Autumn Has Arrived

Wednesday, September 24, 2014
10:07 pm

Good evening everybody, I just got back from my first day of school. I’m actually pretty exhausted, so this will be a quickie. Only nine more months to go…

We officially had our Autumnal Equinox here in Seattle at 7:29 pm on Monday, September 22. I’m sure you all think you know how seasons work, but take some time and think about it, and I’m sure your confidence will fall. 😉

The Earth takes 365.25 days to revolve around the sun. That’s why we have a leap year every four years [(365 + 365 + 365 + 366)/4 = 365.25]. The Earth is also tilted at a 23.5 degree angle as it rotates in a more-or-less circular orbit (not perfectly circular, actually ellipsoidal, with the Earth being closer to the sun in the Northern Hemisphere winter). If we assume that our ellipse is a circle and the rotational speed of the Earth around the sun is constant, then due to the axial tilt of the Earth, one hemisphere will be pointed more directly at the sun, and the other one will receive more of a glancing blow. Check out the visuals below, a picture’s worth a trillion words when trying to explain this stuff. Don’t worry about the dates of the equinoxes not matching in the two pictures below; they can vary from year to year.

You can see that on the Northern Hemisphere summer solstice (June 21), the Northern Hemisphere is pointed toward the sun, and that the North Pole remains in sunshine all day long as the earth spins about its axis. Conversely, on December 21, the winter solstice for the Northern Hemisphere, the Northern Hemisphere is tilted away from the sun, and the Southern Hemisphere is tilted towards it. It’s now Antarctica’s turn to experience the midnight sun.

These two events happen when the sun is over the Tropic of Cancer (23° 26′ N) and Tropic of Capricorn (23° 26′ S), respectively. Somehow, the sun’s gotta travel between these two places, so it crosses the equator twice a year doing so. These are the vernal (spring) and autumnal equinoxes. During these times, both hemispheres are equally pointed towards the sun, and the day is 12 hours long throughout the entire globe. Cool fact for math nerds: rate of change of day length varies sinusoidally throughout the year, with day lengths changing slowest at the solstices and fastest at the equinoxes.

The picture below shows the hours of daylight for different latitudes north of the equator and how they vary throughout the year. Note the sinusoidal shape, and the “midnight sun from June to mid-July at 70° N.

I love seasons, so if you have any more questions about seasons, throw ’em at me.

Weather-wise, the transition from fall to winter is much faster than winter to spring. In other words, we cool off faster than we warm up. And we sure got a early rainstorm to jump-start our autumn. It should be noted that “meteorological winter” here is actually the November-January period, even though geographical/astronomical winter goes through March, so it won’t be long before it feels like winter. After this record-breaking summer, doesn’t that sound a little abrupt? I sure think so.

Look at all the rain that fell from this past storm over the last 48 hours. The purples denote over two inches of rain. Sea-Tac got 1.54 inches. My weather station up on South Whidbey Island got 0.88 inches. And Sequim was dry.

Our winter is supposed to be warm and dry, but this is sure a way to kick off fall!
Charlie