932 



Night Frosts in Spring. 



[FEB, 



meter and wind are watched, it will be noticed that after rain, 

 with a falling barometer and a southerly or south-westerly 

 wind, the wind veers to the west, north-west, or north, and 

 becomes apparently drier, and the weather clears and becomes 

 cold. If this change occurs towards evening, and the wind 

 drops when the sky clears, a frosty night is almost certain. 

 (3) The third recognised cause refers especially to inight 

 frosts, which may occur with destructive effect if the night 

 is clear, even after a warm sunny day, and the destruction 

 is more complete if the day which follows the cold night is 

 itself sunny and warm. The most destructive frosts occur 

 when the causes here noted as the second and third combine, 

 when cold, clear weather, with a calm night, follows a bois- 

 terous day, with a veering of the wind to the north-west or 

 north. 



During clear weather the day temperature is increased by 

 the warm sunshine, but the night temperature is lowered. 

 After the sun is gone, and when the earth and its covering 

 herbage are exposed to a clear sky, they lose heat and get 

 colder than the air. They may cover themselves with dew 

 or hoar frost, this being a sure sign of their having been 

 cold. In turn they cool the air next to them, and the cooled 

 air in its turn trickles like water downhill to the valleys. 



In these circumstances the plants on the tops of the hills 

 are fortunate, for the air which replaces that which has been 

 cooled and has trickled away is practically part of the original 

 undisturbed supply and is comparatively warm. The plants 

 on the hillside get the air which trickles down from above, and 

 which is consequently colder than that enjoyed by the plants 

 at the top. The cooling goes on as the air flows down to 

 the valleys. But the worst fate awaits the plants in the valleys, 

 where pools of cold air form. Thither the coldest air 

 gravitates, and for the plants at the bottom the air is stagnant ; 

 consequently they may cool by exposure to the open sky. 



The effects of this process of cooling may be very different in 

 situations which are quite near to one another. Meteorologists 

 are accustomed to note such effects by having one ther- 

 mometer "on the grass," supported on a couple of forked 

 twigs close to the ground, and another "in the screen," which 

 means that it is kept in a louvred box at a height of four 

 feet from the ground. On calm, clear nights the effect of the 



