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Why does a home feel dark in winter when the windows are the same?

The windows haven't changed, but the daylight has: the day is under seven hours long, the sun sits around 11 degrees high, and an overcast winter sky delivers a fraction of summer's brightness.

1. The day is under half as long

Day length can be worked out from cos(hour angle) = −tan(latitude) × tan(declination), after which the day lasts 2 × hour angle ÷ 15 hours.

Take a home at 55.7° north — Copenhagen's latitude, shared with southern Scotland and the Baltic states — at the winter solstice: tan(55.7°) = 1.465, tan(−23.4°) = −0.434, and the signed product comes to 0.634, giving an hour angle of 50.7° and a day of 6.7 hours. The same sum at the summer solstice gives 17.3 hours.

So the winter day is under 40% of the summer one. And it sits symmetrically around solar noon, the point in the day when the sun stands highest. If solar noon falls around 12 pm, the sun doesn't rise until around 8 am and is down again by 3 or 4 pm. For most people that means leaving home in the dark and getting home in the dark, and the only daylight the home receives falls while nobody's there to see it.

2. The sun sits five times lower

The sun's height at midday comes from 90° − latitude + declination. At 55.7° north: 90 − 55.7 + 23.4 = 57.7° at the summer solstice, and 90 − 55.7 − 23.4 = 10.9° at the winter solstice.

Indoors, the consequence cuts two ways. On one hand, the low winter sun reaches deep into a room and strikes a vertical south-facing pane almost head-on. Every hour of clear sky therefore delivers more light through a south window in winter than in summer, when the sun hits that same vertical glass at a shallow angle. It's just that the winter day is short and clear skies are rarer, so over a whole month the total still comes out lower. On the other hand, that low sun gets blocked by everything in its way: shadow length works out as height ÷ tan(11°) = height × 5.15, so an 8-metre neighbouring house casts a 41-metre shadow, and a 1.8-metre hedge shades 9 metres into the garden.

In summer, with the sun at 57°, that same hedge casts a shadow only 1.2 metres long. That's why a plot can feel open in July and closed-in by November, without anything having been built at all.

3. The light has to cross more atmosphere

When the sun sits low, its rays travel through the atmosphere at a shallow angle and cover a far longer path than when the sun is high. That scatters and dims the light — the same mechanism that turns the sun red at sunset.

Winter sun is therefore both lower and weaker per ray. It makes the light warmer in colour and softer at the edges — beautiful, but markedly less powerful than the same sun high in the sky.

4. The sky is often grey — and a grey sky is far weaker

Most of the daylight in a home at these latitudes doesn't come from the sun's disc at all, but from the sky. And there's a huge difference in what the sky actually delivers. As an order of magnitude: outdoor illuminance on a clear summer day runs into the tens of thousands of lux and can reach around 100,000 lux in direct sun, while a heavily overcast winter day can sit down around a few thousand lux. Indoors, a couple of metres from the window, only a small percentage of the outdoor level is left.

For comparison, ordinary artificial room lighting typically runs to the order of 100-300 lux. That explains the paradox: an overcast winter day can still be, outdoors, many times brighter than a table lamp, yet inside the room so little of it is left that the light goes on at 2 pm — and the room feels grey even when it isn't switched off.

What you can do about it

• Free up the south-facing windows. South is the only compass point that delivers direct sun in December. Move whatever's standing in front: a carport, a tall hedge, a covered terrace, a lean-to roof. Trim a hedge from 1.8 to 1.2 metres and its winter shadow shortens from 9 metres to 6.
• Get the curtains fully clear of the glass. A curtain rail wider than the window, and blinds pulled all the way up. A curtain covering a quarter of the pane all day costs a quarter of winter's daylight.
• Light surfaces, both outside and in. When direct sun is rare, reflected light matters more. A white ceiling, pale walls and a light-coloured surface outside the window pull noticeably more daylight into the room than dark surfaces do.
• Raise the window head, if you're renovating. Daylight reaches roughly 2 to 2.5 times the height from floor to window head into a room. A transom window or a taller frame reaches deeper than a wider one.
• Move the rooms you use during the day. If the home gets sun upstairs and shade at ground level in winter, the home office can be worth moving upstairs from November to February.
• Make peace with grey light where it suits. A north-facing room gets no direct sun, but a perfectly steady light with no glare. For screen work it's often the best room in the house — even in December.

How to find out where the light is disappearing to

Test it, rather than guessing. Stand at each window around midday in the winter half of the year and look south. Is there open sky? Or a roof, a tree, a gable end, a fence?

The shadow angle works out as arctan(height ÷ distance) — for instance arctan(6 ÷ 15) = 21.8° for a 6-metre neighbouring gable 15 metres away. If that figure is bigger than the winter sun's scant 11 degrees, you'll get no direct sun through that window in December, however large it is.

Have the home's floor plan measured and correctly placed on the map, and the whole house can be checked at once. In HouseSense, sunlight in the 3D model is calculated from the address's position, the house's north direction, and a date and time you choose yourself — so you can see the home's December in the middle of August, and work out which rooms make it through winter before you move in.