September is a hot month in the rainy season in Puerto Rico with usually muggy conditions. Mornings and evenings are the best time to visit beaches, swim, walk or relax on the white sands. The probability of rainfall is the highest during the afternoon, which is also the period of stifling hot weather. San Juan registers 5.8" (147mm), but the rainiest places are the slopes of the Cordillera Central and the rainforests. Art museums find many connoisseurs in the wet season when it is either too hot or too wet outside. The sun beats the land for 8 hours a day, and it is mandatory to carry sunglasses and hats, as is an umbrella. Expect wind speeds to be at the lowest during the rainy season, and many days see only a light breeze. Expect the heat index to increase with each passing day in September in Puerto Rico.