The vet calls it by the fancy name actinic dermatitis, but let’s call it what it is: sunburn. Most cats won’t burn, but the ones most likely to are white cats with blue eyes. (In other words, cats who correspond to blue-eyed, fair-skinned, easy-to-burn humans.) A sunburned cat shows redness around the ears, eyelids, nose and mouth.
With a bad burn, there may be hair loss, peeling and itching. Needless to say, it is more common in summer than in winter. Over time, the “fair” cats who have been overexposed to the sun can develop skin cancers, which is also true for sun-worshipping humans. One obvious way to avoid this is keep the cat inside, especially at midday.