It's Raining, But the Radar is Clear. Why?

Saturday afternoon before taking a pre-game run, I took a look at the radar--no echoes over Seattle.

But hitting the trail, it was obviously raining lightly and my sweatshirt got quite wet.

Here is a photo at the time I left, around 2:15 PM, you see the drops on the tree?

Now here is the radar.  No rain over north Seattle where I live.

How could this happen?

And during the big Seahawks game it was clearly drizzling or raining lightly, but the radar showed nothing.

So is something wrong with the weather radar?   The answer is no...  The origin of this discrepancy lies in the type of precipitation and the limitations of weather radar.

On Saturday the precipitation was light rain and drizzle, and these small droplets were descending out of shallow clouds.  The vertical sounding at Quillayute suggested the low-level moist layer was only about 5000 ft thick (the layer where temperature and dew point were the same up to about 850 hPa):

This shallow layer produces small droplets and small droplets are hard to see in normal weather radar.  Also the intensity was light, again providing fewer droplets and thus less of a target.  The droplets got bigger as they descended from the cloud towards the ground.

But even more important is the height of the radar beam.  The closest radar to Seattle is the one at Camano Island, about 70 km (45 miles) away.   As a radar beam leaves the antenna, it rises above the earth surface.  Thus, the farther away from the radar, the higher the beam becomes (see figure).

The lowest radar beam from the Camano Island radar scans at .5 degrees above the horizon (dark blue color aloft).   The beam both rises and expands as it travels away from the radar.  As shown by the figure, by the time the bean gets 45 miles away (over Seattle) the lower part of the beam is at least 2000 feet above the surface, with the center of the beam between 3000 and 4000 ft above the surface.
Thus, the radar beam would miss the lower (and larger) raindrops, the ones that would have better chance of reflecting the radar beam back to the receiver.

Thus, it is not surprising that the radar missed the low-level drizzle that damped the surface.

So why does normal rain show well in weather radar?  Because the clouds and precipitation extend far higher and the drops are larger.