Yes, you can absolutely visit Pompeii in the rain, and for many, the experience is actually more atmospheric and less crowded. The archaeological site is open "rain or shine." Because the ancient Roman streets were built with large, raised stepping stones and deep volcanic stone gutters to handle heavy storms, the drainage is surprisingly efficient, meaning you won't encounter deep mud. However, the ancient cobblestones become extremely slippery when wet, so wearing shoes with excellent traction is a safety requirement. You should prioritize visiting the "villas" and "covered houses" (like the Villa of the Mysteries) during heavy downpours, as these offer shelter and preserved indoor frescoes. It is highly recommended to wear a lightweight raincoat or poncho rather than an umbrella, as the winds coming off the Bay of Naples can be quite strong and the paths are often too narrow to navigate with an open umbrella alongside other tourists.