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 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.”