For the island of Ambergris Caye, summer temperatures average between 75 to 90 F, but are moderated by onshore winds, while winter temperatures range between 60 and 80 F and can be lower due to cold damp Nortners that blow in November through January.
Ambergris Caye, like most of Belize, has a sub-tropical climate. Its quite sunny, and you need to have protection. Click here for details on protection from the sun for your skin. Frosts and freezes are unknown. Weather in San Pedro is similar to that in South Florida, with temps ranging from the mid 50s F on a cool winter day to above 90 on a hot summer day. From November through March, expect lows in the low to mid 60s and highs in the low to mid 80s. The rest of the year, expect lows in the low 70s and highs in the high 80s. Prevailing winds off the water keep beach and shore areas pleasant most of the time. Winds sometimes go calm in summer, especially in August and September. Occasionally during the winter "Northers" blow in, chilling the air and kicking up winds. Ambergris Caye gets about 55 to 60 inches of rain a year. July through November normally is the rainiest time on the island, but even during this "rainy season" it is unusual to have long rainy periods, and sunny weather returns after a day or two. Humidity is fairly high, being highest in the summer at above 80%. June through November technically is hurricane season, but the August through October period is the most likely time for tropical storms and hurricanes. In the last 50 years, only about ten hurricanes have struck Belize, with the worst being Hattie in 1961. Click here for more on hurricanes in Belize.
During Northers, one should be careful about sailing, boating or swimming as strong winds and currents make it difficult to return to shore (in Creole a Norther is a "Joe North" and in Spanish "el Norte"). One should never swim to the reef without a boat (with anchor) of some kind, which will provide refuge in case of exhaustion, accident or a change in winds and currents. There are excellent local guides and it is recommended that their services be sought.
There is a dry season (February to June) and a wet season (July to January) with a mini-dry in August. Rainfall averages 60" per year.
The Tropical sun is intense, and coupled with the sun's reflection off the water, sunburn can occur quickly. Protective clothing or sun screen is always in order.
One caveat: Experienced Divers, who come to SCUBA and Snorkel, have long understood that, for some people, mechanical air conditioning can play havoc with the nasal cavities in the face. If you are a diver that suffers from mask squeeze, move to accommodations on the beach that have good flow through ventilation so that you can rest easily at night and avoid air conditioning as much as possible. Divers from cooler climes, that cannot get comfortable enough due to humidity or temperature, to sleep without air conditioning - may need to take decongestants (with their accompanying side effects) in order to be able to go diving. Be very careful to understand the dangers of using decongestant remedies in order to go diving. And, be aware, mixing decongestants with alcohol, never recommended when diving, is sometimes lethal under pressure. In addition: Divers should never use analgesics at all, as a means to counter the effects of air conditioning on the nasal passages, because their side effects are magnified under pressure and because analgesics often reduce pain but do not always clear nasal cavities - thus making sever, permanent damage to the nasal passages possible. Best bet: Prevention - avoid air conditioning as much as possible and its accompanying nasal drip side effects, when diving.
The lessons are: Stay in accommodations that encourage flow through ventilation. Divers with sensitive nasal passages should avoid air conditioning.