Here are some cloudless days chose close to the main seasons Summer,
Autumn, Winter and Spring. This will give you some comparison as to
how much you might get on a clear day across the year. Cloud can
reduce the UV significantly, but while you may feel a lot cooler,
there is still appreciable UV coming through the cloud and it is
easy to get burnt skin on these days.
Examples of days in the Seasons with low cloud to show
the general shape