September weather forecast
São Luís, Brazil

With mere 23mm (0.91") of rain, September mirrors August's patterns closely and continues to assert São Luís's dry season. There are marginal fluctuations in temperature compared to August, highlighting the ongoing cooler phase. Despite the receding daylight hours, shrinking to 12.1, the UV index elevates back to 12, as in the city's summer months. September maintains the qualitative features of São Luís's dry season with reduced rainfall and cooler temperatures.

Temperature

The dawn of September sees an average high-temperature of a still hot 31°C (87.8°F), subtly contrasting with August's 30.7°C (87.3°F). São Luís records an average nighttime low-temperature of 23.5°C (74.3°F) throughout September.

Heat index

For most parts of September, the heat index is appraised at a scorching 41°C (105.8°F). Additional precautions are required to avert heat cramps and heat exhaustion. Persistent activity may culminate in heatstroke.
Heat index evaluations are typically in the context of light winds and shaded environments. A rise of up to 15 Fahrenheit (8 Celsius) degrees in the heat index could result from direct sunlight.
Note: The heat index, also known as 'real feel' or 'feels like', is a calculation that merges air temperature and relative humidity to depict the sensation of warmth. The impact of weather is individual, with diverse individuals experiencing it differently due to variations in body mass, height, and activity levels. Recognize that direct exposure to the sun can escalate the heat effect, potentially adding 15 Fahrenheit (8 Celsius) degrees to the heat index. Heat index values are particularly meaningful for babies and toddlers. Younglings frequently fail to recognize the requirement to rest and rehydrate. Thirst emerges late in dehydration - it is imperative to keep hydration levels high, particularly during extended physical exertions.
The physiological way the human body responds to excessive heat is by inducing perspiration, using sweat evaporation as a cooling mechanism. Under high air temperature and high humidity (high heat index) conditions, the body's ability to perspire is compromised, increasing the sensation of heat. When body temperature rises due to an imbalance in heat gain, there's a potential for dehydration and overheating.

Humidity

In São Luís, the average relative humidity in September is 81%.

Rainfall

In São Luís, in September, during 2 rainfall days, 23mm (0.91") of precipitation is typically accumulated. Throughout the year, in São Luís, there are 130 rainfall days, and 2290mm (90.16") of precipitation is accumulated.

Sea temperature

September is the month with the coldest seawater in São Luís, Brazil, with an average sea temperature of 27.3°C (81.1°F).
Note: Swimming, diving, and other water-related activities in temperatures between 25°C (77°F) and 29°C (84.2°F) are considered highly enjoyable and satisfying for longer durations without uneasiness.

Daylight

In September, the average length of the day is 12h and 6min.
On the first day of September in São Luís, Brazil, sunrise is at 05:54 and sunset at 17:58. On the last day of the month, sunrise is at 05:42 and sunset at 17:50 -03.

Sunshine

The average sunshine in September in São Luís is 8.3h.

UV index

January through April, September through December, with an average maximum UV index of 12, are months with the highest UV index in São Luís. A UV Index reading of 11, and higher, represents an ultimate health vulnerability from unsafe exposure to UV radiation for the ordinary person.
Note: The maximum daily UV index, 12 in September, converts into the following recommendations:
Undertake all critical precautions. Protection from sun harm is vital. Try to minimize direct Sun exposure between 10 a.m. and 4 p.m., the peak period for UV radiation. Also note that objects like parasols or canopies might not provide complete sun protection. A wide-brimmed hat provides unparalleled sun protection for sensitive areas like the eyes, ears, and neck. Sunscreen with an SPF of 30 or higher should be applied generously and frequently reapplied even on cloudy days, especially after each swim or outdoor workout. Be cautious! Sand and water surfaces make the Sun's UV rays more potent.