It's the tips of their ears that catch the burn. To begin with, the tissues of the ear become sore and tender but if left untreated these early tissue changes progress to an unpleasant skin cancer, squamous carcinoma. Treatment can involve hormonal steroid administration to slow up the rate of tissue change or, at worst, amputation of the affected ear. So it is better by far to prevent the sun causing biological havoc by applying high factor sun block to the tips of the ears. It may sound odd but your cat will be grateful and it is a simple preventive measure that all owners of white cats or cats with white-tipped ears should consider.
Sunburn can be painful for dogs too.
It is the pink around the nostrils that is at risk from the sun. Increasingly, vets are tattooing the pink with black pigments to make them safe, although regular application of modern sunblocks can be just as effective. Interestingly, it's not only domestic pets that are at risk from sunburn. Cattle,particularly Friesians, are great consumers of calamine lotion in the summer months. It's all to do with what they eat.