becomes
higher and in both the eastern hemisphere and the western hemisphere overspreads towards higher latitudes, while in the warmer seasons the high pressure either diminishes or disappears and low pressure takes its place.
As far as is known the great cold waves which form over the northern parts of the continents in winter are the outcome of the radiation of heat from the land, and lead to anticyclonic conditions. In normal seasons the anticyclones and cold waves thus formed in the north, move southeastward to lower latitudes, alternating with but outbalancing the low areas which move eastward, carried with the general drift of the primary system of circulation.
The mean barometric charts show how the resultant of atmospheric changes and movements is high pressure over the continents in winter while in the North Pacific and North Atlantic the barometer is low. These low centres have been termed centres of action.
But a study of monthly charts of single winters shows a marked variation in the contrast between the continental high pressure and the ocean low. In some winters in Canada, it is high pressure in northern latitudes which dominates our weather, while in other winters the continental high seems to be overpowered by the Pacific low which most persistently in the form of deep cyclonic areas prevents the formation of anticyclones over Yukon and the Mackenzie River Basin, and the result is the formation of barometric gradients which produce a general flow of air from south to north over Canada and hence higher temperature than the normal.
In the early part of the winter of 1917-18 the anticyclonic type was more pronounced than in any other winter on record, and with most persistent areas of high barometer coming in over Yukon, perhaps indeed offshoots from the great Siberian winter anticyclone, the North Pacific low pressure was situated much further south than usual and its offshoots, in the form of travelling low areas passed into Canada over southern British Columbia and thence kept away to the southward, and the result was a prevalence of northerly winds, not only in the western provinces but also in Eastern Canada.
During the winter just closed we find conditions in strong contrast to those of the previous year; the North Pacific low was