Climate of Puerto Rico

The climate of Puerto Rico in the Köppen climate classification is predominantly tropical rainforest. Temperatures throughout the year are warm to hot, averaging near 85 F in lower elevations and 70 F in the mountains. Easterly trade winds pass across the island year round while the rainy season stretches from April into November. The relatively cool trade winds are blocked by the mountains of the Cordillera Central which causes rain shadows and sharp variations in the temperature and wind speed over short distances. About a quarter of the average annual rainfall for Puerto Rico occurs during tropical cyclones, which are more frequent during La Niña years.

Temperature
Temperatures range from 70 to 90 F in the lower elevations, while higher elevations in the central part of the island experience temperatures between 61 and 80 F year round. The temperature in the south is a few degrees higher than the north. Between winter and summer, there is only a temperature swing of around 6 F-change. Coastal water temperatures average between 77 F in February to 87 F in August. The highest temperature ever recorded was 103 F at San Lorenzo, while the lowest temperature ever recorded was 38 F at Aibonito. Frost and freezes have occurred in the highest mountains at almost twice the elevation of Aibonito, where the low temperature in winter is in the low 40s (5C). The average temperature in San Juan is over 80°F (27C). It is the only location in plant hardiness zone 13b on the USDA map, with temperatures rarely falling to the low 60s.

Dust from the Sahara
For years, meteorologists have been studying dust that arrives from the Sahara Desert, and they believe the dust which is carried over the Atlantic to Puerto Rico causes a whole series of changes in Puerto Rico's climate and environment, from haze to heat.

Wind
Persistent 19 kn trade winds move from east to west across the island year round. When the trades winds are lighter, sea breeze and land breeze circulations dominate. Every five years or so a hurricane brings high intensity wind.

The island's infrastructure is extremely susceptible to high winds. For example, when Hurricane Maria made landfall on the island in 2017 at Category 5 strength, 100% of the power grid was lost.

Rainfall
There is a pronounced rainy season from April to November and a dry season from December to March, causing some drought. Due to the island's topography, rainfall varies greatly across the island. Pico del Este averages 171.09 in of rainfall yearly while Magueyes Island averages only 29.32 in a year.

Severe weather
Approximately 80 percent of the territory's agriculture was lost due to the hurricane, with agricultural losses estimated at $780 million. The hurricane completely destroyed the island's power grid, leaving all 3.4 million residents without electricity. Hurricane Maria caused a humanitarian crisis in Puerto Rico, with damages estimated in 90 billion dollars and a loss of human life close to three thousand.
 * Puerto Rico experiences the Atlantic hurricane season, similar to the remainder of the Caribbean Sea and North Atlantic oceans. On average, a quarter of its annual rainfall is contributed by tropical cyclones, which is more prevalent during periods of La Niña than El Niño. A cyclone of tropical storm strength passes near Puerto Rico, on average, every five years. A hurricane passes in the vicinity of the island, on average, every 11 years. Two Category 5 hurricanes have struck the island since 1851: the Lake Okeechobee Hurricane of September 1928, and Hurricane Irma of September 2017.
 * Sometimes waterspouts form off the coast during showers and thunderstorms, particularly off the west coast, with tornadoes and hailstorms possible in the inner sea. These thunderstorms may be formed due to tropical waves, tropical cyclones, and frontal boundaries which become stationary across the region between fall and spring.
 * In September 2017, Puerto Rico was hit by Category 5 Hurricane Irma, which had maximum sustained winds of 290 km/h (180 mph). It was and still is the strongest hurricane to hit the island in recorded history. While the eye itself stayed offshore (passing about 60 miles from land), the eye-wall skirted the island's northeastern portion, including the population centre of San Juan. Nearly 70% of the island lost power from the hurricane. The power and infrastructure grid were severely weakened by this storm, leading to more damage from Hurricane Maria, two weeks later.
 * Hurricane Maria made landfall on Puerto Rico on Wednesday, September 20, 2017 near the Yabucoa municipality at 10:15 UTC (6:15am local time) as a high-end Category 4 hurricane with winds of 155 mph. In addition, heavy rainfall occurred throughout the territory, peaking at 37.9 inch in Caguas. The eyewall replacement cycle that caused María to weaken to Category 4 strength also caused the eye to triple in size as the diameter expanded 9 - 28 nmi prior to landfall. This change in size caused the area exposed to high-intensity winds on the island to be far greater. Widespread flooding affected San Juan, waist-deep in some areas, and numerous structures lost their roof.

Weather by season
The Wet Season begins in April and lasts to November. The weather is hot and humid with thunderstorms in the afternoon, especially in August and September. Trade winds bring cool summer breezes in the north and east of Puerto Rico, but due to the topography of the island, these winds do not reach the coast south and west, causing much higher temperatures than in the north. During humid summer days, the heat index can reach 115 °F (47 °C) and actual temperatures can reach 100 °F (38 °C). Sometimes the wind shifts from the south, causing an inverse pattern, where the heat south of the island moves to the north, causing temperatures in the mid 90s (35 °C) in San Juan and low humidity. June is usually the driest month of the summer. During June and July the precipitation is mostly caused by the effects of humidity and heat of the island. The peak of the hurricane season comes in September. Weather conditions can be very rainy as tropical lows pass near the island, with frequent strong thunderstorms. By early November the tropical cyclone season fades and the weather becomes drier and generally sunny.

December brings the Dry Season which lasts through March. Weakening cool fronts from the north lower temperatures and humidity, and dry and sunny conditions prevail. In San Juan in the winter months, high temperatures reach 77–87 °F (25–30 °C) with an average high of 83 °F and lows down to 66–76 °F (19–24 °C), giving an average of 71 °F (22 °C). In the central area of the island the elevation and reduced sea effects bring the minimum temperatures to 55–65 °F (13–18 °C) on average. After fronts pass the temperature may drop to the 40s (4–9 °C) in the mountains, in the 50s (10–15 °C) in the valleys, and in the low 60s (16 °C to 20 °C) in the coastal zone. In general the climate in the dry season is sunny, with little change from day to day. February and March are often quite dry, sunny and hot.

Sun
The sunrise varies from 5:30 a.m. in summer and 7:00 a.m. in winter. Sunset varies between 5:40 p.m. in winter and 7:10 p.m. in summer. Puerto Rico does not use daylight saving time. The ultraviolet index or UV index varies from 10+ in the summer and a maximum of 7 in winter.