That is a good question I guess it depends on how high the UV is?
Clouds are known to magnify UV rays which is why you should still wear a hat on a cloudy day.
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Magnify? I don’t think so. Some UV can pass through some clouds so you can get burned when it’s cloudy, though it’s not common. Maybe that got telephoned into magnified by people.
A tinfoil hat?
I've never heard that clouds magnify UV. I will have to research that.
I guess it depends on the thickness and the type of clouds? https://www.abc.net.au/science/articles/2012/01/24/3413924.htm
Cloud factor
There hasn't been much research about the effect of cloud and the types of cloud on UV so at the moment the Bureau only gives a maximum clear sky value, and the times of the day when the UV index is expected to be above level three, he says.
"UV is affected by many things. One of those is cloud, but it depends upon what type of cloud it is, the thickness of the cloud and whether it is deep right through the atmosphere."
UV radiation can penetrate through thin cloud, so you can still get high levels of UV at ground level on overcast days, he says.
Patchy clouds can also intensify UV levels because radiation is reflected off the clouds' edge.
"In other words you get a mirror effect and the UV can bounce off the clouds and focus on the ground. You can get quite high UV levels at the ground during partly cloudy days when there are breaks in the cloud.
"You have to be very, very careful on those sorts of days not to assume you won't get those sorts of extreme levels."
I kinda want to know the answer now so I must wait here for someone who knows.
I'd say a sunny day easily, but it depends on what there is to absorb the sunlight. Snow is a great insulator and reflects plenty of light, but if there's Even a few specks of dirt on the snow they will condense and melt the snow faster. Same with anything that absorbs light. If it's a barren Arctic desert with only snow, then the temperature would be more important.
This is based on observation, I'm no scientist until a peer reviews this
Answer: 45 and cloudy will melt snow faster in most cases. Snow is pretty reflective, so the heat transfer is more efficient when it comes from the air than straight from the sun, especially when there is wind to move and replace cooled air.
Since snow-melting is a thermodynamic process the investigation pertained largely to a consideration of the various factors influencing the transmission of heat to the snow-mantle. Of these factors, it was found that for high melting rates the heat contributed by convection and condensation of moisture through turbulent diffusion of warm moist air are the important heat-sources. The problem is, then, largely a consideration of the upper limit values of air-temperature, humidity, and wind-velocity compatible with an adequate snow-cover, and the relationship of these values to the rate of snow-melt. Light, 1941
Very interesting. Thank you.
Probably depends on the intensity and angle of the sun relative to the ground. But I’m not sure. Probably some complicated math you could do to find out.