All around the city of Florence there are hills. Close the Fiesole one and far Chianti end Casentino.
It's geographic reality influences the weather. During the Winter it's very cold, even if it's difficult that it snowing. Contrary during the Summer, it's very hot and humid.
Best period to visit Florence is Spring and Autumn, even if those period it can be cloudy and raining.
About tourists, in Spring start the fist school busses and from May the tourist from north of Europe. Month of June, July and August, are the Italian tourist Month, because in this period Italian schools are close.
The best day period to visit Florence is early in the morning and late in the afternoon, when there ia a good light but not a lot of tourists, but it's important remember that Museum, Churches and monument are usually open from 9.00 am to 6 pm
About the weather in Tuscany: Climate and Meteo in Florence