566 Principles of Weather Forecasting. 



of the sun fall alternately 2 3% degrees north and south 

 of the equator, the regions of greatest heating must also 

 move north and south with the apparent shifting of the 

 sun, and this causes the equatorial and tropical calm belts 

 to move north and south. As a result of this shifting there 

 is a tendency to develop two rainy and two dry seasons each 

 year in the regions over which the calm belts travel twice. 



CONTINENTAL WINDS. 



721. Continents Disturb the World System of Winds and 

 Weather. The small specific heat of the land, its opaque 

 nature and the absence of currents of all kinds in it cause 

 the land surface to warm rapidly in the day and during 

 summer, and to cool rapidly at night and during the win- 

 ter. On the other hand the transparency of the oceans, 

 which allows the sunshine to be distributed through a great 

 depth of water ; their high specific heat and the horizontal 

 and vertical currents to which they are subject, all con- 

 spire to make the oceans, relative to the lands in the same 

 latitude, warm in winter and cool in summer. 



During the long days of summer and short nights, in 

 high latitudes, the land becomes much warmer than the 

 water and tends to develop ascending currents and a low 

 air pressure, causing the winds to tend to blow toward the 

 land at the surface and away from the land above in sum- 

 mer ; but in winter, when the nights are long and the days 

 short, the ground becomes very cold and the air contracts, 

 causing the upper air to blow in over the continents above, 

 thus developing high pressure, which forces the surface 

 winds to move from the land toward the ocean in winter. 



There is therefore a tendency for the weather of conti- 

 nents to be rainy and cloudy in summer and dry and sunny 

 in winter, and for the oceans to be dry and sunny in sum- 

 mer and wet and cloudy in winter. This is a very for- 

 tunate relation, because it diminishes the evaporation on 

 the land and increases that on the ocean a:id thus makeu 



