May 07, 2016

Rain Off the Coast in Clear Skies?

The weather radar Friday evening at 8 PM looked really wet, with moderate to heavy rain offshore.

But the strange thing is that the visible satellite imagery showed clear skies.  Rain without clouds?  Very strange.

It turns out that the origin of this wacky situation was due to anomalous propagation of the radar signal from the Langley Hill radar near Hoquiam.  The radar beam was doing strange things because there was a stable layer aloft that bent (or refracted) the radar beam down to the ocean surface, where it was reflected back to the radar.  Thus, we were seeing the ocean surface, not precipitation.

Here is the vertical sounding from the radiosonde at Quillayute at 5 PM Friday.  There was a region where the temperature (red line) did not decline much with altitude between roughly 900 and 800 hPa (roughly between 3000 and 6000 ft).  In the lower layer beneath, temperature decreased rapidly with height.  Such an elevated stable layer caused the radar beam to be bent downward.

This situation is called ducting and is illustrated in the figure below.

This ducting configuration was associated with easterly flow aloft (causing warming) surmounting a cooler northerly flow near the surface.


  1. Hey, thanks Cliff! I was wondering why. The Answer Man. Always a step ahead. Have a wonderful Derby Day. Or Boating Day. Or celebrating the "thisclose" to being independent day. What sweet relief to know the listeners have spoken!

  2. What happened on Friday - it was gorgeous in the Willamette Valley, but very gray and windy here in Bend, which is kind of the opposite of how things usually work. What causes that?


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