This report describes the typical weather at the Juneau International Airport (Juneau, Alaska, United States) weather station over the course of an average September. It is based on the historical records from 1974 to 2012. Earlier records are either unavailable or unreliable.
Juneau, Alaska has a mild humid temperate climate with warm summers and no dry season. The area within 25 mi of this station is covered by tundra (37%), oceans and seas (34%), and forests (28%).
The month of September is characterized by falling daily high temperatures, with daily highs decreasing from 59°F to 51°F over the course of the month, exceeding 65°F or dropping below 48°F only one day in ten.
Over the course of September, the length of the day is very rapidly decreasing. From the start to the end of the month, the length of the day decreases by 2:26 hours, implying an average day-over-day decrease of 4.9 minutes.
The shortest day of the month is September 30 with 11:30 hours of daylight; the longest day is September 1 with 13:57 hours of daylight.
The earliest sunrise is at 5:59am on September 1; the latest sunset is at 7:56pm on September 1; the latest sunrise is at 7:02am on September 30; the earliest sunset is at 6:33pm on September 30.
Daylight saving time is observed in 2012 but it neither starts nor ends in September, so the entire month is in daylight time.
The median cloud cover is 100% (overcast) and does not vary substantially over the course of the month.
On September 15, a representative day, the sky is clear or mostly clear 12% of the time, partly cloudy 7% of the time, and mostly cloudy or overcast 81% of the time.
The probability that precipitation will be observed at this location varies throughout the month. Precipitation is most likely around September 30, occurring in 81% of days. Precipitation is least likely around September 1, occurring in 74% of days.
Throughout September, the most common forms of precipitation are light rain and moderate rain.
Light rain is the most severe precipitation observed during 58% of those days with precipitation. It is most likely around September 25, when it is observed during 46% of all days.
Moderate rain is the most severe precipitation observed during 35% of those days with precipitation. It is most likely around September 26, when it is observed during 28% of all days.
The likelihood of snow falling is highest around September 30, occurring in 3% of days.
Accumulation at this location in September is extremely unlikely. The chances of there being snow on the ground are highest around September 30, occurring 2% of the time.
The snow is typically at its deepest on September 29, with a median depth of 2.0"; the depth exceeds 2.7" only one year out of ten.
The relative humidity typically ranges from 71% (humid) to 98% (very humid) over the course of a typical September, rarely dropping below 48% (comfortable) and reaching as high as 100% (very humid).
The air is driest around September 7, at which time the relative humidity drops below 84% (humid) three days out of four; it is most humid around September 24, rising above 97% (very humid) three days out of four.
Dew point is often a better measure of how comfortable a person will find the weather than relative humidity because it more directly relates to whether perspiration will evaporate from the skin, thereby cooling the body. Lower dew points feel drier and higher dew points feel more humid.
Over the course of a typical September, the dew point typically varies from 40°F (dry) to 52°F (very comfortable) and is rarely below 32°F (dry) or above 55°F (very comfortable).
Over the course of September typical wind speeds vary from 0 mph to 15 mph (calm to moderate breeze), rarely exceeding 26 mph (strong breeze).
The highest average wind speed of 8 mph (gentle breeze) occurs around September 29, at which time the average daily maximum wind speed is 15 mph (moderate breeze).
The lowest average wind speed of 7 mph (light breeze) occurs around September 2, at which time the average daily maximum wind speed is 13 mph (moderate breeze).