The Day of No Weather and SuperBlue Skies

When I get up in the morning, one of the first things I do is check the latest satellite image.

Today's imagery (below) was stunning: not a single cloud in the sky

Nearly all my blogs are about some interesting weather feature...the more severe the better.  

But what to talk about on the most boring weather day in years?

When I got to the UW this morning, I took pictures of the sky ..the most perfect blue skies I could remember.

Viewed from the Seattle PanoCam this am, the light from the blue sky on Puget Sound gave the water a super-blue tint.

So why was the sky so clear?

The upper-level (500 hPa, about 18,000 ft) weather map at 11 AM  explaines a lot.   There was a strong ridge or high offshore, and such features have powerful downward motion on their eastern side...which prevents cloud formation.

Sinking air causes warming by compression and relative humidity to drop (since warm air can "hold" more water vapor than cooler air).  This prevents cloud formation.

The predicted relative humidity at around 10,000 ft (700 hPa pressure) was extraordinarily dry, with dark blue colors indicating relative humidities below 30%.

The solar radiation today was unimpeded, resulting in the variation over the day being almost a perfect cosine shape (the values at Snohomish are shown below).

Why is the sky blue on clear days?

Light from the sun has all wavelengths of visible light.   Atmospheric molecules scatter short wavelengths (like blue) more than longer wavelengths (like orange and red).  As shown in the image below, this preferential scattering of blue light makes the sky look blue.

Water droplets and particles (such as from wildfire smoke) scatter all wavelengths similarly, giving the sky a whitest cast.

As you can imagine, small particle concentrations were very low today, as shown by the PM2.5 numbers (concentrations of particles of 2.5 microns or less) around Seattle (below).  With clear skies, low relative humidity, and clean air, no wonder our skies were amazingly blue.

Enjoy the blue skies....clouds move in over the weekend.

Potential Aurora Tonight

 I have held off mentioning this because I did not want to "cry wolf", but there is an increasing chance that some of you might see some auroral activity tonight.

The latest guidance from the NOAA Space Weather Center (YES,  NOAA!!) indicates the critical index of potential auroral activity (Kp) has now reached 6, which I have found to be the minimum required for even a minor light show around here (see plot below).

The updated Kp forecast takes the index to nearly 7 between 8 and 11 PM tonight...see below.

The graphic from the Space Weather Center shows potential coverage reaching at least northern Washington.

The big question is clouds.   Right now, there are some high clouds overhead (see image around 6 PM), which would be fatal for good viewing, but the latest animation suggests the clouds will be through in 3 hours.

We have a chance!   I will certainly be going outside for a look around 10-11PM tonight.  Good aurora hunting!