THAT FROST!

Ashburton Guardian, Volume 67, Issue 159, 18 April 1947, Page 4

THAT FROST!

METEOROLOGICAL POSITION STATED TEMPERATURE OF UNDER 32 DEGREES Jn recent years il has been a sore point among a section of weather-conscious AshbmTonians Unit the frost readings reported by ibe meteorological station in the Ashburton Domain have been less Ilian they would have expected. It has been the practice of late in Ashburton to record the occurrence of a frost when the grass minimum temperature fell to 30.3 degrees or less, and not 32 degrees, as had previously been tlie case. A “notice to observers” from the Meteorological Office, Wellington, under a date of several years ago, clarities the position. “A ‘frost’ occurs when the temperature of the air falls below freezing point of water, which is 32deg. F.,” says the notice. '•‘When tlie temperature measured inside a screen drops below 32deg. the occasion is counted as ‘ a day of frost in screen, or, more shortly, ‘as “a frost day.’ •‘ln climatic summaries, the frequency of ‘ground frosts’ provides useful information. Data in regard 1o these are derived from the readings of a minimum thermometer freely exposed to tin* sky and held an inch above level ground surfaced with short grass, two Y-shaped pieces of wood being used as supports. The ‘grass minimum’ temperature overnight is recorded at the morning observation hour. If tlie value is 30.3 deg. F. or lower, a ‘ ground frost ’ is said to have occurred and ‘ a day of ground frost’ is counted. “The term ‘degrees of frost,’ which has a considerable popular usage, can still he used to express the temperature fall below 32 degrees. “It should he noted that this definition of a ‘ground frost’ by reference to the readings of grass minimum thermometers js purely a convention, hut one which has been long adopted in Great Britain, and js followed by the New Zealand Meteorological Service. A good reason lay behind this original choice, in that the tissues of growing plants are not likely to sustain damage until temperatures fall appreciably below 32 degrees. Experience suggested 30.3 degrees as a suitable limit, and this has been used ever since for statistical purposes.” The readings that have appeared in the “Guardian” are those of “ground frosts,” hut it seems that any figure under 32 degrees can he described as a frost.