Having lived in southern California, I know that some areas on the planet can be absurdly sunny and clear throughout the year, and having lived several other places, I know some areas can be chronically overcast.

The article Laser Light to leverage free-space optics in space for Optical Satellite as a Service describes a proposed group satellites in medium Earth orbit (MEO) that would maintain high speed optical data links with a large number of Earth stations. I did some further reading, and found the map shown below. I assume this is still conceptual and the final ground station locations may be different.

But I am wondering, besides that one station in southern California, do these appear to be in meteorologically favorable locations for continuous optical data links to satellites? Are these particularly sunny/clear sky locations?

Does a map of clear-sky hours per year - independent of day or night - exist?

As pointed out in this comment and this article the network can work by reconfiguring depending on atmospheric conditions. But do these appear to be chosen by access to clear skies, or access to fiber networks?

enter image description here

above: Graphic from The Speed of Light: Laser Light and Optus Explain Optical Communications Partnership to Via Satellite Magazine


2 Answers 2


Yes, mean cloud cover is routinely measured from satellites. Like all satellite data (and in fact all measurements), it does have an uncertainty, but for the purpose of this question the satellite product of mean cloud coverage is good enough. Personally, I would hesitate to trust the data at very high latitudes with frequent cover of snow or ice, because those are hard to distinguish from clouds with any form of passive down-looking remote sensing.

The figure below is released by ESA and derived from the MERIS and AATSR instruments on the former ENVISAT satellite. Although the caption does not specify, I suspect that it uses visible channels and that it therefore only relates to daytime cloudiness. It would be interesting to see how it compares to 24-hour cloud cover!

Mean cloud cover, ESA

Full image caption and credit:

Global annual mean cloud cover derived from three years (2007–09) of Envisat data. The map shows areas with little to no cloud coverage (blue) as well as areas that are almost always cloudy (red). Clear skies are immediately visible over deserts (such as the Sahara, Namib and over the Atacama). Regions with constant high-pressure systems are detectable, such as the Pacific. On the other hand, the North Atlantic Ocean in the mid-latitudes is mostly covered with clouds, where storms develop. The UK is often cloudy, whereas the Mediterranean shows sunny skies. Data from both the MERIS and AATSR instruments on Envisat were used.

Credits: ESA/Cloud–CCI

(On behalf of ESA and the Cloud-CCI I would like to apoligise for the poor choice of colourmap, but this figure predates the time at which many of us were educated on its poor suitability)

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    $\begingroup$ Very interesting! OK I can see some correlations right away. But mostly I am surprised at the variation - I guess I'm not surprised scientifically, but wow, some people live life under clouds and others see blue sky so much of their lives. Your comment about colormaps is appreciated. See the two links under the donut-shaped GIF in this question: physics.stackexchange.com/q/278664/83380 for example. Haiku no less! (note added in proof - the original Haiku tweet has been deleted) $\endgroup$
    – uhoh
    Commented Oct 13, 2016 at 11:38
  • $\begingroup$ I think you can use this to answer Highest amount of sun hours in the ocean as well $\endgroup$
    – uhoh
    Commented Nov 10, 2022 at 17:53

This is just supplemental information to the accepted answer by @gerrit

I took the ESA cloud cover fraction map provided there, and added dots that represent the approximate locations of the ground stations shown in the 2014 map shown in the question, just to get a rough idea.

I used the extremely cool Submarine Cable Map site to identify the likely fiber optic 'hub locations' that are likely to correspond to the ground station location.

So indeed, a large number (but not all) of these sites appear to coincide with low cloud coverage fraction areas of the earth, as well as submarine optical fiber "landing sites."

enter image description here


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