diamond geezer

 Friday, December 07, 2018

December is a gloomy month, sunshinewise. Indeed in the northern hemisphere December is officially the gloomiest.

Here's a graph showing how much solar radiation reaches the earth's surface month by month. Data is for London, averaged over a number of years.

The official term for what's being measured here is "insolation", defined as the amount of solar energy received across a certain area over a specific time. The units on the graph's vertical axis are kWh/m²/day, if that helps.

At the height of summer a typical square metre in London gets about 5 kilowatt hours of solar energy a day. But in November, December and January that plummets to below 1, with this month the lowest of all. Insolation in December is about eight times weaker than in June or July. That's why Seasonal Affective Disorder is a thing.

Insolation (kWh/m²/day)     LONDON

Insolation data is especially important to people with solar panels on their roof. Numerous factors affect how much radiation solar panels receive, including nearby obstructions, leaf cover and orientation, but nothing can compete with the changing of the seasons.

At London's latitude, midwinter days have less than half the daylight hours of midsummer. That's one reason it feels gloomy at the moment.

Hours of daylight (1st of the month)     LONDON

The elevation of the sun in the sky is another.

Highest elevation of the Sun (1st day of the month)     LONDON

In summer the sun rises steeply across the sky, whereas in winter the angle is much more gentle. The summer sun can exceed an elevation of 60° at midday, whereas the winter sun never reaches 20°.

A low sun is bad news for insolation because energy hits the earth's surface at a more oblique angle, spreading out the same amount of solar radiation across a wider area. So even when we do have daylight in the winter it isn't very strong daylight, and that helps makes December feel especially dull.

Today for example the sun rises at 07:50, but it isn't until 08:40 that it reaches 5° above the horizon. As for the loftier benchmark of 10°, the sun won't get that high until 09:33. That's why mornings are feeling particularly gloomy at the moment. Later the sun will sink back below 10° as early as ten past two, then duck below 5° just after three o'clock. That's why afternoons are feeling particularly gloomy at the moment.

This table shows when the sun first reaches 10° above the horizon, month by month. Times are GMT/BST as appropriate.

Solar elevation rises above 10° (1st day of the month)     LONDON

In summer the sun reaches 10° by 6am, so when most people wake up the day already feels bright. But at this time of year it doesn't reach 10° until well after 9am, and so the day starts off dull. The very dullest mornings are at the end of December.

Solar elevation falls below 5° (1st of the month)     LONDON

As for 5°, and evenings, in summer the sun doesn't dip that low until around half past eight. But at this time of year it starts getting noticeably dimmer around three o'clock, so it's all downhill from mid-afternoon. The very dullest afternoons are in the middle of December.

Here's another way of looking at this. Today the elevation of the sun will be below 10° for 3½ hours. If you do the sums, that's 43% of today's total daylight hours. That's glum. And it'll be below 5° for 1 hour 40 minutes, which is 21% of today's daylight hours. That's dreary.

Which has all been leading up to this graph. It shows the proportion of daylight hours that are gloomy, and how that changes over the year. I've drawn one line for below 10°, and another for below 5°.

From April to September the graphs are remarkably flat. In spring and summer the sun's only below 10° for 20% of daylight hours, pretty much consistently, and below 5° for 10%. But from October the graph shoots up, and by December those proportions have doubled. At the moment the sun's less than 10° above the horizon for 40% of daylight hours, and less than 5° above the horizon for 20% of daylight hours. December's daylight is proper weak.

Not forgetting the weather, of course. Not only does December have the lowest average sunshine total of any month of the year, it's also the cloudiest month... and cloud greatly reduces the amount of sunlight getting through. Most clouds reflect the majority of sunlight hitting them, the very thickest cloud reflecting over three-quarters. The technical term is albedo, if you want to research some more.

This December has been unusually cloudy in London, with just four hours of sunshine so far in this first week. In fact we've not even managed ten hours of sunshine over the last fortnight, which is exceptional.

Short daytime hours, low sun and overcast skies combine to make December the gloomiest month. No wonder our ancestors shoehorned in pagan celebrations, and Christmas, to help brighten it up.

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