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The Sunniest and Darkest Places on Earth

from Gerhard Holtkamp, 15. June 2009, 22:22
It's the week of summer solstice on the northern hemisphere which means that the sun never sets above the Artic Circle and those brave people who stay the winter over in Antarctica are at the midpoint of half a year of total darkness, right? Well it's worth to take a more careful look. Which places get the most sunshine and how dark does it ever get?

Let's for the moment ignore clouds. Without those clouds we can use astronomical formulae to calculate sunrise and sunset all over the planet. There are still some problems we have to get out of the way. Firstly, the sun isn't just a point in the sky but is a disk half a degree wide. Sunrise is defined as the moment when the first piece of the sun that is to say its upper limb appears on the horizon and at sunset this upper limb disappears. Secondly, there is atmospheric refraction. When our calculations say that the sun is just below the horizon we might still be able to see it because the sun's light gets bend around in the atmosphere. Sunrise of partially eclipsed Sun. Coral Bay, Western AustraliaTo account for these two effects one typically calculates the times of sunrise and sunset as the moment at which the center of the sun is -0.83° below the horizon (ignoring refraction). Those values appear in almanacs, newspapers etc. There is a slight variation of the sun's apparent radius which we can ignore (the Sun - Earth distance varies a little) and a more pronounced variation in refraction due to temperature and air pressure which we also ignore because we usually don't know them too well in advance. But that really doesn't matter because unless we stand at the beach overlooking a large ocean the sun will disappear behind some hills, trees or houses before the almanac tells us it should or we might be on top of a mountain and see the sun a little longer.

One consequence of refraction is that you can see the midnight sun even slightly below the Artic Circle. In the year 2009 the Arctic Circle gets a total of 4647 hours of sunshine (meaning between sunrise and sunset). This figure will be almost the same in other years (except leap years). This is equivalent to 193.6 days. Without clouds the Arctic Circle would be the sunniest place on Earth. The North Pole gets 4575 hours, at 45° North the value is 4463 hours and the equator comes in with just 4422 hours - this is 9 days less sunshine that at the Artic Circle!

You might think that the southern hemisphere would be a mirror image of the northern hemisphere. But now another effect comes into play: The Earth moves around the Sun in an elliptical orbit. It is a little closer to the Sun at the beginning of January and it also moves a little faster then and it is farthest at the beginning of July and moving a little slower. As a result northern summers are a few days longer than southern summers. The Antarctic Circle does come out number one in sunshine hours on the southern hemisphere but a total of 4530 hours makes it 5 days less than the figure for the Arctic Circle.

But all those astronomical calculations about sunshine mean little if the sky is overcast. Thanks to Earth observation satellites we have a global picture of cloud cover and this gives you a good idea about the actually observed sunshine.

Mean annual cloud cover.

Here is a world map of mean annual cloud cover measured over a 14 year period. As you might expect the desert regions in the north and south have very few clouds but also the interiour of Antarctica and Greenland are often clear. It is interesting to note that while tropical land areas like the Amazon and the Kongo basins are often cloudy the stretches over the tropical oceans are clear.

A high number of sunshine hours may lift your spirit and put you into a better mood but it doesn't mean that you necessarily receive a lot of energy from the sun. You probably have made the painful experience that you get sunburned a lot faster if the sun is high in the sky. This is due to the geometric effect of projection and also due to the shorter way that the light has to travel through the atmosphere. Absorbed Solar Radiation.The combined effects of sunshine hours, cloud cover, surface reflection and angle of incidence of the sunlight explain the map of the actually absorbed solar radiation. Now the tropics show up as the main powerhouse (but note the discrepancy between the often cloud covered continents and the cloud free oceans) while the polar regions are cold as expected.

Once the sun has set it doesn't get dark right away. Some sunlight scattered in the atmosphere still reaches us and we experience twilight. By the time the sun is 6° below the horizon it has become dark enough that the brightest stars show up and the streetlights are turned on. This marks the end of civil twilight and is usually what most people would refer to if they talk about twilight. But there are two more types of twilight. The end of nautical twilight is reached when the sun is 12° below the horizon. Up to that moment ships at sea might still discern the coastline and find their way. Now it starts to get really dark on the ground. But the sky is still rather bright. More stars are showing up than before but you wouldn't see the Milky Way.

Once the sun is 18° below the horizon to mark the end of astronomical twilight the sky is as dark as it will get. No matter how much further the sun sinks the sky doesn't get any darker because now the portion of scattered sunlight still reaching us is less than the background brightness of the sky. This background comes from the weak light of the stars and by something called airglow. Airglow is caused by the recombination of atoms (mostly oxygen and nitrogen) which have been ionized during the day by the ultraviolet radiation of the sun.

Airglow. STS-99 Endeavour. NASA.

Astronauts in space can see it as a ring surrounding the Earth. A particularly nice picture of airglow was taken by the crew of the Shuttle Endeavor in 1999. See how the stars in Orion still shine through even the thickest layer of airglow. So even if you are at a place far away from city lights the sky doesn't get completely dark. "I didn't know what black was until I've been in space" was the remark of an astronaut who had just returned from a Shuttle mission.

To make astronomers happy we want to have as dark a sky as possible. We already learned that the sun should be more than 18° below the horizon for that and we have to stay away from the light pollution of civilization. But there is one more thing to take care of - the Moon. With a bright Moon in the sky you will see fewer stars and no Milky Way or other dim objects. Who gets the most hours of astronomically dark skies without the Moon? The 2009 figures (variations are possible in other years due to the orbit of the Moon) show the equator with a maximum of 1721 hours. At 80° North it is just 825 hours and the minimum for all latitudes is 807 hours (this amounts to just little over a month) at 80° South. So Antarctica isn't all that dark even in winter! Add to that the fact that it is covered in snow and ice which lightens up the landscape and that there may be some aurora brightening up the sky this is the least-dark place on Earth. By contrast the equator is the darkest place especially as there is often some dense vegetation which seems to swallow what little light is left.

But dont't let all these contemplations spoil your romantic moonlit nights!

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  1. aj smith sunniest & warmest, still green [non-desert] with low humidity factor
    11.07.2009 | 22:49

    Where in the U.S. are the above-described regions??? I refer to annually,
    not just in a given season.

    That is, I don't want to move to a dry climate like AZ, and prefer that there be local farming near to where I live-work... Little rain- & snowfall as possible... A change of seasons each season, if possible, but that's not as essential as the above aspects...

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