Weather Blog

Sun looms large (literally!) as it finally makes Seattle appearance

Sun looms large (literally!) as it finally makes Seattle appearance

It's January, and the sun is finally out after literally taking all of December off!

No really, it did. Did you know the greatest amount of sunshine we had on any day in December was still considered 70% cloud cover? According to Sea-Tac's official climate data, there were 4 days 70% cloudy, 3 days 80% cloudy, 13 days 90% cloudy and a whopping 11 days completely overcast.

But now that the sun is out, does it seem a little larger than usual? You might think it was just because we forgot what it looked like, but there is some truth to the notion that the sun is larger.

Don't worry, it's like this every January though.

The Earth's orbit around the sun isn't a perfect circle -- it's more of an ellipse. Around New Year's Day, the Earth is at its closest point from the sun -- called the "perihelion". This year, that occurred at 9 p.m. PDT on January 1, and at that moment, we were 3.1 million miles closer to the sun than we will be on "aphelion" -- the farthest point which will occur this summer on July 5 (the first day of Seattle's summer, right?)

The next question you might ask is -- does that mean our winters are warmer than those in the Southern Hemisphere, since we are closer to the sun than their winter in July?

The logic is sound, but it's not the case. The Earth does receive 7 percent more energy from the sun in January than July, but most of the Earth's land is concentrated in the Northern Hemisphere. Land does a much better job than ocean of absorbing and releasing the sun's energy -- i.e., ground heats up faster and more easily than water.

So even though we are getting more of the sun's energy now, in the summer we have more land in the Northern Hemisphere to heat up.  Add it all up, and Earth is about 4 degrees cooler in January than July, even though there's more sun energy shining on us.

(Note that the Earth's tilt on its axis is way more influential to the seasons and global temperature than the minor change in the distance from the sun.)

Another stumper: the Earth's orbit speed is slightly faster this time of year then when the Earth is farthest away from the sun. That means virtually speaking, our summer ends up being about 3 days longer than the Southern Hemisphere summer/Northern Hemisphere winter.

Of course, around here, you might be hard pressed to convince people that summer is three days longer than winter.

For more information, check out this great article from NASA. It's from 2001 but still rings true today, although the dates fluctuate a little each year.

And to find out exactly when it will be aphelion or perihelion, click here. Just remember that is on UTC time (the old GMT time) which is +8 from PST in the winter and +7 from PDT in the summer.

Other signs summer is coming... eventually

Aside from actually seeing the sun, there are a few other subtle clues that winter has begun its march to summer.

On Dec. 25, the average high temperature in Seatte bottomed out at 45.1 degrees F. On Dec. 26, it began its climb at 45.2 degrees and was up to 45.6 on Jan. 3.  It'll eventually top out at 77.4 degrees in early August before then making its gradual trek down again.

Also, the sun ries on Jan. 4. will be 7:57 a.m. after rising at 7:58 a.m. on Jan. 3. It marks the first date the sunrise has been earlier since summer. Note that sunset is at 4:31 p.m. which is 13 minutes later than its earliest sunset of 4:18 p.m. on Dec. 17.

Why are neither of these dates on the winter solstice of Dec. 21? It's related to that whole perihelion/aphelion thing -- the Earth's orbital speed around the sun varies a little during the year. It's called The Equation of Time.