September weather forecast
Key West, FL

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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

Weather in September

September's weather in Key West, Florida, comes with mixed surprises. The month start signals the approach of the end of the wet, hot season, and people start to look forward to cooler weather. The later parts of the month feature the intense heat lessening gradually. This period, however, brings the highest recorded levels of precipitation, peaking at 5.12" (130mm), resulting in an interesting weather dynamic.

Temperature

The commencement of September notes an average high-temperature of a still warm 84.4°F (29.1°C), exhibiting little difference from the previous month. Registering an almost steady temperature, Key West's nights in September see an average low of 82.2°F (27.9°C).

Heat index

The heat index value during September is computed to be a hot 93.2°F (34°C). Implement additional safety measures to avoid heat cramps and heat exhaustion. Persistent activity might trigger heatstroke.
Note that heat index values are valued for light wind and location in the shade. A direct sunlight exposure may boost the heat index by 15 Fahrenheit (8 Celsius) degrees.
Note: The heat index, also known as 'real feel' or 'felt air temperature', blends air temperature and moisture content to represent the felt temperature to humans. This impact is subjective, influenced by the person's physical activity and individual heat perception, affected by factors including wind, attire, and metabolic differences. It is essential to realize that direct sunlight can escalate the perceived temperature, increasing the heat index by 15 Fahrenheit (8 Celsius) degrees. Heat index values are extremely significant for babies and toddlers. Children often fail to recognize the requirement to rest and rehydrate. Thirst, being a late symptom of dehydration, underlines the importance of maintaining hydration, especially during ongoing physical activities.

Humidity

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

Rainfall

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.

Ocean temperature

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

The average length of the day in September in Key West is 12h and 17min.
On the first day of the month, sunrise is at 7:08 am and sunset at 7:45 pm. On the last day of September, sunrise is at 7:18 am and sunset at 7:14 pm EDT.

Sunshine

In September, the average sunshine is 9h.

UV index

The months with the highest UV index are April through October, with an average maximum UV index of 7. A UV Index estimate of 6 to 7 represents a high health risk from unsafe exposure to UV radiation for the average person.
Note: During September, 7 as the daily maximum UV index converts into the following advice:
Adopt measures and apply sun safety strategies. Shielding from skin and eye damage is necessary. Avoid direct Sun exposure from 10 a.m. to 4 p.m., a period when UV radiation is most intense, and remember that not all shade structures provide full protection. Arm yourself against UV rays' harm with protective apparel and UV-filtering shades. Watch out! Sand and water enhance the UV exposure you receive.

Frequently asked questions

How hot is September?

The high-temperature, in September, in Key West, is 84.4°F (29.1°C), while the low-temperature averages to 82.2°F (27.9°C). In September, the average heat index (a.k.a. 'feels like', 'apparent temperature'), which represents the influence of relative humidity on air temperature, is computed to be 93.2°F (34°C).

What is the least humid month?

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

What is the average humidity in September?

The average relative humidity in September is 74%.

What is the wettest month?

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 many days does it rain in September?

In September, the rain falls for 23.1 days. Throughout September, 5.12" (130mm) of precipitation is accumulated.

How warm is the water in September?

In Key West, the average seawater temperature in September is 85.6°F (29.8°C). For swimming, diving, and other water activities, temperatures between 77°F (25°C) and 84.2°F (29°C) are perceived as very enjoyable and pleasurable for prolonged periods without feeling uncomfortable.

What is the average length of the day in September?

In Key West, the average day length in September is 12h and 17min.

When is the sunrise / sunset in September?

On the first day of September, sunrise is at 7:08 am and sunset at 7:45 pm. On the last day of the month, sunrise is at 7:18 am and sunset at 7:14 pm EDT.

How long on average sun shines in September?

The average sunshine duration in September is 9h.

When is the highest UV index?

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

What is the UV index reading in September?

The average daily maximum UV index in September is 7. A UV Index value of 6 to 7 symbolizes a high health risk from exposure to the Sun's UV radiation.

Average temperature in September
Key West, FL

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

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

Average wind speed in September
Key West, FL

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

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

Average rainfall in September
Key West, FL

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

The 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 - 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 / Average sunshine in September - Key West, FL
  • Average daylight in September: 12h and 2min

The month with the longest days is June (Average daylight: 13h and 36min).
The month with the shortest days is December (Average daylight: 10h and 36min).

  • Average sunshine in September: 9h and 0min

The month with the most sunshine is July (Average sunshine: 11h and 0min).
The month with the least sunshine is December (Average sunshine: 6h and 18min).

Average sunshine days in September
Key West, FL

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

The 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 - 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 - Key West, FL
  • 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).

Key West, Florida, USA

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