Just like humans, cats can get sunburn too. Over time the damage caused can increase the risk of developing squamous cell carcinoma which is a skin cancer.
White cats, cats with white ears, noses or bald cats are most susceptible to sunburn. This is because of the lack of melanin (skin and hair pigment), and protective hair in these areas.
The ears are most commonly affected, although eyelids and noses can also be affected.
Solar dermatitis (also known as actinic dermatitis) is a common disorder in areas with warm, sunny climates.
What are the symptoms of solar dermatitis in cats?
Initially, exposure causes mild redness, hair loss along the ear margins. Scaling, thickening of the skin and itching occur over time. This may cause your cat to shake his head and scratch at the affected area causing bleeding and possibly infection. Dermatitis becomes progressively worse each summer until persistent ulceration and squamous cell carcinoma can develop. 
How is solar dermatitis treated?
If you believe your cat has been sunburned seek veterinary attention immediately. Treatment depends on the severity of the burn.
- Antibiotics may be required if an infection is present.
- If the sunburn is severe, topical or oral steroid cream may be prescribed.
- If squamous cell carcinoma has developed, surgery to remove the cancerous cells (partial pinnectomy) and chemotherapy or radiation therapy will be required.
Prevention is better than cure in this case and the following should be followed with vulnerable cats:
- Limiting your cat's exposure to the sun by confining him/her indoors during the hours of 10.00am - 4.00pm.
- Application of a waterproof sun block. Speak to your veterinarian about which brands are safe to use on cats.
- Early detection is important, so your veterinarian may wish to take a biopsy to determine if your cat has sunburn or squamous cell carcinoma.
 The Cornell Book of Cats