The trade winds, which blow northeast from the coast of Africa across most of the Caribbean, have a huge effect on the region's weather. They make temperatures in the Windward Islands (Martinique, Dominica, Grenada, St. Lucia, St. Vincent and the Grenadines) more mild than those in the Leeward Islands (Puerto Rico, U.S. Virgin Islands, Guadeloupe, St. Eustatius and Saba, St. Maarten/St. Martin, St. Kitts and Nevis, Antigua and Barbuda, Anguilla, Montserrat, and the British Virgin Islands).
Generally speaking, the extreme southern Caribbean has the most stable and predictable weather; here, the trade winds blow steady and strong, sometimes bringing a brief afternoon shower. But destinations like Aruba tend to be dry to the point of arid, with desert-like features in some places.
The northern Caribbean tends to have more seasonal variations in temperature, but winters also tend to be less humid and breezier, making beach conditions even more pleasant than in summer. Year-round in the Caribbean, however, temperatures never go above 100 degrees Fahrenheit, and dip into the 60s or below only rarely and at high elevations, such as in the mountains of Cuba and Jamaica.
At sea level, where most Caribbean resorts are located, average temperatures are remarkably steady year-round, just like (and mostly because of) ocean temperatures that are consistently warm. You should expect temperatures in the 70s and 80s all year everywhere except Bermuda, which has a subtropical climate similar to North Carolina's and can get down into the 60s and 70s during the winter. (Jamaica does have a few Blue Mountain resorts that could also get a tad chilly at times).
Mountainous islands like Jamaica, Cuba, and St. Lucia also get more rainfall: Lush, tropical Dominica leads the region, getting more than 300 inches of rain annually. The mountains of Cuba and Jamaica typically get 2-3 times more rain than falls at sea level; on islands like Jamaica, Barbados, and Trinidad, you'll also notice that the windward sides of the island get more rain than the leeward side. May to October tend to be the wettest months in the Caribbean.