The weather in the spring and fall is ideal for sightseeing. Winters are relatively mild, too, except in mountainous areas, and tourist attractions are much less crowded.
Except for the coastal and mountainous areas, summer in Italy can be very warm. For example, the average daytime temperature in Rome in July is 85F / 29,5 C.
Many hotels have air-conditioning, but it may not be central air as we know it. Most Italian hotels and homes do not have screens on their windows, and, believe it or not, they are not needed (most of the time).