diamond geezer

 Friday, December 01, 2017

Today's post is about why it's dull and gloomy in the winter.

Specifically it's a post about the elevation of the sun. If you drew a line to the horizon and a line to the sun, the elevation would be the angle inbetween. Think of it as a measure of the height of the sun in the sky.

The elevation of the sun at sunrise or sunset is always 0°.
The elevation of the sun at noon on the equator at the equinox is 90°.

The sun never gets to be overhead in the UK, so the best it manages here in the summer is nearer 60°. But in the winter it doesn't even scrape 20°, which is why it feels so dull and gloomy, and ultimately why winters are cold.

Here's a table showing the elevation of the sun here in London, hour by hour, on 1st December.

Elevation of the Sun, London, 1st December

The sun rises today around quarter to eight, so at 8am the elevation of the sun is only just above zero. By 9am it's reached 8°, which is roughly half the height it'll reach later in the day. The sun is highest in the sky around noon, at which point its elevation reaches 17°. And then it's in decline for the rest of the afternoon, dipping beneath the horizon before 4pm.

Here's the same information in a graph.

17° isn't particularly high in the sky, only about a fifth of the way up compared to 'overhead'. The sun will only be higher than 15° for about two hours today. The sun's elevation won't reach 10° until quarter past nine this morning, and will sink back below that by quarter past two this afternoon, a good 90 minutes before sunset. This is why you often have to squint when driving in the winter, because the sun is very close to the horizon for a long period of time.

Here's a table showing the elevation of the sun in London at the winter solstice.

Elevation of the Sun, London, 21st December

On 21st December the highest the sun gets in the sky is only 15°, and that's the lowest maximum of the year. There's not actually very much difference between 21st December's figures and today's - the figures don't change very much in December, and the Sun is low in the sky all month.

For a very different set of figures, here's what happens at the summer solstice.

Elevation of the Sun, London, 21st June

These times are BST, which is why the highest elevation is at 1pm, not noon. But just look at how much higher the sun gets, more than four times higher, reaching 62°.

It's not just better in the middle of the day. The sun is higher than 45° (that's 'halfway up the sky') from 10am to 4pm. The sun is higher than 30° from 8:15am to 5:45pm. The sun is higher than 17° (that's 'better than December') from 7am to 7pm. The sun is considerably higher, for longer, in the summer than it is in the winter. This graph shows that quite well.

Look in particular at the area under each curve. In summer the area under the curve is large, so lots of solar radiation gets through, which is why it gets warm. But in winter the area under the curve is tiny, so very little solar radiation gets through, which is why it gets cold.

Here's a table showing maximum solar elevation throughout the year.

Highest elevation of the Sun on the 21st of the month, London
Time Jan  Feb  Mar  Apr  May  Jun  Jul  Aug  Sep  Oct  Nov  Dec 

There are five months in the year when the sun's elevation tops 50°. Hurrah. But there are also three months when the sun doesn't reach 20°, and we're in the depths of that low season right now. See how the figures change fastest in spring and autumn, and stay roughly the same in summer and winter. Here's a graph.

Here's another more technical graph, specifically a polar sun chart. Each of the blue curves shows the path of the sun across the sky, month by month, with maximum elevation where it crosses the vertical line. You can generate your own polar sun charts here or here. You can see single curves for London here.

And finally, lets refocus on December, and consider other parts of the country. All of the figures so far have been for London, and apply to anywhere else with a latitude of 51½°N. For every degree of latitude you go south, the sun is one degree higher at noon. And for every degree of latitude you go north, the sun is one degree lower at noon. Sorry Birmingham, Merseyside, and Scotland, it really is duller where you are. Africa, you win.

Maximum solar elevation
1st December
Milton Keynes52°N16°

And that's why it's dull and gloomy in the winter. Hang on in there, the sun'll be getting higher again soon.

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