Earliest Sunset Of The Season Starts Today. Not Even The Winter Solstice Yet, Here's Why

The winter solstice in Vermont last year, on Decembe 21, 2018 was typically
dark and gloomy in Vermont. This is a view of the Winooski River
in Burlington, Vermont's Intervale. 

Today in Burlington, Vermont, the sun will set at 4:13 p.m.  The sun will set at that time each day through December 14.

These are the earliest sunsets of the year.

Which seems not to make complete sense.  The winter solstice doesnn't hit until  December 21 this year.  On that date in Burlington, the sun will set at 4:16 p.m., three minutes later than it will this afternoon.

So why is sunset coming three minutes later than it does today, when December 21 is supposedly the shortest day of the year? Why is this happening?

It's complicated, but I'll give it a shot here. This is not meteorology, my forte, but astronomy. But later in this post, I'll get into some weather too, since this is, um a weather and climate blog.

We regard a day as lasting 24 hours.  But that's not exactly true, depending upon the time of year. According to timeanddate.com:

"The length of a solar day, as this duration is called, is not exactly 24 hours long. It varies throughout the year because of the ellpitial shape of Earth's orbit and it's axial tilt. It is longer than 24 hours around the summer and winter solstices, and shorter than 24 hours aroud the spring and fall equinoxes.

"On most days, solar noon does not occur at the same time as noon on your watch. Around the solstices, solar noon occurs a few minutes later than the previous day. For example, on December 21, 2019, - the day of the Northern Hemisphere winter solstice - solar noon in New York will be at 11:53 a.m. On January 4, 2020 the day of the latest sunrise solar noon will take place six minutes later, at 12  noon.

As solar noons increasingly occur later, sunrises and sunsets also steadily occur later each day after the winter solstice. This is why a location's earliest sunset occurs before, and the latest sunrise occurs after the winter solstice."

This state of affairs is true throughout the northern hemisphere. In Burlington, Vermont, even as the sun starts to set later in the day around the winter solstice, sunrises keep occuring later.  The latest sunrises of the season in Burlington ar at 7:29 a.m. All those late sunrises come between December 28 and January 8, after the winter solstice.

Despite coming after the shortest day of the year, January is typically
colder than at the winter solstice. On the bright side, it's a bit
sunnier in January and February than it was in December. It was
 below zero on a sunny  late afternoon in Janaury, 2018 when I
took this photo of Mount Mansfield, Vermont. 

Now for the meteorology.

As most winter weary Vermonters know, the coldest weather of the winter usually doesn't happen around the solstice on December 21, but later in the winter when we get well into January.

If the northern hemisphere has the least sun on December 21, shouldn't that be be the coldest time of year?  

Um, no.

The atmosphere changes slowly with the seasons and there's a lag time of about four to six week until temperatures bottom out.  Of course you can get the worst cold wave of the year around Christmas, but usually it waits until January or early February.

Sunlight, though increasing, is still relatively puny going through January. Meanwhile, it takes the land and especially the oceans to release heat that was built up last summer.  The land and oceans are still releasing heat from months ago in late December.  With more of that heat gone by January and early February, it will be colder then. In general, anyway.

Usually, but not always, in mid to late February, you begin to notice winter's sting is beginning to fade. That's because by then, the increasing sun angle is beginning to overpower the heat being released from land and oceans. The northern hemisphere begins to soak in the sun's rays, and we begin the climb into spring.

By the way, the same kind of stuff happens in the summer. The latest sunset of the year is not on the summer solstice, and the worst heat of summer comes in July and early August.

One other thing you'll notice in Vermont.  November and December are made darker by incessant cloud cover.  It's the cloudiest time of the year. That's partly because the Great Lakes, lakes in Canada and parts of Hudson Bay aren't frozen yet.

Moisture from these bodies of water rises into the cold air and quickly condenses into clouds, which then move overhead in the Green Mountain State.

In January and February, it's a little sunnier. Sure, we still get a lot of clouds from frequent passing storms. But the bodies of water to our north and west are getting more and more ice covered by then.  There's less open water to release moisture ot the skies, which means less cloudiness to darken our weekend ski trips.