Climate and Weather in Atlanta:
Many visitors to Atlanta expect a true tropical environment here and they are quite disappointed when they get off the plane and realize they aren't surrounded by palm trees. In fact, the American south does not become tropical until you reach nearly a quarter of the way through Florida, so if you are visiting Atlanta in December, January or February, bring long pants and shirts as well as a warm coat.
Summers in Atlanta can be very warm, with temps in the 90'sF or in the 30sC. Falls and springs would be considered warm by most people's standards, but that doesn't mean there aren't times when things get a little chilly.
The biggest weather related bother you will have to worry about in Atlanta is rain. It does rain a bit more than in most places, and there isn't one particular time of year you can visit where the rain is significantly less than other times. Rain in the summer usually comes with thunderstorms, but these squalls normally only last a half an hour to an hour, and the rest of the evening is usually fine. Bring an umbrella with you when you go out and try to dress in layers unless you are there in the middle of a hot summer day.