-11°C a “sharp frost” I don’t think so

Courtesy UKMO

The forecast for tonight from the Met Office for the Highlands says that there will be a “sharp frost” with a minimum air temperature of -11°C. Sharp is not a recognized term for describing the severity of frost and never has been and the Met Office should know better. The Met Office I left behind would never have described -11°C as a “sharp frost”, but the new corporate version seems to finds no problem in doing so. Once upon a time each outstation had its own detailed local staff instructions [LSI’s] that described what the responsibilities of each member of staff were and how they were to perform their duties, it now seems in this PC world that we live in anything goes. There’s a reason why we stick with terms such as slight, moderate and severe, rather than touch or sharp, and that’s consistency. Here is the correct way that the severity of a frost should be gauged according to the Bible that we called the Meteorological Office Glossary:-

Frost occurs when the temperature of the air in contact with the ground, or at thermometer-screen level, is below the freezing-point of water (‘ground frost’ or ‘air frost’, respectively). The term is also used of the icy deposits which may form on the ground and on objects in such temperature conditions (GLAZE, HOAR-FROST). Since the sensation of cold depends not only on air temperature but also on the accompanying wind speed, the fourfold classification of frosts used in forecasts of this condition in the United Kingdom is varied with wind speed. Thus, frost is classified as ‘slight‘, ‘moderate‘, ‘severe‘, or ‘very severe‘ for screen temperature ranges of 0.1 to -3.5°C, -3.6 to -6.4°C, -6.5 to -11.5°C, or below -11.5°C, respectively, if the accompanying wind speed is less than 10kt; and for screen temperature ranges of -0.1 to -0.4°C, -0.5 to -2.4°C, -2.5 to -5.5°C, or below -5.5°C, respectively, if the wind speed is 10kt or more.

Met Office Glossary (1991).
Courtesy UKMO

The coldest place in IONA last night was at Loch Glascarnoch with a minimum temperature of -9.1°C, that coupled with a three or four knot wind meant that it was a severe frost bordering on very severe frost.

I’ve written a similar blog every couple of years making the same plea and I know full well that no one’s listening but I still do it. Is that a definition of madness or some kind of obsessive compulsive disorder – maybe.

3 thoughts on “-11°C a “sharp frost” I don’t think so”

  1. It seems that how the frost appears goes to what people call it.
    The other night we had a min of -2.2°C and a couple of knots wind.
    That would be called a slight frost by Met Glossary definition but it does not tell the whole story.
    Every surface was covered in a thick white frost and people were calling it a hard frost.
    The availability of moisture seems to come into the equation as well.
    I have seen -2°C and a couple of knots wind with far less frost deposited before.

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