128 The Realm of Nature CHAP. 



are caused by the action of condensing and evaporating 

 water in the atmosphere ; the afternoon minimum is caused 

 by a bodily removal of air upward and to the east, the 

 evening maximum results from the sinking down and piling 

 up of air from the west. 



184. Land and Sea Breezes. The different heating 

 and cooling of land and sea ( 164) produces a regular 

 change in the daily winds of tropical coasts and islands, and 

 in very calm clear weather similar effects may be observed 

 in all latitudes. An island or strip of coast when heated 

 by the Sun gives rise to ascending currents of air (Fig. 25). 

 About 10 A.M. these ascending currents, having carried the 



FIG. 25. Sea-Breeze during sunshine. FIG. 26. Land-Breeze, at night. 



air into the upper regions, produce a fall of pressure over the 

 land compared with that over the cooler sea, and a sea- 

 breeze sets in, at first as a very gentle air, but gradually 

 increasing in force until about 3 P.M., when the land surface 

 is most highly heated. After that hour the land cools down 

 more quickly than the sea, and as the atmospheric pressure 

 becomes equalised the sea-breeze dies away. The air over 

 the land continues to cool down and to sink ; more air con- 

 sequently flows in above, and the pressure over the land 

 thus becomes greater than that over the sea. A surface 

 land-breeze (Fig. 26) sets in about 8 P.M., often with sudden 

 squalls, which are dangerous to boats. It gradually 

 increases in strength as the land grows cooler until it 

 reaches a maximum about 3 A.M. In the trade-wind 



