Charleston's climate, like the rest of coastal South Carolina, is mild and humid up until the beginning of summer, when the heat index jumps. January temperatures are moderate, but most range 37-59 F/3-14 C. Expect temperatures to run 53-76 F/11-24 C along the coast in April, 61-83 F/16-27 C in May. Severe thunderstorms can occur in spring.
Visitors expecting to enjoy the beaches will find pleasant conditions through the summer, though July and August are hot (high 80s F/30 C to above 90 F/32 C) and humid. Humidity at 60%-80% in high heat can take a lot out of you, so pace your sightseeing. Drink plenty of water and dress lightly. Be aware that the season for hurricanes and tropical storms is June-November.
The fall sees less humidity, and temperatures range 55-84 F/12-28 C. Indian summer lasts through November, with dry days and temperatures from the low 40s F/5 C to the upper 60s F/18 C.