THE PLANT ECOLOGY OF THE COAST BELT OF NATAL. 381 



blows from the land in the early mornina;, being due to the 

 land cooling down faster at night. Such laud- and sea- 

 breezes are too well known to need further description. They 

 occur throughout the year except when they are obscured by 

 the stronger easterly or southerly winds already described. 



(d) During the winter months anticyclonic conditions 

 generally prevail over jSTatal. The high pressure brings cold 

 nights, and with a still atmosphere cold air drainage down the 

 valleys at night is a regular phenomenon. Actual frosts, as 

 already pointed out, are rare, and cold valley mists are not a 

 regular occurrence during winter nights on the coast belt as 

 they ai'e in the midlands. Nevertheless, since the vegetation 

 is so much more tropical on the coast belt, cold air drainage 

 at night is a factor of considerable importance. The cold air 

 flows down the valley like a river and often its upper limits 

 are very sharply defined. As one descends a valley at night 

 one plunges into it suddenly. The plants at a certain level 

 may have to endure temperatures several degrees lower than 

 those a few yards higher up. 



(e) Foehn Winds. — These are the dry hot winds of Natal. 

 They are most frequent in autumn as the anticyclonic con- 

 ditions disappear and cyclones occur more frequently off the 

 coast. The air is drawn out from the mouths of the valleys 

 and the air from above descends to take its place. Being 

 w^armed by compression it becomes hotter and hotter the 

 deeper the valley. Foehn winds are most pronounced over the 

 deep valleys of the midlands. They occur only occasionally 

 over the south coast, but they are more frequent over the 

 north coast opposite the great river valleys of the Umvoti and 

 Tugela. As already pointed out the cyclones generally move 

 from the north-east and recurve in towards Dui-ban. On the 

 south coast they push over the land of the coast belt and 

 pressure decreases. This probably explains why "hot winds ^' 

 are infrequent on the south coast. It is difficult to over- 

 estimate the effect of these dry, hot winds. They are 

 probably of the utmost importance in determining the dis- 

 tribution of thorn veld and thorn scrub, Avhich are common 



