September weather forecast
O'Fallon, MO

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Weather in September

As we venture into September, a decline in temperature clearly signifies the arrival of the autumn season. Even though the weather cools down considerably from August, the midday sun offers a warm respite, making it pleasant for outdoor activities. This month sees a reduction in rainfall amounting to 1.42" (36mm), bringing drier days than the wet August. The UV index drops to 5, which means sunshine is less harsh but protective measures are still recommended. It also rings the last bell for snow-free period before the arrival of the late autumn chill.

Temperature

As September arrives, O'Fallon's weather displays a minor decrease in high-temperatures, transitioning from August's warm 85.5°F (29.7°C) to a still moderately hot 79.5°F (26.4°C). O'Fallon usually reports an average low-temperature of 60.1°F (15.6°C) during the month of September.

Humidity

In O'Fallon, the average relative humidity in September is 73%.

Rainfall

In O'Fallon, in September, during 11.3 rainfall days, 1.42" (36mm) of precipitation is typically accumulated. In O'Fallon, Missouri, during the entire year, the rain falls for 132.3 days and collects up to 21.06" (535mm) of precipitation.

Snowfall

In O'Fallon, snow does not fall in April through October.

Daylight

In O'Fallon, the average length of the day in September is 12h and 26min.
On the first day of the month, sunrise is at 6:32 am and sunset at 7:32 pm. On the last day of September, sunrise is at 6:58 am and sunset at 6:46 pm CDT.

Sunshine

In September, the average sunshine is 9.5h.

UV index

The average daily maximum UV index in September is 5. A UV Index reading of 3 to 5 represents a medium health hazard from exposure to the Sun's UV radiation for average individuals.
Note: In September, the maximum UV index of 5 suggests this advice:
Be mindful. Individuals with a lighter skin tone may suffer burns in less than 30 minutes. Minimize direct exposure to the Sun during midday when its radiation is at its peak. Clothes that are both tight-knit and comfortably loose are prime choices for sun protection.

Frequently asked questions

What is the usual temperature for O'Fallon in September?

An average high-temperature, in September, in O'Fallon, is 79.5°F (26.4°C) and the average low-temperature is 60.1°F (15.6°C). In August, the average heat index (a.k.a. 'apparent temperature', 'felt air temperature'), which combines relative humidity and air temperature, is appraised at 95°F (35°C).

What is the average humidity in September?

In September, the average relative humidity is 73%.

How much does it rain in September?

In O'Fallon, in September, it is raining for 11.3 days, with typically 1.42" (36mm) of accumulated precipitation.

When it does not snow?

April through October are months without snowfall.

What is the average length of the day in September in O'Fallon?

In O'Fallon, the average day length in September is 12h and 26min.

When are sunrise and sunset in September?

On the first day of September, sunrise is at 6:32 am and sunset at 7:32 pm. On the last day of the month, sunrise is at 6:58 am and sunset at 6:46 pm CDT.

How long on average sun shines in September?

In September, the average sunshine duration is 9.5h.

What is the UV index reading in September?

The average daily maximum UV index in September is 5. A UV Index value of 3 to 5 symbolizes a medium hazard to health from exposure to Sun's UV rays.
Average humidity
Average rainfall days
Average snowfall
Average snowfall days
Average daylight
Average sunshine
Average sunshine days
Average UV index
Average cloud cover
Average visibility

Average temperature in September
O'Fallon, MO

Average temperature in September - O
  • Average high temperature in September: 79.5°F

The warmest month (with the highest average high temperature) is July (87.1°F).
The month with the lowest average high temperature is January (37.4°F).

  • Average low temperature in September: 60.1°F

The month with the highest average low temperature is July (68.5°F).
The coldest month (with the lowest average low temperature) is January (23.7°F).

Average pressure in September
O'Fallon, MO

Average pressure in September - O
  • Average pressure in September: 30.02"Hg

The month with the highest atmospheric pressure is January (30.16"Hg).
The month with the lowest atmospheric pressure is June (29.92"Hg).

Average wind speed in September
O'Fallon, MO

Average wind speed in September - O
  • Average wind speed in September: 6.1mph

The windiest month (with the highest average wind speed) is January (9.1mph).
The calmest month (with the lowest average wind speed) is July (5.5mph).

Average humidity in September
O'Fallon, MO

Average humidity in September - O
  • Average humidity in September: 73%

The month with the highest relative humidity is May (79%).
The month with the lowest relative humidity is October (70%).

Average rainfall in September
O'Fallon, MO

Average rainfall in September - O
  • Average rainfall in September: 1.42"

The wettest month (with the highest rainfall) is May (2.76").
The driest month (with the least rainfall) is December (0.91").

Average rainfall days in September
O'Fallon, MO

Average rainfall days in September - O
  • Average rainfall days in September: 11.3 days

The month with the highest number of rainy days is May (15.7 days).
The month with the least rainy days is February (6.8 days).

Average snowfall in September
O'Fallon, MO

Average snowfall in September - O
  • Average snowfall in September: 0"

The month with the highest snowfall is February (1.61").
The months with the least snowfall are April, May, June, July, August, September and October (0").

Average snowfall days in September
O'Fallon, MO

Average snowfall days in September - O
  • Average snowfall days in September: 0 days

The months with the highest number of snowfall days are January and February (5.8 days).
The months with the least snowfall days are May, June, July, August, September and October (0 days).

Average daylight in September /
Average sunshine in September
O'Fallon, MO

Average daylight in September / Average sunshine in September - O
  • Average daylight in September: 12h and 2min

The month with the longest days is June (Average daylight: 14h and 48min).
The month with the shortest days is December (Average daylight: 9h and 30min).

  • Average sunshine in September: 9h and 3min

The month with the most sunshine is June (Average sunshine: 10h and 30min).
The months with the least sunshine are January and February (Average sunshine: 4h and 42min).

Average sunshine days in September
O'Fallon, MO

Average sunshine days in September - O
  • Average sunshine days in September: 16.7 days

The month with the most sunshine days is December (20.6 days).
The month with the least sunshine days is July (12.6 days).

Average UV index in September
O'Fallon, MO

Average UV index in September - O
  • Average UV index in September: 5

The months with the highest UV index are May, June, July and August (UV index 6).
The months with the lowest UV index are January, February, November and December (UV index 2).

Average cloud cover in September
O'Fallon, MO

Average cloud cover in September - O
  • Average cloud cover in September: 28

The month with the most cloud cover is January (Cloud cover 50).
The months with the least cloud cover are June, July, August and September (Cloud cover 28).

Average visibility in September
O'Fallon, MO

Average visibility in September - O
  • 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).

O'Fallon, Missouri, USA

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