diamond geezer

 Saturday, October 27, 2018

It's a bit nippy for October, with an Arctic blast expected to keep temperatures in single figures throughout the day. But how normal is this? When does autumn's first cold spell usually kick in?

In meteorology, there is no such thing as normal. But what I can do is look back at recent autumns to see when the first chilly day arrived. Specifically I've checked back through every year since 2010, using data from Hampstead, to find the first autumn day with a maximum temperature below 10.0°C.
2010: 20th Oct
2011: 16th Nov
2012: 26th Oct
2013: 4th Nov
2014: 17th Nov
2015: 20th Nov
2016: 2nd Nov
2017: 5th Nov
The earliest date in the list is 20th October and the latest is 20th November. In statistical terms it's usually good to remove outliers, so let's cut those two, which leaves 26th October - 17th November as the "normal" period during which temperatures first return to single figures. That three week period begins right around now. Today's Arctic blast is not extraordinary.

I can perform a similar analysis at various borderline temperatures.
The first autumn day not to reach 15°C: 9th Sep - 7th Oct
The first autumn day not to reach 10°C: 26th Oct - 17th Nov
The first autumn day not to reach 5°C: 30th Nov - 28th Dec
Roughly speaking, the first autumn day not to reach 15°C tends to be in the second half of September, i.e. the switchover between summer and autumn. The first day not to reach 10°C tends to be near the start of November, i.e. just after the clocks go back. And the first day not to reach 5°C tends to be in December (although in 2015 it slipped into mid-January). There are no clear patterns for "the first day not to get above freezing", as this is a much rarer event.

Those were firsts. How about lasts?
The last day to exceed 25°C: 7th Aug - 15th Sep
The last day to exceed 20°C: 15th Sep - 16th Oct
The last day to exceed 15°C: 28th Oct - 23rd Nov
Summer's last heatwave could be at any time in August or the first half of September. The last warm day (20°C or above) is a late September/early October thing. And the final mild day (15°C or above) is probably due during the next three weeks.

Then there's the other end of the year.
The first day to reach 15°C: 23rd Feb - 2nd Apr
The first day to reach 20°C: 6th Apr - 30th Apr
The first day to reach 25°C: 19th Apr - 12th Jun
The first day to reach 30°C: 27th Jun - 18th Jul
London's first 15°C day is usually in March, 20°C in April, 25°C from mid-April to mid-June, and 30°C in July. If you live some distance from Hampstead, or elsewhere in Britain, I don't need to remind you that your dates may be different.
The last day not to exceed 5°C: 11th Feb - 19th Mar
The last day not to exceed 10°C: 26th Mar - 11th May
The last day not to exceed 15°C: 16th May - 17th Jun
If today is the first day this winter not to reach double figures, then the last such day is likely to be in April, maybe even early May. Only six months to wait. Wrap up warm.

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