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Historically we would expect the first sub-2C reading around early to mid October and the first air frost around mid to late October.
However there have been so many autumn seasons since the turn of the century with horrible cloudy/windy mild nights that mean the first frosts are almost always late now. No increase in warm daytime weather though. Just endless average day temperatures with mild nights particularly during the Octobers of 2005, 2006, 2009, 2011 and 2014. All dreadful months.
I expect 2017 will be more like those rather than 2008, 2010 and 2012 - the only decent crisp and pleasant Octobers recently that managed a frost. Nothing below 8C so far this month which is just ridiculous, given the simultaneous lack of warmth, but just in keeping with the general trend.
First Sub-2C minimum:
2005: 9 Nov
2006: 3 Nov
2007: 9 Nov
2008: 2 Oct
2009: 5 Oct
2010: 20 Oct
2011: 6 Nov
2012: 23 Sept
2013: 16 Oct
2014: 29 Oct
2015: 1 Nov
2016: 8 Nov
Average: 23 Oct
First Air Frost:
2005: 18 Nov
2006: 19 Nov
2007: 24 Nov
2008: 28 Oct
2009: 9 Nov
2010: 25 Oct
2011: 2 Dec
2012: 16 Oct
2013: 5 Nov
2014: 10 Nov
2015: 22 Nov
2016: 11 Nov
Good point. I have seen and heard of plants being more damaged on a cold clear night with temps not going below 32F, but an air frost, vs a windier night with a low of say 30F. A number of years here we had a few nights in with low temps between 28F and 32F, and my plants didn't miss a beat and kept flowering cause there was no frost on the plants. Not unusual at all to see annuals flowering in Philly well into December.
Very true. I grow tamarillos and papaya, and they can handle temperatures down to -3C/-4C with the downslope wind that is quite common here during winter, with no damage. The same plants will suffer major frost burn at 1C/0C, without the wind.
Average date for first overnight air frost (> 0C) at Trondheim Airport Værnes is Oct 9th according to our met.office.
Since Sep has been a little warmer than average so far, with coldest overnight low 5C so far, and many overnight lows above 10C, I will guess Oct 14th this year.
The first hard freeze (>-2C) is harder, no statistic for that. Looking at the years from 2000 onwards, there are three October months with no hard freezes, and thus 14 with coldest low -2C or colder. Always in Nov. So I will guess Oct 25th.
To put it in perspective...
Røros, inland and at 625 m asl, has average first air frost Aug 24th. Can also see air frost in July, and had several nights with air frost in July this summer.
Ytterøyane's average first air frost is on Dec 6th, and several winters never see hard freeze (<-2C), did not record that last winter (-1.8C coldest).
Because freezes are extremely rare here I'm going to post my prediction for the first sub-50F temp.
Since 2000
2000: Dec 20
2001: Dec 27
2002: Jan 4 (2003)
2003: Dec 7
2004: Dec 15
2005: Jan 7 (2006)
2006: Nov 22
2007: Jan 3 (2008)
2008: Jan 21 (2009)
2009: Dec 29
2010: Dec 7
2011: Jan 7 (2012)
2012: Dec 22
2013: Jan 17 (2014)
2014: Nov 2
2015: Jan 24 (2016)
2016: Jan 8 (2017)
Daily mean below freezing - December 8 (12 days later)
Ice day - December 27 (18 days later)
2018 records:
Frost: October 21
Freeze: November 10
Hard freeze: November 10
Very hard freeze: November 11
Daily mean below freezing: November 27
Ice day: November 27 (only ice day in late 2018, roffle)
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