September weather forecast and climate
Key West, FL

A beach pier in Key West, Florida, with palm trees in the background
A beach pier in Key West, Florida, with palm trees in the background
September, the first month of the autumn, in Key West, is still a warm month, with temperature in the range of an average low of 82.2°F (27.9°C) and an average high of 84.4°F (29.1°C).

Temperature

In Key West, the average high-temperature in September is relatively the same as in August - a still warm 84.4°F (29.1°C). In Key West, Florida, in September, the average low-temperature is 82.2°F (27.9°C).

Heat index

The average heat index is evaluated at a tropical 92.7°F (33.7°C). Take extra precautions, as heat cramps and heat exhaustion are possible. Sustained activity may result in heatstroke.
Note that heat index values are valued for light wind and location in the shade. With the exposure to direct sunlight, the heat index may be increased by up to 15 Fahrenheit (8 Celsius) degrees.
Note: The heat index, also known as 'apparent temperature', 'felt air temperature', 'feels like', or 'real feel', is what the temperature feels like to the human body when the air temperature is combined with the relative humidity. Additional factors like clothing, metabolic differences, pregnancy, and activity influence one's weather impression. Be aware that direct exposure to sunlight increases heat impact, and may raise the heat index additionally, by up to 15 Fahrenheit (8 Celsius) degrees. Heat index values are particularly important for children. Children are often less aware of the need to rest and re-hydrate. Thirst is a late sign of dehydration - it is vital to remain hydrated, particularly during prolonged physical activities.

Humidity

July through September, with an average relative humidity of 74%, are the least humid months.

Rainfall

The month with the most rainfall in Key West is September when the rain falls for 23.1 days and typically aggregates up to 5.12" (130mm) of precipitation.

Ocean temperature

In Key West, the average seawater temperature is 85.6°F (29.8°C).
Note: 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.

Daylight

In Key West, the average length of the day is 12.3h.

Sunshine

In September, the average sunshine in Key West is 9h.

UV index

April through October, 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 risk from unprotected exposure to the Sun's UV rays for average individuals.
Note: The daily maximum UV index of 7 in September interpret into the following advice:
Take precautions against overexposure. Protection against sun damage is needed. Seek shade and minimize exposure to direct Sun in a period between 10 a.m. and 4 p.m. when UV radiation is most intense; keep in mind that shade structures like parasols or canopies do not provide perfect sun protection. On bright days sunglasses that block both UVA and UVB rays should be worn. A hat with a wide brim is extremely helpful, as it can prevent roughly 50% of UV radiation from reaching the eyes. Beware! The strength of the UV rays is significantly increased by the reflection of the water and sand.

What is the temperature in September in Key West?

In September, the average high-temperature is 84.4°F (29.1°C), and the average low-temperature is 82.2°F (27.9°C). The average heat index (a.k.a. 'feels like', 'apparent temperature'), that factores the actual air temperature with the relative humidity, in September is calculated to 92.7°F (33.7°C).

What is the least humid month in Key West?

July through September, with an average relative humidity of 74%, are the least humid months.

What is the average humidity in September in Key West?

The average relative humidity in September in Key West is 74%.

How much does it rain in Key West in September?

In Key West, Florida, in September, it is raining for 23.1 days, with typically 5.12" (130mm) of accumulated precipitation.

What is the wettest month in Key West?

The month with the most rainfall is September when the rain falls for 23.1 days and typically aggregates up to 5.12" (130mm) of precipitation.

How warm is the water in September in Key West?

In Key West, the average seawater temperature in September is 85.6°F (29.8°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 Key West have in September?

In Key West, Florida, the average length of the day in September is 12.3h.

How long on average sun shines in September in Key West?

The average sunshine in September is 9h.

What is the month with the highest UV index in Key West?

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

How high is the UV index in September in Key West?

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 the Sun's UV rays.
Average temperature in September - Key West, FL
  • Average high temperature in September: 84.4°F

The warmest month (with the highest average high temperature) is August (85.3°F).
The month with the lowest average high temperature is January (72.7°F).

  • Average low temperature in September: 82.2°F

The month with the highest average low temperature is August (82.9°F).
The coldest month (with the lowest average low temperature) is January (68.5°F).

Average pressure in September - Key West, FL
  • Average pressure in September: 29.93"

The month with the highest atmospheric pressure is January (30.09").
Months with the lowest atmospheric pressure are September and October (29.93").

Average wind speed in September - Key West, FL
  • Average wind speed in September: 10.3mph

The windiest months (with the highest average wind speed) are January and November (14.4mph).
The calmest month (with the lowest average wind speed) is July (9.5mph).

Average humidity in September - Key West, FL
  • Average humidity in September: 74%

The month with the highest relative humidity is February (78%).
Months with the lowest relative humidity are July, August and September (74%).

Average rainfall in September - Key West, FL
  • Average rainfall in September: 5.12"

The wettest month (with the highest rainfall) is September (5.12").
The driest month (with the least rainfall) is March (0.67").

Average rainfall days in September - Key West, FL
  • Average rainfall days in September: 23.1 days

Months with the highest number of rainy days are August and September (23.1 days).
The month with the least rainy days is February (11 days).

Average sea temperature in September - Key West, FL
  • Average sea temperature in September: 85.6°F

The best month for swimming (with the highest average sea temperature) is August (86.9°F).
The coldest months (with the lowest average sea temperature) are January and February (71.2°F).

Average daylight in September / Average sunshine in September - Key West, FL
  • Average daylight in September: 12.3h

The month with the longest days is June (Average daylight: 13.6h).
The month with the shortest days is December (Average daylight: 10.6h).

  • Average sunshine in September: 9h

The month with the most sunshine is July (Average sunshine: 11h).
The month with the least sunshine is December (Average sunshine: 6.3h).

Average sunshine days in September - Key West, FL
  • Average sunshine days in September: 4.5 days

The month with the most sunshine days is March (17 days).
The month with the least sunshine days is September (4.5 days).

Average UV index in September - Key West, FL
  • Average UV index in September: 7

Months with the highest UV index are April, May, June, July, August, September and October (UV index 7).
The month with the lowest UV index is January (UV index 5).

Average cloud cover in September - Key West, FL
  • Average cloud cover in September: 29

The month with the most cloud cover is January (Cloud cover 34).
The month with the least cloud cover is April (Cloud cover 23).

Average visibility in September - Key West, FL
  • Average visibility in September: 6mi

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