September: A dramatic month of weather changes
By the numbers, September is one of the most interesting months of the year. It's not the snowiest or the coldest. It doesn't boast the most rain or even the most heat. In fact, it doesn't even see the greatest amount of temperature difference start to finish. But it is the month that takes us from summer to winter, in more ways than one.
Summer to Fall
For most, fall officially starts Sunday, Sept. 22 on the autumnal equinox. The season runs through the winter solstice in December, but for some — autumn starts a bit earlier. Meteorologists define the start of fall as Sept. 1.
Meteorologists use this date for a few reasons. It's easier to keep track of records when a season is defined by the first day of the month. The solstice and equinox change dates, making it more difficult to have consistent dates for record keeping.
By defining summer as the warmest three months of the year, winter as the coldest three, and spring and fall as the transitions between, the dates never change and the months correlate closely with each season.
September has the most daylight loss of the year. At an average of 5 minutes, 39 seconds of lost daylight each day, the month as a whole loses almost 2 hours, 45 minutes of light.
To put that in context, sunrise on Sept. 1 happens at 6:51 a.m., and the sun doesn't set until 9:06 p.m. Just 30 days later that same sunrise happens at 8:03 a.m., and the sun sets at a relatively early 7:34 p.m.
Temperature is a lagging indicator of daylight, meaning the warmest days of the year typically follow the days with the most daylight, and the coldest days of the year typically follow the days with the least amount of daylight.
By the time September rolls around, Anchorage is already down more than 5 hours of daylight since the solstice and still losing more than 5 minutes of daylight each day.
It’s no wonder then that temperatures start falling. At the beginning of the month, the average high temperature is a comfortable 60 degrees. As the month closes out a short four weeks later, that daily high drops to a blustery 49 degrees.
September is also home to the average first sub-freezing night. Just a few weeks into the month, Anchorage typically sees its first 32-degree night of the season.
On average the temperature drops to 32 degrees on Sept. 24 for the first time before settling in to the cold reality of the biting cold days to come.
Although September isn't the wettest month on record, it still brings its fair share of storms to the region. The month typically brings about a weather pattern that favors storms moving in from the Gulf of Alaska, dumping rain across the southern portion of the state.
With an average of 2.99 inches of rain during the month, September boasts more than 17% of the yearly rainfall in Anchorage.
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