Friday, October 18, 2013

What happens when the weather radar sees the sun?

Our weather radars are constantly rotating around, scanning the skies at various angles above the horizon.   What happens when the radar is directly aimed at the sun?   What does it see?  Weather on old sol?

The answer.   This does happen and what we see are intense triangular features known as sun spikes.

These are caused by the radar receiving microwave radiation emitted by the sun, not microwave radiation emitted by the radar, hitting the sun and returning.

Here is an example from this morning when the Langley Hill radar near Hoquiam caught the rising sun (image at 7:44 AM):
Pretty neat.  The other echoes you see are mainly ground clutter (return from terrain) and a few birds still in the sky.

Or how about last night near sunset?  Here is an example from the Portland radar around 6:19 PM:

Not surprisingly, it is facing the west-southwest.

These sun spikes are particularly noticeable now because there is no precipitation to cover it up.  And when there is no precipitation the radars are in a hyper-sensitive clear air mode.

Ah yes...the forecast.   The latest model runs suggest it will be dry through next Friday, with only a weak disturbance on Monday and Tuesday bringing a bit of middle/high clouds.   And, yes....there is a good chance of morning low clouds on several of these dry days.  This is the Faustian bargain meteorologist make with the weather gods--- we trade low clouds for no rain.   If you don't like this, you make the deals.

But one thing is sure...the mountains should be spectacular with lots of sun and warmer than normal temperatures.




10 comments:

Mike said...

Isn't calling these sun spikes "echos" misleading? The radar is not detecting a bounce of its signal off the sun's surface, it is detecting solar radiation, right?

franzamador said...

I assume this means that the radar is receiving microwave radiation generated by the sun, not that it's bouncing its own waves off the sun, which would take over 16 minutes.

Cliff Mass said...

Mike and franamador...good points and I have amended the blog to ensure the point is clear...cliff

Grant Wells said...

Hi,

I have another question please. why is the tempature in Packwood always the one the highest and lowest in western washington. today, 10/18, the temp shows as 73 and a low this morning of 29.
http://www.cookscountry.com/recipes/Chuck%ADRoast%ADin%ADFoil/27308?extcode=L3KN3AA00

Thanks

dbostrom said...

Reminds of days gone by when among other things I was responsible for network downlinks for a radio station. Each year we'd get a notice from our network advising of the few minutes we'd need to fill with local content, because our dishes ended up pointed directly into the sun. A satellite with the sun directly behind it just wasn't audible to the receiving equipment on the ground.

Great post, Cliff! Thanks.

Rod said...

Hi Cliff:

The election is close. Please re-run your thoughts on the Seattle School Board. Thanks.

smokejumper said...

I haven't been checking the weather very often because this must be what's its like to forecast weather in San Diego. Sunny and upper 60s. Could not ask for better weather during apple harvest. Everyone is having backyard parties and BBQs. Still applying the sunblock.

But I notice in the observations most people in the Seattle area, it's the upper 40s. My condolences. What a nuisance amidst what should be beautiful weather. Hey we'll get ours in January.

Upupaepops said...

drove over to Blewett Pass for the Larch. In the fog at the pass 830am 35 degrees

returning home at 3pmish 65 degrees

in Redmond all is blah and gray

head east my friends!

Rod said...

Perhaps the low clouds and fog might burn off by next July 15th, or so. Eh?

darly said...

I put together a simple animation of the solar spike on NEXRAD over the course of a year, I should improve it someday to not change color curves, etc.

http://www.youtube.com/watch?v=6882RAfmQlA