The best time to visit Miami Beach is the spring. Between March and May, the sun shines with high 70s and low 80s during the day. Also, you'll miss the peak winter rates. This part of Florida enjoys a subtropical climate, meaning it's usually warm year-round and a bit rainy. Pack an umbrella if you visit in summer or fall; at the very least, you can expect some afternoon showers. In the worse-case scenario, you could experience torrential downpours since hurricane season runs from about June to November.
Summer means that families will converge on the beach; however, they won't stick around for the evenings. With temperatures in the high 80s and lower 90s, you should bring lots of sunscreen and take frequent dips in the ocean.
The time to see and be seen, winter brings out the best in Miami Beach. You'll enjoy the people-watching, while sitting on the outdoor patio of an exclusive restaurant with temps in the mid 70s. Miami Beach also saves its best events for the winter, especially Art Basel. Everyone (who is anyone) wants to be here during this time, so bringing the check book and booking three to four months in advance are musts.