Day Length and Solar Energy

This page is part of a series on insolation-factors. An overview is provided at insolation. The pages are:
sun angle,
air mass,
day length, and clouds and pollution.

The relationship between season, day-length and solar radiation is all pretty straight-forward. As you get closer to winter, your days get shorter (assuming you don't live on the equator) and having less hours of daylight reduces the amount of solar energy you receive over the course of a day.

There are of course other factors involved and so the longest day of the year isn't necessarily the sunniest.

A sunny day in Oslo.  Boats on the water and a city-scape.
Oslo in the sunshine: "Enjoy it now!"
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On this page, we first explain how many different factors combine to keep winter dark and then give an extreme example of this (in Oslo, Norway).

Some readers may want to skip that discussion and just check out our Day Length Table where we show how day-length varies depending on your latitude.

Many Conspirators Block the Sun

At mid-latitudes the effects of solar angle, air mass and day length all reduce solar radiation more and more as you get closer and closer to winter.

For this reason, shorter days often coincide with (and contribute to) the days when a place receives the least solar radiation.

This situation coupled with the fact that the winter skies are often cloudy helps us understand why, for example, in Oslo, Norway (60 North), the average daily insolation on a horizontal surface is .24 kWh/m2/day in December and 5.54 kWh/m2/day in June.

In July, Oslo gets an average 17.8 hours of daylight a day. In December, it only sees sunlight for an average of 6.21 hours a day.1

[If solar panels are tilted optimally for each month, the insolation on the panels goes up to .75 kWh/m2/day in December and 5.71 kWh/m2/day in June. The reason tilting solar panels influences solar radiation is discussed on sun angle and insolation.]

Latitude (N) longest day (hrs) shortest day (hrs) shortest/
longest
65 3.58 hrs 21.98 hrs .163
60 5.87 hrs 18.87 hrs .311
55 7.17 hrs 17.37 hrs .41
50 8.07 hrs 16.37 hrs .493
45 8.77 hrs 15.62 hrs .561
40 9.33 hrs 15.02 hrs .62
35 9.8 hrs 14.52 hrs .67
30 10.22 hrs 14.08 hrs .73
25 10.58 hrs 13.7 hrs .77
20 10.92 hrs 13.35 hrs .82
15 11.23 hrs 13.02 hrs .86
10 11.55 hrs 12.7 hrs .91
5 11.83 hrs 12.42 hrs .95
0 12 hrs 12 hrs 1

Day Length Table

The table compares different latitudes, showing both their shortest day of the year and their longest. (We even threw in the ratio of shortest/longest days as a free bonus!)

As you can see, the closer you get to the equator, the more uniform your day-length is throughout the year. The rest of this page details how day-length varies with season.

At any given moment, about one-half of the globe is in the sun and one-half is not. For this reason, if the earth wasn't tilted on its axis, everywhere on the globe would have sunlight for about 12 hours (half the time of the earth's rotation) and darkness for about 12 hours.

Day Length and Time of Year

However, the revolution of the earth around the sun and the (currently approximately) 23.26 tilt of the earth on its axis combine to make sure that a place's day relationship to the sun is constantly changing and so day-length changes from day-to-day. Well, not quite. On the equator every day is always about 12 hours long.

And at latitudes somewhat greater than the Arctic and Antarctic Circles (currently about latitudes 66 North and South, respectively), there will be some number of consecutive winter days when the sun never comes up (0 hours of day length) and some number of consecutive summer days when the sun never goes down (24 hours of day length).

In the Northern Hemisphere, all locations have their shortest day at the Winter Solstice (late December - when the earth's axial tilt is tilting the Northern Hemisphere about 23.26 degrees away from the sun). Everything is reversed in the Southern Hemisphere and so people there have their longest day of the year on the Winter Solstice.

After that, in the Northern Hemisphere the days keep getting longer and longer and in the Southern Hemisphere they keep getting shorter and shorter.

On the Spring Equinox (late March - when the earth's axial tilt is not tilting any part of the globe towards or away from the sun), there is close to 12 hours of daylight and 12 hours of night everywhere on the globe (night and day aren't perfectly equal on the Equinoxes because a couple of optical effects make the sun visible a little while before a location is actually pointed towards the sun).

Since the Poles only have like two days a year, that last generalization doesn't quite apply to them. However, it is around the Equinox that the sun rises for the first time in six months on the North Pole and sets for the first time in six months on the South Pole.

In the Northern Hemisphere, the days continue to get longer and longer until they reach the longest day of the year on the Summer Soltice (late June - when the earth's axial tilt is tilting the Northern Hemisphere about 23.26 degrees towards the sun). Locations in the Southern Hemisphere reach their shortest day of the year at this time.

After that, the days in the Northern Hemisphere start getting shorter and shorter, reaching about 12 hours of daylight on the Autumnal Equinox (late September - the earth's axial tilt is once again not tilting any part of the globe towards or away from the sun). The days continue to get shorter until the Winter Solstice when the whole thing starts over again. As always, the opposite situation is found South of the Equator.

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Footnotes

1. This data was found using Oslo's latitude/longitude coordinates using resources from NASA at http://eosweb.larc.nasa.gov/ The exact page used was
http://eosweb.larc.nasa.gov/cgi-bin/sse/grid.cgi?email=
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