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

September welcomes an average high-temperature of a still warm 84.9°F (29.4°C), not much different from the preceding month. San Juan records a slight downturn to an average a still warm 79.2°F (26.2°C) during the nights of September.

Heat index

The average heat index in September is computed to be a torrid 95°F (35°C). Ensure additional precautions, heat exhaustion and heat cramps may be expected. Heatstroke could occur from sustained activity.
The standards for the heat index are set for locations in shade and under light breezes. When exposed to the direct sun, the heat index may augment by 15 Fahrenheit (8 Celsius) degrees.
Note: The heat index, also known as 'apparent temperature' or 'real feel', reflects the blend of air temperature and relative humidity to give a temperature impression. The effect is personal, with varied weather perceptions among individuals due to differences in body mass, stature, and physical activity. It is noteworthy to mention that direct sunlight exposure can augment the felt temperature, leading to an increase in the heat index by 15 Fahrenheit (8 Celsius) degrees. Heat index values are essentially critical for children. Children are often less aware of the need to rest and re-hydrate. Thirst is a late sign of dehydration - it's vital to remain hydrated, particularly during prolonged physical activities.
The human body normally cools itself by perspiration. Excessive warmth is eliminated from the body by evaporation of sweat. An increase in relative humidity hinders the body's normal cooling function by reducing evaporation, thereby decreasing the rate of body cooling and enhancing the sense of warmth. Overheating risks increase with rising body temperatures, especially if heat release isn't efficient.

Humidity

In San Juan, the average relative humidity in September 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, there are 159.9 rainfall days, and 12.01" (305mm) of precipitation is accumulated.

Ocean temperature

September is the month with the warmest seawater, with an average ocean temperature of 84.9°F (29.4°C).
Note: Water activities in temperatures ranging from 77°F (25°C) to 84.2°F (29°C) are pleasurable, without feeling uncomfortable even during extended periods.

Daylight

In San Juan, the average length of the day in September is 12h and 14min.
On the first day of September, 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.

Sunshine

In San Juan, Puerto Rico, the average sunshine in September is 9.4h.

UV index

April through November, with an average maximum UV index of 7, are months with the highest UV index. A UV Index estimate of 6 to 7 represents a high health vulnerability from exposure to the Sun's UV rays for average individuals.
Note: The daily maximum UV index of 7 in September interprets into the following advice:
Sidestep overexposure. Individuals with fair complexions risk burning in fewer than 20 minutes. Limit your exposure to the sun's UV radiation, which is strongest between 10 a.m. and 4 p.m., and note that parasols or canopies may not provide full sun protection. Boost your sun resistance with garments that are densely woven yet spacious. Caution! The mirror effect of sand and water ups the UV radiation strength.
Updated: Jul 18, 2024
Published by: Weather U.S. | About Us
Data Sources | Weather Forecasting & Climate

Frequently asked questions

How hot is it in September?

An average high-temperature, in September, in San Juan, is 84.9°F (29.4°C) and the average low-temperature is 79.2°F (26.2°C). The average heat index (a.k.a. 'felt air temperature', 'apparent temperature'), which combines relative humidity and air temperature, in September is computed to be 95°F (35°C).

What is the average humidity in September?

In September, the average relative humidity is 78%.

How much does it rain in September?

In San Juan, in September, it is raining for 11.7 days, with typically 1.18" (30mm) of accumulated precipitation.

What is the month with the warmest ocean?

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

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.

What is the average length of the day in September?

In San Juan, the average day length in September is 12h and 14min.

When are sunrise and sunset in September?

On the first day of September, 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.

What is the UV index reading in September in San Juan?

In September, the average daily maximum UV index in San Juan is 7. A UV Index of 6 to 7 symbolizes a high hazard to health from unprotected exposure to UV radiation.

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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