St. Petersburg is generally warm and humid year-round, but there is a noticeable variation between summer and winter. In the warmer months, daytime highs average in the low to mid-80s F/28-30 C, but the sticky conditions can make things somewhat uncomfortable. You'll find it a little cooler along the Gulf shore, where there's a steady breeze.
Winters are mild, with daytime highs usually reaching the 60s F/15-18 C and often into the 70s F/21-26 C, though occasional cold snaps are possible. Lows near the freezing point aren't uncommon, but the warmth usually returns quickly. No matter when you visit, be sure to use sunscreen. Be aware that hurricane season is June-November.