Contents
- Weather in September
- Frequently asked questions
- Average temperature in September
- Average pressure in September
- Average wind speed in September
- Average humidity in September
- Average rainfall in September
- Average rainfall days in September
- Average snowfall in September
- Average snowfall days in September
- Average daylight in September
- Average sunshine in September
- Average sunshine days in September
- Average UV index in September
- Average cloud cover in September
- Average visibility in September
Weather in September
Temperature
Mercer Island's transition into September involves a gentle shift in temperature, receding from a moderately hot 76.6°F (24.8°C) in August to an agreeable 70°F (21.1°C). In September, Mercer Island's temperature tends to fall to an average low of 50°F (10°C) during the night.Humidity
In Mercer Island, the average relative humidity in September is 74%.Rainfall
In Mercer Island, in September, it is raining for 7.5 days, with typically 0.63" (16mm) of accumulated precipitation. In Mercer Island, during the entire year, the rain falls for 119.1 days and collects up to 11.97" (304mm) of precipitation.Snowfall
April through October are months without snowfall.Daylight
In September, the average length of the day in Mercer Island is 12h and 33min.On the first day of September in Mercer Island, sunrise is at 6:28 am and sunset at 7:48 pm. On the last day of the month, sunrise is at 7:07 am and sunset at 6:49 pm PDT.
Sunshine
In September, the average sunshine is 9.3h.UV index
In September, the average daily maximum UV index is 4. A UV Index reading of 3 to 5 represents a moderate health risk from unprotected exposure to Sun's UV rays for the average person.Note: The maximum UV index of 4 during September leads to these instructions:
Be mindful. Individuals with a lighter skin tone may suffer burns in less than 30 minutes. The Sun is at its harshest during midday; protect yourself by staying in shaded areas and minimizing exposure. Wearing sunglasses with both UVA and UVB protection effectively reduces sun-induced eye damage.
Published by: Weather U.S. | About Us
Data Sources | Weather Forecasting & Climate
Frequently asked questions
What is the usual temperature for Mercer Island in September?
What is the average humidity in September in Mercer Island?
How many days does it rain in September in Mercer Island?
When it does not snow in Mercer Island?
What is the average length of the day in September in Mercer Island?
When are sunrise and sunset in September in Mercer Island?
How long on average sun shines in September in Mercer Island?
What is the UV index intensity in September in Mercer Island?
Average temperature in September
Mercer Island, WA
- Average high temperature in September: 70°F
The warmest month (with the highest average high temperature) is August (76.6°F).
The month with the lowest average high temperature is December (42.8°F).
- Average low temperature in September: 50°F
The month with the highest average low temperature is August (53.4°F).
The coldest month (with the lowest average low temperature) is February (34.5°F).
Average pressure in September
Mercer Island, WA
- Average pressure in September: 30.01"Hg
The month with the highest atmospheric pressure is January (30.1"Hg).
The month with the lowest atmospheric pressure is August (30"Hg).
Average wind speed in September
Mercer Island, WA
- Average wind speed in September: 3.7mph
The windiest month (with the highest average wind speed) is March (5.1mph).
The calmest month (with the lowest average wind speed) is August (3.4mph).
Average humidity in September
Mercer Island, WA
- Average humidity in September: 74%
The month with the highest relative humidity is January (85%).
The month with the lowest relative humidity is July (69%).
Average rainfall in September
Mercer Island, WA
- Average rainfall in September: 0.63"
The wettest month (with the highest rainfall) is December (2.01").
The driest month (with the least rainfall) is July (0.24").
Average rainfall days in September
Mercer Island, WA
- Average rainfall days in September: 7.5 days
The month with the highest number of rainy days is March (13.7 days).
The month with the least rainy days is August (4.1 days).
Average snowfall in September
Mercer Island, WA
- Average snowfall in September: 0"
The month with the highest snowfall is February (1.77").
The months with the least snowfall are April, May, June, July, August, September and October (0").
Average snowfall days in September
Mercer Island, WA
- Average snowfall days in September: 0 days
The month with the highest number of snowfall days is February (3 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
Mercer Island, WA
- Average daylight in September: 12h and 3min
The month with the longest days is June (Average daylight: 15h and 54min).
The month with the shortest days is December (Average daylight: 8h and 30min).
- Average sunshine in September: 9h and 2min
The month with the most sunshine is August (Average sunshine: 10h and 42min).
The month with the least sunshine is December (Average sunshine: 4h and 12min).
Average sunshine days in September
Mercer Island, WA
- Average sunshine days in September: 20.5 days
The month with the most sunshine days is August (24.9 days).
The month with the least sunshine days is February (13.8 days).
Average UV index in September
Mercer Island, WA
- Average UV index in September: 4
The month with the highest UV index is July (UV index 6).
The months with the lowest UV index are January, February, November and December (UV index 2).
Average cloud cover in September
Mercer Island, WA
- Average cloud cover in September: 34
The month with the most cloud cover is December (Cloud cover 56).
The month with the least cloud cover is August (Cloud cover 25).
Average visibility in September
Mercer Island, WA
- Average visibility in September: 6mi
The months with the highest visibility are May, June, July, August and September (6mi).
The months with the lowest visibility are January, February, March, April, October, November and December (5mi).
Mercer Island, Washington, USA
Weather forecast for your location
2. Allow the app to use your location