FROSTS

Rotorua Morning Post, Volume 2, Issue 301, 15 August 1932, Page 2

FROSTS

INCIDENCE OP FROSTS IN THE D0MINION SUMMARISED. WHERE FROSTS OCCUR. The proud possessors of maximum and miiiiinum thermometers delight on these cold and frosty nights to put their instruments on or near the grass level. Then, in the morning they look at the indicators which show how far the mercury had descended during the night. Subtracting the number of degrees indicated from thirty-two, freezing point," they arrive at the number of degree of frost, and with an air of authority inform their friends in the morning train or tram that "last night there was a frost of so many degrees — the worst one I have recorded this •winter." For ordinary purposes such a method of measuring frost, counting one degree of frost for every ddgree the thermometer j falls bslow thirty-two degrees (Fah- i renheit), is near enough. In fact, that is the general way frosts are recorded, but meteorologists, for subtle reasons of their own which need not be 'entered into here, have a slightly different way of reckoning frosts. In accordance with a European convention, a ground frost is considered to have occurred when the temperature recorded by a minimum thermometer one inch above the ground surface has fallen below 30.4 degrees Fahrenheit. The thermometer under such circumstaneas must be freely exposed to the sky above a grass surface. Influence of Ground For1" , Most people are firmly convinced by this time, especially Lower Hutt residents, that there have been an exceptional number of frosts this win- , ter, and they may be right. In

discussing the frequency of frost m New Zealand, Dr. E. Kidson, Director of tha Dominion Meteorological Services, in a recent paper, points out that frost is a very local phenomenon, and therefore the frequency of occurrence may vary within wide limits over a comparatively small area. A level plain will be more subject to frost than a hill alone, and a hollow in a plain still more so. An area which receives the drainage of cold air from higher levels, too, is likely to be subject to frost. If, for some reason, there is a continuous wind at night-time, the liability to frost is reduced. Proximity to the sea leads to fairly uniform temparatures and reduces the liability to frost. From these remarlcs, be says, it will be seen that records would be necessary from.an enormous number of sites, and a very large map would be required if the frequency of occurrence of frosts in New Zealand were to be shown in complete detail. Frequent Frosts. From December to March the number of frosts is everywhere small and the amount of damage done is seldom serious anywhere, and in many districts negligible. From May to August the number is large, and in the interior of the South Island particularly all clear nights throughout the year is not quibe equal to both Islands, frosts tending- to occur later in the winter in the North Island than in the South. In the South Island June and July are about equally subject to frost, but in the North Island, on the other hand, July has the greatest number of frosts, while August has more than June. "Frost Days" Many of the ground frosts referred to in the preceding paragraphs are confined to a very shallow layer at the surface, and most plants are tall enough to escape their effects. This is especially the case as regards fruit trees. A better idea of the liability to damage to orchards or to tall or hardy plants is given by the frequency of occurrence of temperatures below freezing (32 degrees F.) in the standard thermometer screen at the standard height of 4 feot above the ground. On such occasions the air below that level would all be at a temperature below freezing. In Europe such days are called "frost days" by meteorologists. The contrast between the North and South Islands in the case of "frost days" is less than in the case of ground frosts. In the North Island there are practically none in January or February. The percentage of the total falling in the different months is given below: - North South