September weather forecast
San Juan, PR

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An old fort building and a view of the stunning San Juan city, Puerto Rico
An old fort building and a view of the stunning San Juan city, Puerto Rico

Weather in September

September in San Juan witnesses a minor pullback from the robust heat of summer. The city enjoys the highest sea temperature of the year at 84.9°F (29.4°C), soothing the still active summer heat. A shift is evident in the wind speeds, which decrease to 8.1mph (13km/h). Rainfall touches its peak at 1.18" (30mm) over 11.7 days, signaling a change in the season. Quintessentially, the month represents a gradual transition towards cooler weather.

Temperature

With September's arrival, San Juan's weather records an average high-temperature of a still warm 84.9°F (29.4°C), marking little difference from August's 85.1°F (29.5°C). During September's nights, San Juan notes a minor temperature decline, reaching an average of 79.2°F (26.2°C).

Heat index

The heat index value for September is estimated at a torrid 95°F (35°C). Exercise heightened safety, heat cramps and heat exhaustion are probable. Persistent activity may provoke heatstroke.
For clarity, heat index numbers account for light winds and areas under shade. The heat index values could be amplified by up to 15 Fahrenheit (8 Celsius) degrees in direct sunlight.
Note: The heat index, also known as 'felt air temperature' or 'feels like', is a calculation that merges air temperature and relative humidity to depict the sensation of warmth. The effect of weather is individual, with a variety of people experiencing it differently due to distinctions in body mass, height, and level of activity. When exposed to direct sunlight, it's possible for the heat index to increase by as much as 15 Fahrenheit (8 Celsius) degrees. Heat index values are extremely vital to babies and toddlers. Youngsters generally face more threats than adults given their lesser ability to sweat. Coupled with their larger skin surface relative to their small bodies and higher heat production due to their activities, they are more endangered.
Evaporating sweat, an outcome of perspiration, is key in assisting the human body to moderate its temperature. With high air temperature and humidity (high heat index) conditions, sweat production is restricted, leading to an amplified perception of heat. Overheating risks increase with rising body temperatures, especially if heat release isn't efficient.

Humidity

In September, the average relative humidity is 78%.

Rainfall

In San Juan, in September, it is raining for 11.7 days, with typically 1.18" (30mm) of accumulated precipitation. Throughout the year, in San Juan, there are 159.9 rainfall days, and 12.01" (305mm) of precipitation is accumulated.

Ocean temperature

The month with the warmest seawater is September, with an average ocean temperature of 84.9°F (29.4°C).
Note: Water temperatures between 77°F (25°C) and 84.2°F (29°C) are regarded as highly pleasant, making all water activities enjoyable without any discomfort over long durations.

Daylight

The average length of the day in September is 12h and 14min.
On the first day of the month, sunrise is at 6:09 am and sunset at 6:38 pm. On the last day of September, sunrise is at 6:14 am and sunset at 6:13 pm AST.

Sunshine

In September, the average sunshine is 9.4h.

UV index

The months with the highest UV index are April through November, with an average maximum UV index of 7. A UV Index value of 6 to 7 symbolizes a high health risk from unprotected exposure to Sun's UV rays for ordinary individuals.
Note: In September, the maximum UV index of 7 suggests this advice:
Shun overexposure. Protection from sun ravages is indispensable. Stay out of direct sunlight between 10 a.m. and 4 p.m., when UV radiation is most intense, and remember that not all shade structures provide complete protection. Shield yourself from the harmful effects of UV radiation with sun-protective attire, a hat, and quality sunglasses. Attention! The reflection of both water and sand can increase the intensity of UV rays.

Frequently asked questions

Is it hot during September?

In San Juan, 84.9°F (29.4°C) is the average high-temperature and 79.2°F (26.2°C) is the average low-temperature during September. In September, the average heat index (a.k.a. 'felt air temperature', 'real feel'), which combines air temperature and relative humidity, is estimated at 95°F (35°C).

What is the average humidity in September?

The average relative humidity in September is 78%.

How much does it rain in September?

In September, the rain falls for 11.7 days. Throughout September, 1.18" (30mm) of precipitation is accumulated.

When is the ocean warmest?

With an average ocean temperature of 84.9°F (29.4°C), September is the month with the warmest seawater.

How warm is the water in September?

In September, the average ocean temperature is 84.9°F (29.4°C). For any water activity, surface water temperatures between 77°F (25°C) and 84.2°F (29°C) are considered as excellent and extremely pleasant for lengthened periods.

How many hours of daylight does San Juan have in September?

The average day length in September is 12h and 14min.

When are sunrise and sunset in September?

In September, on the first day of the month, sunrise is at 6:09 am and sunset at 6:38 pm. On the last day of the month, sunrise is at 6:14 am and sunset at 6:13 pm AST.

How long on average sun shines in September?

The average sunshine duration in September is 9.4h.

What are the months with the highest UV index?

April through November, with an average maximum UV index of 7, are months with the highest UV index.

How high is the UV index in September?

In September, the average daily maximum UV index is 7. A UV Index value of 6 to 7 symbolizes a high hazard to health from unsafe exposure to Sun's UV rays.

Average temperature in September
San Juan, PR

Average temperature in September - San Juan, PR
  • Average high temperature in September: 84.9°F

The warmest month (with the highest average high temperature) is August (85.1°F).
The months with the lowest average high temperature are January and March (79.9°F).

  • Average low temperature in September: 79.2°F

The months with the highest average low temperature are August, September and October (79.2°F).
The coldest month (with the lowest average low temperature) is March (73.9°F).

Average pressure in September
San Juan, PR

Average pressure in September - San Juan, PR
  • Average pressure in September: 29.92"Hg

The months with the highest atmospheric pressure are January and February (30.02"Hg).
The month with the lowest atmospheric pressure is October (29.9"Hg).

Average wind speed in September
San Juan, PR

Average wind speed in September - San Juan, PR
  • Average wind speed in September: 8.1mph

The windiest month (with the highest average wind speed) is July (10.7mph).
The calmest month (with the lowest average wind speed) is October (7.4mph).

Average humidity in September
San Juan, PR

Average humidity in September - San Juan, PR
  • Average humidity in September: 78%

The months with the highest relative humidity are May, June and July (79%).
The months with the lowest relative humidity are February and March (76%).

Average rainfall in September
San Juan, PR

Average rainfall in September - San Juan, PR
  • Average rainfall in September: 1.18"

The wettest month (with the highest rainfall) is November (1.57").
The driest month (with the least rainfall) is April (0.55").

Average rainfall days in September
San Juan, PR

Average rainfall days in September - San Juan, PR
  • Average rainfall days in September: 11.7 days

The month with the highest number of rainy days is November (16.1 days).
The month with the least rainy days is June (10.3 days).

Average sea temperature in September
San Juan, PR

Average sea temperature in September - San Juan, PR
  • Average sea temperature in September: 84.9°F

The best month for swimming (with the highest average sea temperature) is September (84.9°F).
The coldest month (with the lowest average sea temperature) is February (78.9°F).

Average daylight in September /
Average sunshine in September
San Juan, PR

Average daylight in September / Average sunshine in September - San Juan, PR
  • Average daylight in September: 12h and 1min

The month with the longest days is June (Average daylight: 13h and 12min).
The month with the shortest days is December (Average daylight: 11h and 0min).

  • Average sunshine in September: 9h and 2min

The month with the most sunshine is June (Average sunshine: 10h and 54min).
The month with the least sunshine is December (Average sunshine: 6h and 36min).

Average sunshine days in September
San Juan, PR

Average sunshine days in September - San Juan, PR
  • Average sunshine days in September: 12.5 days

The month with the most sunshine days is June (13.7 days).
The month with the least sunshine days is November (9.4 days).

Average UV index in September
San Juan, PR

Average UV index in September - San Juan, PR
  • Average UV index in September: 7

The months with the highest UV index are April, May, June, July, August, September, October and November (UV index 7).
The months with the lowest UV index are January, February, March and December (UV index 6).

Average cloud cover in September
San Juan, PR

Average cloud cover in September - San Juan, PR
  • Average cloud cover in September: 26

The month with the most cloud cover is December (Cloud cover 35).
The month with the least cloud cover is September (Cloud cover 26).

Average visibility in September
San Juan, PR

Average visibility in September - San Juan, PR
  • Average visibility in September: 6mi

The months with the highest visibility are January, February, March, April, May, June, July, August, September, October, November and December (6mi).
The months with the lowest visibility are January, February, March, April, May, June, July, August, September, October, November and December (6mi).

San Juan, Puerto Rico, USA

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