UV light from the sun when cloudy?
When i was 9 years old it was a partial sun eclipse but it was very cloudy so couldn’t see it. But i remember me telling my parent and i was convinced i was going to get blind.
Now when i searched the net for answers about how much Clouds are damping the sun s UV do i get really really confused.
Some sites say the UV will go right through the Clouds, which means that eveyr person looking in the sky say a rainy day can get damaged by the sun.. In fact it would be more dangerous to look at a cloud where the sun is behind than looking at the sun itself because the Eye is not closing as much.
So what is really up here. How can it be the SAME thing to look at a cloud in front of the sun and looking into the sun itself..
Shouldn’t the cloud not even damping but also spread out the UV? A laser is dangerous because it is so concentrated but if beam is very big will it not be dangerous.
Answer ( 1 )
There are multiple questions here. One thing is that if the LIGHT is reduced by some filter then the pupils of the eyes open up admitting more of the light. If the UV is not absorbed by that filter then the risk is higher than it might have been without the filter.
Clouds do reduce UV substantially. But thin hazy cloud does not reduce it to zero or anywhere near it. So in those conditions it is possible to stay in the sun ( while sun baking) for a longer period of time and get sunburned due to the increased time of exposure.
If the day is very cloudy there is a negligible amount of any form of solar radiation.
On a sunny day in summer my solar panels produce some 24 kW hr of electricity from the sun.
On a hazy day they can produce at least half of this.
On a day of heavy cloud they produce as low as 3 kWhr or even less.
This is a measure of the total radiant energy they obtain from a day. ( UV plus visible light)
As you point out the diffuse nature of the light passing through heavy cloud means that you would not get the “flash burn” that a welder can get ( or that can come from a laser.). This is a pinpoint of high energy that can overheat and damage a tiny spot on the retina.
So in most cases our eyes ( and nature) do what is necessary. We squint, sneeze or make some reaction to prevent us from looking at a dangerous source. Darwin gave us an explanation as to why this should be the case.