200 Forecastiny Weather. 



45" to 25'J iu the spriug is much more destructive than a change from 35" to 

 lo" in winter, or from 80° to 60" in summer. 



•' Destructive frosts in spring may arise from three separate causes, and on 

 occasions two of these causes may combine 



" The first is the occurrence of an ordinary type of cold, wintry weather. 

 The barometer falls with a northerly, north-easterly or easterly wind, and we 

 get, in consequence, a cold spell, probably with snow. 



" The second of the two causes is noticable in changeable April weather, 

 and may be referred to the passage of barometric depressions. If the barometer 

 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 happens towards evening, and the wind drojjs 

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



" The third recognised cause refers especially to night frosts, which may 

 occur with destructive effect if the night is clear (even after a warm sunny 

 day), and the destruction is the more complete if the day which follows the 

 cold night is itseK 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 comes at the close of a boisterous day, with a 

 veering of the wind to the north-west or north. 



" We may consider a little more in detail the process by which the cooling 

 takes place on calm, clear nights. Over our Midland Counties in April there 

 is on the average a difference of 16-3'^ between the highest temperature of 

 the day and the lowest temperature of the night. In May the difference 

 amounts to 17-7". During clear weather the day temperature is increased 

 by the warm sunshine, but the night temi)erature is lowered.' After the sun is 

 gone, 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, which are a sure sign of their having been cold. But in turn they 

 cool the air next to them, and the cooled air in its turn trickles like water 

 down hill 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 trickled away is 

 practically part of the original undisturbed supply, and is comparatively warm. 

 The plants on the hill side get the air which trickles down from above, and 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 much in the opposite way to 

 that in which a joint roasts in a ' hastener ' before a fii'e. The shape of 

 the ground which causes a pool of stagnant air to form takes the part of the 

 ' hastener.' 



" 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 thermometer ' 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 protection of the screen is very striking. 

 The differences are not altogether due to enclosure in a screen ; the height of 

 the screen above the grotind has something to do vv^ith it because the trickling 

 stream of cold air keeps to the ground, and is not often very thick. 



" The following conclusions regarding the distribution of temperature 

 during frosts on calm nights are well established : — 



" (a) The frost becomes more severe as one goes from the hills down 

 to the valleys, and hollows on the hillsides are colder than the more 

 exposed parts. 



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