Dark, Wet and Windy
28th December 2019
A similar day to yesterday, but darker, milder, wetter, windier and even less snow! There is very little or no snow left on the hills near to the coast. Elsewhere, the snow cover is sparse, patchy and in strips along high coire rims, ridge crests and in runnels, chiefly above 850 – 900m.
Dark mornings. Ever wondered why it continues to get darker in the mornings after the winter solstice? Or in other words, a later sunrise. In fact, it is not until the beginning of January that sun starts to rise earlier in the UK. And in another fact, evenings draw out (later sunset) before the winter solstice, roughly mid December.
A day length is on average 24hrs. A ‘true’ day length – a solar day, varies throughout the year, the shortest (around 23h 59’ 38’’) in mid September and longest (around 24h 00’ 30’’) in late December. So in December, days are around 30’’ longer than our clocks assume and this causes the days to shift around 30’’ later each day. This explains why the evenings start drawing out before the shortest day (winter solstice), but it continues to get darker in the mornings until into the New Year when the solar days start to shorten.
But why does the length of the solar day vary throughout the year I hear you ask? Firstly, the earth moves in an elliptical orbit around the sun and its speed varies according to how close it is to the sun; basically, the closer, the faster. The earth is at its fastest in early January and slowest in early July when it is furthest from the sun. Secondly, the axis of the earth’s rotation is tilted – around 23.5° from the plane of its orbit around the sun, hence the solar day also varies around the globe. OK?
But what is a solar day? Well …..
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