Seattle's Darkest Day in Three Years

Here in Seattle it was extraordinarily dark yesterday.  The darkest day in years.

The total radiation on Tuesday on top of the roof of the atmospheric sciences building was .54 Megajoules (MJ) per meter squared (a joule is a unit of energy).

To give you some perspective, here is a table of the lowest daily radiation amounts since 2004 at the UW, provided by UW Research Meteorologist Mark Albright.  The date of lowest radiation is also noted.  As you can see, yesterday was the darkest since 2015, where the total dropped to .44 MJ per meter squared on December 7th.

  Year           Date        Amount of solar radiation

 2004  1229  0.64 MJ/m**2

 2005  1201  0.61

 2006  1214  0.39

 2007  1202  0.51

 2008  1225  0.79

 2009  1215  0.74

 2010  1223  0.57

 2011  1210  0.68

 2012  1219  0.48

 2013  1220  0.56

 2014  1210  0.55

 2015  1207  0.44

 2016  1219  0.68

 2017  1218  0.69

*2018  1211  0.54

The absolute worst was the obscene value of .39 on December 14, 2006.  On a day like that, you might as well stay in bed.

You will note that the darkest days are all in December, running from December 2 to December 29th.  

So why was yesterday so dark and why are December's the month of most feeble solar radiation reaching the surface?

Two things are happening.  First, a lot of deep clouds, which prevent solar radiation from getting to the surface, and second, this is the period of short days and low sun angle.

The visible satellite image at 2 PM Tuesday indicated a dense frontal cloud band right over western Washington (see below)

And the infrared image at the same time shows that the cloud tops were very high, and thus the clouds were deep.

To make it all worse, the sunrise/sunset table for Seattle shows that we are now experiencing the earliest sunset of the year (4:17 PM).  The day will continue to get shorter until December 21st.

But there is something very positive happening lately:  lots of snow has been falling in the Cascades, with all the ski areas operating this week, although not all the runs are open.    And today (Wednesday) should be a much brighter day, with some intermittent showers.

from Cliff Mass Weather and Climate Blog https://ift.tt/2SFw0cv