Found this on Purrs Cat Care guides
Just as with people, excessive sunlight can be dangerous to pets. It is particularly a threat to cats with white or pale ears or noses, parts which may get sunburn that can even progress to skin cancer. It is best to keep these pets indoors on sunny days between 10am and 4pm. If this is not possible, sunblock (factor 15 or higher) can be used, but it will need repeated applications through the day.
Avoid products containing zinc or salicylates – get your vet to check if a sunblock is suitable. If your cat has reddening, scaling, crusting or scabbing, especially on the ears or nose, seek veterinary advice