South Beach is a flat sandbar. There are no hills, almost no canopy, and the buildings are short enough that by 10 AM in June the sun has cleared the Art Deco rooflines and the sidewalks are open to the sky. The breeze off the Atlantic helps for the first half-hour and then turns into a hairdryer. Walking here at noon is a posture choice.
Before about 8:30, the same blocks are walkable. The east-facing buildings on Collins and Washington throw long morning shadows across the sidewalks. The little side streets between the hotels — Espanola Way, the row south of 14th — stay cool an extra half hour. The picks below are graded for that window. Most are short by design; you are not trying to put in steps, you are trying to get somewhere before the sun is on you.