San Juan
Puerto Rico, USA
Long-term weather forecast
Long-term weather forecast

September weather forecast and climate
San Juan, PR

An old fort building and a view of the stunning San Juan city, Puerto Rico, with a clear view of the calm waterfront
An old fort building and a view of the stunning San Juan city, Puerto Rico, with a clear view of the calm waterfront
September, the first month of the autumn, in San Juan, is also a hot month, with temperature in the range of an average high of 89.2°F (31.8°C) and an average low of 77.7°F (25.4°C).

Temperature

August and September, with an average high-temperature of 89.2°F (31.8°C), are the warmest months.

Heat index

The average heat index is calculated to an extremely hot 108°F (42.2°C). Beware - heat cramps and heat exhaustion are very likely. Heatstroke is imminent with prolonged activity.
Note that heat index values are valued for light wind and location in the shade. Exposure to direct sunshine can increase heat index values by up to 15 Fahrenheit (8 Celsius) degrees.
Note: The heat index, also known as 'felt air temperature', 'feels like', 'real feel', or 'apparent temperature', combines air temperature and relative humidity to represent a human perceived temperature equivalent. This effect is individual, with diverse people perceiving weather differently for numerous reasons (wind, clothing, metabolic differences), and activity. Bear in mind that direct sunshine exposure increases weather impact, and may raise heat index by up to 15 Fahrenheit (8 Celsius) degrees. Heat index values are particularly important for children. Young children are generally more endangered than adults, as they usually less sweat. And also, due to larger skin surface relative to their small bodies and higher heat production as a result of their activity.
Perspiration is the human body's physiological response to high temperatures and is an attempt to lower body temperature through evaporation of sweat. A higher relative humidity affects normal body cooling by reducing the evaporation rate, subsequently lowering the rate at which the body cools and increasing the perception of heat. If the body can not disperse the excessive heat and body temperature reaches conditions of hyperthermia, above 99.5°F (37.5°C) to 100.9°F (38.3°C), thermal conditions are potentially life-threatening.

Humidity

The average relative humidity in San Juan is 76.4%.

Rainfall

In San Juan, it is raining for 17.4 days, with typically 7.4" (188mm) of accumulated precipitation. In San Juan, during the entire year, the rain falls for 198.5 days and collects up to 70.6" (1794mm) of precipitation.

Ocean temperature

With an average ocean temperature of 84.9°F (29.4°C), September is the month with the warmest seawater in San Juan.
Note: Water temperature between 77°F (25°C) and 84.2°F (29°C) is regarded as very warm and enjoyable by all. With these temperatures, water activities are pleasurable, without feeling uncomfortable for prolonged periods.

Daylight

In September, the average length of the day is 12.2h.

Sunshine

The average sunshine in September is 7.8h.

UV index

March through September, with an average maximum UV index of 12, are months with the highest UV index. A UV Index estimate of 11, and higher, represents an ultimate health vulnerability from the exposure to the Sun's UV radiation for the ordinary person.
Note: In September, the average maximum UV index of 12 translate into the following recommendations:
Take all required precautions, as unprotected skin and eyes can burn in minutes. The Sun's UV radiation is most powerful between 10 a.m. and 4 p.m. To the extent possible, limit exposure to the direct sun during these hours. Sun-protective clothing, a long-sleeved shirt and pants, a wide-brimmed hat, and UVA and UVB-blocking sunglasses are particularly helpful in blocking UV radiation's harmful effects. A broad-spectrum sunscreen of SPF 30+ should be liberally applied and re-applied every two hours, and immediately after an outdoor workout or a swim. Beware! The strength of the UV rays is significantly increased by the reflection of the water and sand.

What are the warmest months in San Juan?

Warmest months in San Juan are August and September, with an average high-temperature of 89.2°F (31.8°C).

What is the temperature in September in San Juan?

In September, the average high-temperature is 89.2°F (31.8°C), and the average low-temperature is 77.7°F (25.4°C). The average heat index (a.k.a. 'felt air temperature', 'real feel'), that factores the actual air temperature with the relative humidity, in September is estimated at 108°F (42.2°C).

What is the average humidity in September in San Juan?

The average relative humidity in September in San Juan is 76.4%.

How many days does it rain in September in San Juan?

In San Juan, Puerto Rico, during September, the rain falls for 17.4 days and regularly aggregates up to 7.4" (188mm) of precipitation.

What month is the ocean warmest in San Juan?

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

How warm is the water in September in San Juan?

In San Juan, Puerto Rico, the average seawater temperature in September is 84.9°F (29.4°C). Water temperature between 77°F (25°C) and 84.2°F (29°C) is regarded as very warm and enjoyable by all. With these temperatures, water activities are pleasurable, without feeling uncomfortable for prolonged periods.

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

In San Juan, the average length of the day in September is 12.2h.

How long on average sun shines in September in San Juan?

In San Juan, the average sunshine in September is 7.8h.

What is the month with the highest UV index in San Juan?

March through September, with an average maximum UV index of 12, are months with the highest UV index in San Juan, Puerto Rico.

How high is the UV index in September in San Juan?

The average daily maximum UV index in September in San Juan, Puerto Rico, is 12. A UV Index estimate of 11, and higher, represents an excessive health risk from unsafe exposure to the Sun's UV rays.
Average temperature in September - San Juan, PR
  • Average high temperature in September: 89.2°F

Warmest months (with the highest average high temperature) are August and September (89.2°F).
The month with the lowest average high temperature is January (83.2°F).

  • Average low temperature in September: 77.8°F

The month with the highest average low temperature is August (78.2°F).
Coldest months (with the lowest average low temperature) are January and February (72°F).

Average humidity in September - San Juan, PR
  • Average humidity in September: 76.4%

The month with the highest relative humidity is October (76.9%).
The month with the lowest relative humidity is March (71%).

Average rainfall in September - San Juan, PR
  • Average rainfall in September: 7.4"

The wettest month (with the highest rainfall) is August (7.7").
The driest month (with the lowest rainfall) is March (3.1").

Average rainfall days in September - San Juan, PR
  • Average rainfall days in September: 17.4 days

Months with the highest number of rainy days are November and December (19.6 days).
The month with the lowest number of rainy days is March (12.2 days).

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: 12.2h

The month with the longest days is June (Average daylight: 13.2h).
The month with shortest days is December (Average daylight: 11h).

  • Average sunshine in September: 7.8h

Months with most sunshine are March and July (Average sunshine: 9.1h).
The month with least sunshine is November (Average sunshine: 6.7h).

Average UV index in September - San Juan, PR
  • Average UV index in September: 12

Months with the highest UV index are March, April, May, June, July, August and September (UV index 12).
Months with the lowest UV index are January and December (UV index 8).