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Friday, 1 March 2013

The Changing of the Seasons

A correspondent notes that today is the first day of meteorological spring. "What on earth is this?" he wonders. Let me explain.

In the first place, I should explain that I am describing the situation in the Northern hemisphere, where winter solstice is round about 21 December and summer solstice in late June. If you live in the southern hemisphere, then your ancestor shouldn't have pinched that handbag in Victorian London, should he?

So - due to the angle of the Earth's polar axis to the plane of the ecliptic - or, to put it in English - because the Earth is tilted in its orbit - in the Northern Hemisphere summer, the North Pole points more generally towards the sun, and in winter, the South Pole does. This means that the end of the Earth pointing at the sun gets more heat and sunlight, and longer days. Indeed, at the Poles you get six months of light and six of darkness.

Now, if we were to take the length of the days as our guide to when the seasons are, then clearly the summer solstice would be the middle of summer. In which case, the season of summer would start about six weeks previously - ie just after Mayday - and would end six weeks later - ie just after Lammas. Likewise winter would start 6 weeks before Solstice - ie just after All Hallows - and end six weeks after Solstice - ie round about Imbolc/Candlemas.

It should be noted in passing that the whole set of quarter- and cross-quarter-days (or "proper special days", as we call them in the Beaker Folk) are about 4 days out from what the sun is actually doing. The best bet is not to try and explain this.

But - the fact is that the height of summer, or the depths of winter, do not coincide with the solar peaks and troughs. On 21 June, when the Sun's about to start digging up the potatoes and bringing in the Harvest Home, it's barely warm. The real heat of of Summer, at least in Husborne Crawley, is from mid July through late August. Likewise you barely ever get a white Christmas, but it doesn't half snow sometimes in February. This is due to thermal lag. The ground and, to a much greater degree, the sea start warming up when the days are long, but there's still warming up in August. The Arctic sea ice is at its greatest extent in March, not in December - and lowest in September. The real seasons lag behind the solar ones.

And so we have this arbitrary 6-week lag in our seasons. "Midsummer", so-called, is the start of summer. Vernal Equinox is the start of Autumn (or "Fall", if you live in the 16th century).

But why does "Meteorological Spring" start on 1 March? Simple. Because meteorologists aren't capable of coping with the concept of seasons starting on erratically-numbered days. They've moved all the seasons to start on the first of the month, so as to make their maths easy. Ironic, for the people with the most powerful computers in the country.


4 comments:

  1. I've never seen such a good explanation! Why couldn't you have taught me when I was at school, I'd have understood so much more! (Answer: because you were a toddler at the time. Mind you, you'd still have made more sense than - never mind!)

    What is that addition to Stonehenge by the way? The Tardis? R2D2? Obviously the big red man is Hellboy as played by Ron Perlman (love that film!) (yes I have very low taste)

    love Mags B x

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    Replies
    1. The big red man is the Wicker Man on the original film poster!

      Stonehenge is all natural - you may be seeing the snow behind if, if it's that white patch you're talking about, or the shadow on one of the stones if it's that gray patch.

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  2. As a dweller in western Washington State, in another continent far, far away, I can well understand what you are saying here, Druidess! We have had little rain so far this season(s) and no snow, like we usually get.
    Waiting for more wet(her)

    Jim of Olym
    Acurally auriel is my spouse but i use her i d when she's not looking

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  3. Interestingly (or not), the descendants of those nineteenth-century handbag tealeaves in Australia (and the Kiwis) officially count the seasons from the first of the month. So 1 March was the first day of autumn, 1 June will begin winter, spring starts on 1 September and summer on 1 December.

    Makes it easier to remember.

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