I've read that 80% of sunlight is passing through clouds. Now there a small clouds and big clouds so it should be some average. Is this true or is the amount of sunlight passing throug a cloud really much more depending on the size of clouds. Is it for instance on a totally cloudy day that only 30% is passing and on a nice summerday when a cloud comes by it is 90%. Probably it depend on how high in length the clouds are.

But perhaps even a more interesting question is, are all wavelengths passing though on the same level? So is uv-light passing even easy as infrared light? In my experience though when walking in the sun it feels that much more than 20% of those warm IR-rays are being blocked by clouds (even if they are very tiny). So is it possible that even 70% of the IR rays are blocked or perhaps relatively moren than visible light or UV-light?

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    $\begingroup$ Having just getting a sunburn on a (very) cloudy day, I can confirm that UV does penetrate clouds. $\endgroup$
    – Gimelist
    Jul 6, 2016 at 11:08

2 Answers 2


Here is not a detailed article, but one you might find some informative thoughts in: http://www.americanscientist.org/issues/pub/sunshine-on-a-cloudy-day

On a basic note, any generalization starting with "clouds block x or x%" are far too general to compare actual percentages. It assumes a cloud is a cloud is a cloud. They are not. What a cloud actually does to block or not block electro-magnetic radiation is not an absolute. It varies by type of cloud, thickness, altitude, angle of the sun and likely many other factors. As a generality, clouds block a part of visible, UV and IR light depending on a number of factors especially depth of the cloud. Some of the EM waves are absorbed, some reflected and others pass through. The UV end of the spectrum tends towards passing through while the IR end is more prone to being reflected which helps to create a greenhouse effect. These are often presented as absolutes, but are actually generalities.


According to Trenberth, Fasullo & Kiehl update of the "Earth's global annual mean energy budget" seems worlds clouds in average reflects about 23% of incoming sun radiation (about 79 W/m2).

Data from 1997 (that could be outdated) estimates clouds blocks approximately 20-25 W/m2 (6-7% of surface radiation). If so, aerosols has a small role in IR block, and the main part is due to CO2, water vapor, methane and other greenhouse gases.

However, the way clouds overlap on the downward flux of heat with other greenhouse gases introduces uncertainties. Another source of error is the amount of water vapor between the surface and the cloud base.

Kate Marvel wrote:

“To accurately model clouds, we’d have to track the behavior of every water droplet and dust grain in the entire atmosphere, and there’s no computer powerful enough to do that,”

In general, seems clouds is the main source of uncertainty in models.


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