Forecast I luj Weather. 201 



" (fc) The frost is most severe on the ground, and becomes less severe 

 at shrub height, still less so at tree height ; so that herbage and low 

 shrubs may be destroyed when higher shrubs and trees are spared. 



" (c) An overcast sky or a light wind generally prevents ground frosts. 

 " Tt is also a well established meteorological fact that on the average wind 

 falls off in the evening, and in settled weather a calm night often follows a 

 day with a good breeze. This is especially the case with an easterly wind. 



" Keferring to the three causes which contluce to the production of sharp 

 night frosts, the first two are easily associated with general meteorological 

 conditions over the country, and to anticipate them forms part of the ordinary 

 duty of weather forecasting. The changes are often very sudden, and while 

 it is as a rule possi})le to anticipate their general chai-acter, it is less easy to 

 form an estimate of the intensity of the changes. The difference between the 

 changes which produce only a chill, and those which cause a frost, is not 

 indicated on the maps used for forecasting. 



" The frosts of calm nights are still more difficult to deal with by forecasts 

 issued from a central office. They are subject to the effects of local 

 peculiarities of site and circumstances, of which account can only be taken 

 by those who are on the spot. A light air, hardly strong enough to be called 

 a wind, will keep away a night frost by preventing stagnation ; a calm, on the 

 other hand, favours frost ; but from the point of view of the weather-forecaster 

 the calm may be an artificial calm due to surrounding trees or buildings, and 

 not at all due to what he would understand by the weather. 



" It has already been pointed out that situation, whether on a hill or hill- 

 side, or a valley, is also of importance. It is, therefore, necessary that persons 

 interested in protecting their crops from frost should make use of their local 

 knowledge in extension of the information to be obtained from forecasts. 



" One of the best aids to the use of local knowledge is the regular study of 

 weather maps. The Meteorological Office issues daily charts of the weather 

 over the British Isles and North-Western Europe, which can be had by any one 

 on payment of the cost of postage and covers (\l. per annum), and the 

 conditions for the occurrence of frosts can be watched much more effectively 

 by an examination of the daily map than by the mere consultation of the 

 forecast which is drawn up for a whole district, and must be limited to about 

 a dozen words. 



" Further information about the probability of a night frost may be got 

 from local observations of the temperature and humidity of the air, the readings 

 of the dry and wet bulb thermometer. From these readings on any occasion 

 the so-called '' dew point " of the air can be computed. The dew point is the 

 temperature at which dew begins to form. 



" Destructive fronts on calm nights are far more likely to occur when the 

 air is dry ; that is when the dew point is far below the air temperature, or the 

 wet bulb is far below the dry, than on nights when it is moist or the dew point 

 and wet bulb temperatures are near the dry. 



" From what has been said above, it will be gathered that any one who is 

 interested in protecting his crops from night frosts, and, therefore, wishes to 

 know beforehand when frosts are likely to occur, will do well to study : — 



"(1) The pecularities of his locality to know whether from being in a 

 cup or valley, it is especially liable to frosts on calm niglits. 



" (2) The daily charts, with or without forecasts by telegraph, in order 

 that he may recognise the meteorological conditions in which the weather 

 is likelj' to hiecome cold or calm. 



•'(3) The readings of the dry and wet l)ulb thermometers, so that he 



may recognise the occasions when ground night frosts are likely to be 



severe on account of the dryness of the air. 



'* The best method of protection for young [jlauts against night frosts is to 



cover them up and thereby prevent first uf all tlio loss of heat, ami sccimdly the 



access of cold air. 



