Nashville tends to stay humid all year long but especially so in summer. The temperatures can routinely get into the 90s F/30s C, and the humidity can make it feel much hotter. For visitors from cooler climes, July and August can be quite uncomfortable.
Winters aren't too cold, although they can be rainy. Temperatures average 30-50 F/0-10 C then, but the moist air makes for a chill. It snows very little in Nashville—maybe two or three times a year. If you visit in winter, take at least one warmer-weather outfit: Although it may be below freezing one day, it could rebound to 70 F/21 C the next.
Spring temperatures usually stay in the 40-70 F/5-20 C range. However, it can be very rainy, sometimes spurring severe storms. Fall is by far the best time of year to visit. Temperatures are mild (50-75 F/10-25 C), and the humidity is at its lowest. Fall foliage puts on a gorgeous display, with late October and November typically the peak times for leaf color.