January -30° F. (as for land); July 23° F.
Supposing that the belt were entirely oceanic, the mean temperature in 60° N. would be:
January 29° F.; July 41° F.
These figures show how enormously effective the land and sea distribution really is. From Appendix it is easy to calculate the probable temperature distribution resulting from any arrangement of land and water masses. Since the geography of the more recent geological periods is now known in some detail, we have thus a means of restoring past climates and discussing the distribution of animals and plants in the light of this knowledge. Of course it is not pretended that no other possible causes of great climatic variation exist, but no others capable of seriously modifying temperature over long periods are known to have been in operation. As we shall see later, there are solar and other astronomical causes capable of modifying climate slightly for a few decades or even centuries, but these are insignificant compared with the mighty fluctuations of geological time.
In applying the results of this “continentality” study to former geological periods the method adopted is that of differences. The present climate is taken as a standard, and the temperatures of, for instance, the Glacial period are calculated by adding to or subtracting from the present temperatures amounts calculated from the change in the land and sea distribution. This has the advantage of conserving the present local peculiarities, such as those due to the presence of the Gulf Drift, but such a procedure would be inapplicable for a totally different land and sea distribution, such as prevailed during the Carboniferous period. That it is applicable for the Quaternary is perhaps best shown by the following comparison of temperatures calculated from the distribution of land, sea and ice with the actual temperatures of the Ice Age as estimated by various authorities (inferred fall):
| Locality. | Author. | Inferred Fall. | Calculated Fall. | ||
| Jan. | July. | Mean. | |||
| °F. | °F. | °F. | °F. | ||
| Scandinavia | J. Geikie | More than 20 | 36 | 18 | 27 |
| East Anglia | C. Reid | 20 | 18 | 13 | 15 |
| Alps | Penck and Brückner | 11 | 13 | 9 | 11 |
| Japan | Simotomai | 7 | 9 | 5 | 7 |
It is seen that the agreement is quite good.
There is one other point to consider, the effect of height. The existence of a great land-mass generally implies that part of it at least has a considerable elevation, perhaps 10,000 or 20,000 feet, and these high lands lave a very different climate to the neighbouring lowlands. Meteorologists have measured this difference in the case of temperature and found that the average fall with height is at the rate of 1° F. in 300 feet. In the lower levels the fall is usually greater in summer than in winter, but at 3000 feet it is fairly uniform throughout the year. Consequently, quite apart from any change in climate due to the increased land area, an elevation of 3000 feet would result in a fall of temperature of 10° F., winter and summer alike. This reinforces the effect of increased land area and aids in the development of ice-sheets or glaciers.
The effect of geographical changes on the distribution of rainfall are much more complicated. The open sea is the great source of the water-vapour in the atmosphere, and since evaporation is very much greater in the hot than in the cold parts of the globe, for considerable precipitation over the world as a whole there must be large water areas in the Tropics. In temperate latitudes the water-vapour is carried over the land by onshore winds, and some of it is precipitated where the air is forced to rise along the slopes of hills or mountains. Some rain falls in thunderstorms and similar local showers, but the greater part of the rain in most temperate countries is associated with the passage of “depressions.” These are our familiar wind- and rain-storms; a depression consists essentially of winds blowing in an anti-clockwise direction round an area of low pressure.
These centres of low pressure move about more or less irregularly, but almost invariably from west to east in the temperate regions. They are usually generated over seas or oceans, and, since a supply of moist air is essential for their continued existence, they tend to keep to the neighbourhood of water masses or, failing that, of large river valleys. In a large dry area depressions weaken or disappear. Their tracks are also very largely governed by the positions of areas of high pressure or anticyclones, which they tend to avoid, moving from west to east on the polar side of a large anticyclone and from east to west on the equatorial side. Since anticyclones are developed over the great land areas in winter, this further restricts the paths of depressions to the neighbourhood of the oceans at that season.