[Physics] How much and what kind of sunlight passes through a cloud

Question

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 (IR) 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?

Answer 1

Based on this article on the website American Scientist Clouds and UV, clouds pass more UV light than you would imagine, which suprised me, as where I live it is cloudy on most days and I thought I received some UV protection as an upside to gloomy weather.

Several studies suggest that reflection off the sides of cumulus clouds is one mechanism by which UV radiation can become enhanced. Also refraction and scattering of direct and diffuse radiation could result in markedly increased enhancement. Thus cloud conditions that include  cirrus clouds thin enough not to completely obscure the solar disk, along with lower-altitude cumulus clouds, may lead to the perfect UV storm.

Just how common is cloud enhancement? The various studies have found that between 1.4 and 8 percent of all measurements show cloud enhancement compared with clear-sky values, depending on geographic location, but as many as 25 percent of those made on partly cloudy days may show it. Most often the enhancement lasts for 10 minutes or less—not a concern for the sun worshiper—but it has been known to persist for an hour.

Based on Wikipedia Infrared Window

Clouds are excellent emitters of infrared radiation. Window radiation from cloud tops arises at altitudes where the air temperature is low, but as seen from those altitudes, the water vapor content of the air above is much lower than that of the air at the land-sea surface. Moreover, the water vapour continuum absorptivity, molecule for molecule, decreases with pressure decrease. Thus water vapour above the clouds, besides being less concentrated, is also less absorptive than water vapour at lower altitudes. Consequently, the effective window as seen from the cloud-top altitudes is more open, with the result that the cloud tops are effectively strong sources of window radiation; that is to say, in effect the clouds obstruct the window only to a small degree.