Why are the dates when we switch between summer and winter times (for example, swapping between Greenwich Mean Time and British Summer Time in the UK) placed so asymmetrically around the solstices?
鈥� British Summer Time enables us to enjoy lighter summer evenings while minimising accidents caused by travelling in the dark in winter, as result of poorer visibility and colder icy conditions. We tend to be more rushed on the morning commute and may also be more tired and less alert. Therefore, we change the clocks primarily to avoid a dark icy morning rush.
So why does it end fewer than two months before the shortest day (21 December) but not restart until more than three months later? That鈥檚 because 21 December has neither the latest sunrise nor earliest sunset. The earliest sunset occurs around 14 December and latest sunrise is around 30 December, and hence the mornings in late January are as dark as those in early December.
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As for icy weather, the coldest part of the year trails behind the shortest day because of thermal lag, the same reason the hottest part of the day is not at noon but late afternoon. At 3 pm the sun is getting lower, but is still adding more heat to the day.
鈥淢ornings in late January in the northern hemisphere are as dark as those in early December鈥�
In January and February, although days are lengthening, there is still insufficient sunshine to warm the day between the long night hours, so February is often colder than November. Therefore, in late October, with sunrise at nearly 8 am, we change clocks to lighten the morning, and then we wait until late March, when sunrise has advanced to just after 6 am and most of the icy weather is over, to start our days earlier again.
Hillary Shaw, Newport, Shropshire, UK
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