No frost on grass at 0.5c?

Andy From Sligo · 09-12-2017 3:08am #1

I have an outdoor digital thermometer and it says tonights temperature is 0.5 degrees centigrade, however there is no frost on the grass or the cement path they are just wet, but there has been times where its around 2 or 3c and there has been frost on the grass and pavement - just curious as to why tonight there appears to be no white frost on the grass seeing as its 0.5c?

Simon Gruber Says · 09-12-2017 6:36am

Frost is caused by moisture in the air condensing and freezing on something that's surface temperature is below freezing. So, either the humidity was very low or the surface temperature of the places where frost would normally occur hasnt reached or has since rose above freezing, in which case you would see dew or condensation.

Gaoth Laidir · 09-12-2017 8:58am

If skies are cloudy then the surface doesn't lose much heat through radiation as it's reflected back off the clouds. I'd imagine it was at least partly cloudy at that time. Had it been a fully clear sky the the surface would have been well below zero and frost would form.

Air temperature at 1.5 metres doesn't really matter for grass frost.

Andy From Sligo · 09-12-2017 3:12pm

Thanks all, very interesting - never knew that

Jpmarn · 14-12-2017 3:52pm

In windy conditions they would not be any frost until the actual air temperature falls below 0C. After that you'll get a severe penetrating frost.

Danno · 14-12-2017 4:27pm

That's why on weather forecasts they warn about frost and ice in "sheltered places". Sheltered from the wind.

No frost on grass at 0.5c?

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