68 



IRISH GARDENING 



Spring Frosts. 



By J. Adams. M.A. 



Of the various hindrances to the work of t he- 

 successful gardener there is probably none of 

 greater consequence than late spring frosts. 

 The various methods of protecting plants 

 against these late frosts are well known, and it- 

 is not my intention to refer to them here, but 

 rather to direct attention to the desirability of 

 compiling records of the occurrence of frosts in 

 certain districts throughout a series of years. If, 

 for example, a ten years' record shows that frosts 

 have occurred in a particular district several 

 times during the month of May. then an alert gar- 

 dener knows that he has to be on his guard, and 

 is not likely to be caught napping. Knowledge in 

 this case is power, and to be forewarned is to be 

 forearmed. In fact, for each district we want 

 an answer to the following questions : — (1) How 

 often during the months of April and May have 

 frosts occurred, and on what dates { (2) What is 

 the latest date for frost to occur in each year ? 

 The longer the period over which the records 

 extend the better, but a ten years' record should 

 be sufficient to give a fair idea of the proba- 

 bilities for the future in each district. The full 

 consideration of the various factors which in- 

 fluence the temperature of the air is too large 

 to be gone into at length here, but the following 

 are some of the more outstanding features : — 



(1) The temperature falls as the distance from 

 the equator increases. This rate of fall would 

 probably be nearly uniform if the land surface 

 were perfectly flat and there were no ocean 

 currents or winds. But the presence of 

 mountains and large bodies of water and the 

 direction of air currents have a profound 

 influence on temperature as well as distance from 

 the equator, so that it is only true in the general 

 sense that the temperature falls as the latitude 

 increases. To take a concrete example : — In the 

 month of January the mean temperature cf 

 North Antrim is about 41 F., while that of 

 South \\esi Cork is 44 F. : in the month of July 

 the mean temperature of North Donegal is 58 

 V.. while that of Wexford is 60° F. 



(2) The temperature falls about 1° F. for every 

 330 feet above sea level. It therefore follows 

 that the higher the elevation the lower is the 

 temperature, and consequently there is greater 

 probability of frost. Unless other influences 

 come into play a place only 50 feet above sea 

 level will, as a rule, have a temperature 2° 

 higher than another place near it at an elevation 

 of 700 feet. But there are some exceptions to 

 the rule. 



(3) An exception to the foregoing statement 

 will be found in valleys surrounded by low hills. 

 The side of the hill may have a higher tempera- 

 ture at night than the bottom of the valley. 

 This may be explained in the following way : 

 If two liquids of different densities are in turn 

 poured gently into a vessel the heavier liquid 

 will settle on the bottom and the lighter will rise 

 to the top. Masses of air of different density 

 will behave in the same way. As we have seen, 

 the temperature falls the higher the elevation 

 above sea level. The air on the side or top of a 

 hill is therefore slightly cooler by day than the 

 air in the valley, a fact which has often been 

 verified by experience altogether apart from the 

 reading of the thermometer. But cold air is 

 heavier than warm air. The draught produced 

 up a chimney is a familiar example of this, the 

 hot air rising up the chimney and cooler air 

 passing in at the bottom to take its place. At 

 night, therefore, there is a steady flow of cool 

 air from the higher elevations down the sides of 

 the hill into the valley, and before the sun has 

 risen the air at the bottom of the valley on a 

 clear, still night may have fallen below the 

 freezing point, white a little higher up the air 

 may be several degrees warmer. If a wind is 

 blowing, however, colder and warmer air become 

 mixed up. and there is little difference between 

 the temperatures of the hillside and the valley. 

 It is probably on this account that fruit planta- 

 tions are more successful if planted on a gently 

 sloping ground, although the question of drainage 

 also has something to do with it. The Ameri- 

 cans distinguish between a frost and a freeze. 

 Frosts occur on still, clear nights, and are more 

 or less local ; freezes are often accompanied by 

 wincis, are general or even Continental in range, 

 and are not marked by the whiteness of 

 frost. 



(4) A region close to a sheet of water is less 

 liable to frosts than a district not so situated. 

 The depth of the water in this case is more 

 important than its area. A body of water is not 

 so easily heated as a mass of jock or a land 

 surface, and. on the other hand, when once 

 heated it does not cool down so rapidly. Con- 

 sequently on a still, clear night in spring when 

 the temperature of the laud may fall below 

 freezing poinl thai of the water and of the air 

 above it will be several degrees higher. The 

 influence of the water will also be felt over a 

 bell of laud some distance from its margin whose 

 width will depend on i he elevation of the land 

 bordering the water, and also oj] the extent of 

 the water surface and on its depth, if an inland 

 lake. Therefore it will appear that a strip of 

 kind bordering a kike may be quite free from 



