September weather forecast
Boulder, CO

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The old building of the University of Colorado in Boulder, Colorado
The old building of the University of Colorado in Boulder, Colorado

Weather in September

As Boulder transitions to fall in September, a significant decline in temperature begins. This month provides a respite from the intense summer heat while still providing plenty of sunshine. Snowfall makes a reappearance but only in trace amounts, setting the stage for the colder months ahead. The progression into October will see further drops in temperature and increased precipitation. The advent of fall in September thus offers a perceptible pause from the summer robustness.

Temperature

The transition into September involves a small decrease in the average high-temperature, dipping slightly from a warm 79.5°F (26.4°C) in August to a pleasant 72.3°F (22.4°C). In Boulder, the September nights descend to a significantly cooler average of 47.8°F (8.8°C) compared to daytime highs.

Humidity

June, August and September, with an average relative humidity of 45%, are the least humid months in Boulder.

Rainfall

In Boulder, in September, during 9.9 rainfall days, 1.02" (26mm) of precipitation is typically accumulated. In Boulder, during the entire year, the rain falls for 117.3 days and collects up to 14.17" (360mm) of precipitation.

Snowfall

June through September are months without snowfall in Boulder.

Daylight

In September, the average length of the day is 12h and 26min.
On the first day of the month, sunrise is at 6:29 am and sunset at 7:32 pm. On the last day of September, sunrise is at 6:57 am and sunset at 6:44 pm MDT.

Sunshine

The average sunshine in September is 10.9h.

UV index

In September, the average daily maximum UV index in Boulder is 4. A UV Index value of 3 to 5 symbolizes a moderate health risk from exposure to the Sun's UV radiation for the average person.
Note: In September, the maximum UV index of 4 suggests these recommendations:
Be cautious. Those with fair skin can be subjected to burns in fewer than 30 minutes. Limiting sun exposure during midday hours helps in reducing the harmful effects of UV radiation. Safeguard your eyes, ears, and face from sun rays with the aid of a wide-brim hat. Be mindful! The reflection from snow can increase the Sun's UV radiation nearly twice.
Updated: Feb 10, 2024
Published by: Weather U.S. | About Us
Data Sources | Weather Forecasting & Climate

Frequently asked questions

What are September temperatures in Boulder?

In September, the average high-temperature and low-temperature in Boulder, Colorado, are 72.3°F (22.4°C) and 47.8°F (8.8°C) respectively.

What is the least humid month in Boulder?

The months with the lowest humidity are June, August and September, with an average relative humidity of 45%.

What is the average humidity in September in Boulder?

In Boulder, Colorado, the average relative humidity in September is 45%.

How many days does it rain in September in Boulder?

In Boulder, Colorado, in September, it is raining for 9.9 days, with typically 1.02" (26mm) of accumulated precipitation.

When it does not snow in Boulder?

In Boulder, snow does not fall in June through September.

Does it snow in Boulder in September?

In Boulder, in September, it doesn't snow. In October, it regularly snows.

What is the average length of the day in September in Boulder?

The average day length in September is 12h and 26min.

When are sunrise and sunset in September in Boulder?

In September, on the first day of the month, sunrise is at 6:29 am and sunset at 7:32 pm. On the last day of the month, in Boulder, Colorado, sunrise is at 6:57 am and sunset at 6:44 pm MDT.

How long on average sun shines in September in Boulder?

The average sunshine duration in September is 10.9h.

How high is the UV index in September in Boulder?

The average daily maximum UV index in September is 4. A UV Index estimate of 3 to 5 represents a medium hazard to health from unsafe 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
Boulder, CO

Average temperature in September - Boulder, CO
  • Average high temperature in September: 72.3°F

The warmest month (with the highest average high temperature) is July (80.4°F).
The month with the lowest average high temperature is December (35.2°F).

  • Average low temperature in September: 47.8°F

The month with the highest average low temperature is July (55.2°F).
The coldest month (with the lowest average low temperature) is February (18°F).

Average pressure in September
Boulder, CO

Average pressure in September - Boulder, CO
  • Average pressure in September: 30.01"Hg

The month with the highest atmospheric pressure is January (30.15"Hg).
The months with the lowest atmospheric pressure are April, May and June (29.92"Hg).

Average wind speed in September
Boulder, CO

Average wind speed in September - Boulder, CO
  • Average wind speed in September: 5.7mph

The windiest month (with the highest average wind speed) is January (7.3mph).
The calmest months (with the lowest average wind speed) are May, July, August and September (5.7mph).

Average humidity in September
Boulder, CO

Average humidity in September - Boulder, CO
  • Average humidity in September: 45%

The month with the highest relative humidity is February (70%).
The months with the lowest relative humidity are June, August and September (45%).

Average rainfall in September
Boulder, CO

Average rainfall in September - Boulder, CO
  • Average rainfall in September: 1.02"

The wettest month (with the highest rainfall) is May (2.48").
The driest month (with the least rainfall) is January (0.39").

Average rainfall days in September
Boulder, CO

Average rainfall days in September - Boulder, CO
  • Average rainfall days in September: 9.9 days

The month with the highest number of rainy days is July (17.8 days).
The month with the least rainy days is November (3.3 days).

Average snowfall in September
Boulder, CO

Average snowfall in September - Boulder, CO
  • Average snowfall in September: 0"

The month with the highest snowfall is April (7.76").
The months with the least snowfall are June, July, August and September (0").

Average snowfall days in September
Boulder, CO

Average snowfall days in September - Boulder, CO
  • Average snowfall days in September: 0.1 days

The month with the highest number of snowfall days is February (10.3 days).
The months with the least snowfall days are June, July and August (0 days).

Average daylight in September /
Average sunshine in September
Boulder, CO

Average daylight in September / Average sunshine in September - Boulder, CO
  • Average daylight in September: 12h and 2min

The month with the longest days is June (Average daylight: 15h and 0min).
The month with the shortest days is December (Average daylight: 9h and 24min).

  • Average sunshine in September: 10h and 5min

The months with the most sunshine are June and July (Average sunshine: 11h and 42min).
The month with the least sunshine is February (Average sunshine: 6h and 36min).

Average sunshine days in September
Boulder, CO

Average sunshine days in September - Boulder, CO
  • Average sunshine days in September: 17.6 days

The month with the most sunshine days is January (22.2 days).
The month with the least sunshine days is July (9.9 days).

Average UV index in September
Boulder, CO

Average UV index in September - Boulder, CO
  • Average UV index in September: 4

The months with the highest UV index are June, July and August (UV index 5).
The months with the lowest UV index are January, February, November and December (UV index 2).

Average cloud cover in September
Boulder, CO

Average cloud cover in September - Boulder, CO
  • Average cloud cover in September: 16

The months with the most cloud cover are April and May (Cloud cover 28).
The month with the least cloud cover is September (Cloud cover 16).

Average visibility in September
Boulder, CO

Average visibility in September - Boulder, CO
  • 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).

Boulder, Colorado, USA

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