September weather forecastBoulder, CO
September
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
September's onset in Boulder brings a negligible change in high-temperatures, falling from a moderately hot 79.5°F (26.4°C) in August to a pleasant 72.3°F (22.4°C). September nights show a substantial disparity from the daytime peak with an average low-temperature of 47.8°F (8.8°C).Humidity
June, August and September, with an average relative humidity of 45%, are the least humid months.Rainfall
In Boulder, during September, the rain falls for 9.9 days and regularly aggregates up to 1.02" (26mm) of precipitation. Throughout the year, there are 117.3 rainfall days, and 14.17" (360mm) of precipitation is accumulated.Snowfall
In Boulder, snow does not fall in June through September.Daylight
The average length of the day in September 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 is 4. A UV Index reading of 3 to 5 represents a moderate threat to health from exposure to the Sun's UV rays for average individuals.Note: The maximum daily UV index of 4 during September translates into the following directions:
Safety measures are advised. Light-skinned individuals may experience burns in under half an hour. Midday is the peak time for the Sun's radiation; stay in the shade and protect yourself. Outfit yourself in sun-safe attire, like long sleeves, pants, a hat, and UV-blocking sunglasses. Guard yourself! Snow reflection can almost double the UV radiation from the Sun.
Published by: Weather U.S. | About Us
Data Sources | Weather Forecasting & Climate
Frequently asked questions
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: 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: 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: 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: 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: 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: 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: 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: 12h and 24min
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 54min
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: 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: 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: 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: 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).
