Miami where it is

The climate of Miami, the southernmost and largest city in Florida, is tropical, with pleasantly warm winters and long and muggy summers.
The average temperature ranges from 20 °C (68 °F) in January to 29 °C (84 °F) in July and August. Here are the average temperatures.
Miami - Average temperatures
Min (°C)141516192123242424221815
Max (°C)242527283031323231292725
Min (°F)575961667073757575726459
Max (°F)757781828688909088848177

Rainfall amounts to about 1,500 millimeters (60 inches) per year. There is no real dry season, although the least rainy season is winter, from December to February. In summer, from May to October, the rains are abundant, but they occur in the form of showers and thunderstorms, which release a lot of rain in a short time and therefore do not reduce too much the sunshine hours. Here is the average precipitation.
Miami - Average precipitation

Winter, from December to February, is pleasantly warm. Rainfall is fairly rare and is brought by cold fronts that sometimes can move from the continent to southern Florida. The cold waves typical of large areas of the United States do not arrive this far south; after all, we are almost at the Tropic. Every so often, however, from December to March, a bit of cold air can arrive here, bringing mainly a bit of cold at night for two or three days, around 5/8 °C (41/46 °F), while the days remain very mild, around 18/20 °C (64/68 °F). In January 1977, when many American states were hit by an intense cold wave, the temperature in Miami dropped for a couple of nights to -0.5 °C (31 °F), but even in those days of low-temperature records, daytime temperatures remained about 15 °C (59 °F). On January 22, 1985, a similar situation occurred, with a minimum temperature of -1°C (30 °F) and a maximum temperature of 15 °C (59 °F).

The short spring, in March and April, and the even shorter autumn, in November, are warm, with almost summery weather but without the sticky heat and the frequent thunderstorms of summer.

The long summer, from May to October, is hot, but above all, it's sultry, although sea breezes give a bit of relief. Showers and thunderstorms are frequent and sometimes may be intense, but for the rest, the sun regularly shines.

South Beach

The amount of sunshine in Miami is very good all year round. The sunniest months (also considering the length of the days) are March and April, while the least sunny are those of winter, when the sun still often shines. In summer, the sky is not always clear: scattered clouds can form quite often, and can sometimes swell, leading to showers and thunderstorms, but after the sun will shine again.
Miami - Sunshine

The sea in Miami is warm enough to swim in all year round, in fact, it does not drop below 24 °C (75 °F) in winter, while in summer it gets very warm, so as to allow long swims. Here are the average sea temperatures.
Miami - Sea temperature
Temp (°C)242424252729293029282625
Temp (°F)757575778184848684827977

From June to November, Miami, like the rest of Florida and the Gulf of Mexico, can be affected by hurricanes, which, however, are more frequent from August to October. In the past, some hurricanes have directly hit southern Florida and have been destructive, such as the "Great Miami Hurricane" in September 1926, Hurricane Okeechobee in September 1928, and Hurricane Andrew in August 1992.

Best Time

The best time to visit the city of Miami and explore the nature that surrounds it, like the Keys or the Everglades, goes from December to April. In winter, Miami is definitely the warmest and most sheltered place in the United States, and there are many sunny days, but, as we said, sometimes the weather can be a bit cool, windy or rainy.
So, the weather in winter is not always ideal for a beach holiday, although, as mentioned, the sea is warm enough for swimming even in this season. The best period is probably from mid-March to late April. In May, the heat begins to be felt and thunderstorms increase in frequency, but you can still go. You could also go in the long summer, but the weather is hot and muggy, and there's also the risk, which should not be underestimated though statistically unlikely, of hurricanes.

See also the climate of Florida.