Monday, 30 October 2017

Winter is coming

Just occasionally I find something I thought I knew, and have known for all my life, comes in to question, and today it is the very definition of the word "Winter".

To me, that has always been "the cold bit of the year". There are four seasons (we're talking UK here). I had not really considered if they are equal parts of the year of if Spring and Autumn are somehow smaller or not. But for me, Winter has always been the bit where it is cold, and Summer the bit where it is hot.

But when exactly does winter start and end? Well, if I had to think about it I would clearly put the middle of Winter as the Winter solstice - around 21st December. It is the point when the days finally start getting longer and so there is more sun and more heat and we are finally "coming out" of winter.

Indeed, a bit of research shows the term "midwinter" dates back to the 12th Century, and is "around the winter solstice". That makes it the middle of winter.

Therefore, if I had to pin any dates on it, I would say it just over six weeks before to just over six weeks after the Winter solstice. So starting early November and going on until late February.

Interestingly, the UK met office define winter as the whole months December, January and February, largely for convenience of making it whole calendar months for each season. In some ways this fits with the fact I would say February is usually colder than November, simpler from my own memory and experience.

I found a nice graph which seems to show that February is colder than November, which fits my feeling on the matter. I am told this is partly because the seasons lag slightly due to the way the Earth retains heat (or lack thereof). So logically the middle of Winter may be the Winter solstice, but temperatures lag by a few weeks.

Even so, from a purely astronomical point of view, I would still have firmly places my idea of Winter centred around the Winter solstice.

The met office, and several other sources, say this is not the case! They say that from an astronomical point of view Winter STARTS on the Winter Solstice and runs to the Spring equinox. Summer STARTS on the Summer solstice and runs to the Autumn equinox.

This just does not fit with my view of winter, or of "midwinter" being the Winter solstice, a view that clearly dates back hundreds of years.

How did this happen - how did astronomers decide that the shortest day, which is clearly somewhere within the "cold bit of the year" would be the START of Winter, and not the middle?

It makes no sense to me, sorry. It can only mean I have misunderstood what "Winter" actually means for my whole life. Am I alone? What next, making Pluto a planet?


  1. The seasons don't lag because of the Earth (rock doesn't retain very much heat and releases it so incredibly slowly that it has next to no effect on a timescale of mere years). The seasons lag because of the *oceans*. As such, people in the middle of a continent will often experience the coldest months as being around the Equinox, as you suggest: it's only people stuck out on the edge of major oceans who experience the coldest month as being months later.

    1. Interesting and even more reason for an “astronomical” definition to be centred on the solstice even if a local meteoralogical one is lagged.

    2. And this is indeed why there is a difference between "continental weather" (extremes of temperature throughout the year) and "island weather".

      In the UK we have "island weather" in that the sea around us has a huge thermal mass and acts to moderate the temperatures throughout the year. So we have comparatively warm winters and cool summers.

      Compare to locations at similar latitudes but in the middle of a continent, such as the centres of Germany and Poland, Southern Canada, etc. With no sea nearby, they receive warm summers and much colder winters than we do.

      As someone who windsurfs in the sea throughout the year, the "lag" of sea temperatures is very apparent. I would usually be switching to a summer wetsuit in May even though the air temperature is quite reasonable through April - the sea is still pretty cold. Conversely, I'd be thinking about switching to a winter wetsuit in November because the air is cold, but the water is still comparatively warm.

  2. It's funny, I've never wondered about the definition of Winter. :-)

    - Howard Winter (yes, really!)