------------+--------------------+--------------+----------------- | | | Calculated Fall. Locality. | Author. |Inferred 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.