Weather Blog

Why does the radar show rain on a clear night?

Why does the radar show rain on a clear night?

It might be a real head-scratcher. It's crystal clear outside, the stars are out -- even clear enough to see a green comet making a pass by Earth.

So why on Earth is the radar showing rain Tuesday night?

The image above is our radar image from 12:04 a.m. Wednesday morning. Note local observations confirm clear skies at the time.

Sure looks like it is pouring rain outside. But it's obviously not. What's happening? The radar is going to the birds. Or maybe better put: The birds are going to the radar.

UW Atmospheric Sciences Professor Cliff Mass has written a few excellent stories on his blog detailing this mystery, but basically when it is clear outside, the National Weather Service runs the radar in a super sensitive mode to help better track wind patterns.

But the radar also picks up a lot of things in this sensitive mode -- including bird migration patterns. So most of what you are seeing is the radar detecting all the birds heading south for the winter. Note that this pattern occurs mainly from about sunset through just after midnight, then tends to ease toward dawn.

Mass figures the birds travel at night to avoid being sighted by predators, which makes sense.

But did you know when is the best time birds love to make the journey?

According to a story on, it turns out, the day after a cold front passes is the best time, with north winds, rising pressure, dropping temperatures and clearing skies bringing favorable flying conditions.

Read the entire story here

Speaking of mysterious green blobs...

Comet 103P/Hartley 2, also known as "The Green Comet" is making its closest encounter to Earth on Wednesday -- a mere 11 million miles away.

Thus, this is now the best time to catch a glimpse at the comet, fog permitting.

The peak viewing time is in the few hours just before dawn. Just go outside and look straight up. You'll find it near the bright star Capella, according to If you're having trouble finding it, here is a star map

And here is a video of what you are looking for, courtesy of Tom Gwilym of the Eastside Astronomy Society. He took this from his backyard in Renton:

It'll be hard to find with the naked eye -- especially near city lights. But if you have a set of binoculars, you should get a pretty decent view. And it's even better with a telescope.

Mainy clear skies should hold through Thursday morning, so pre-dawn Thursday is a solid bet for being cloud-free in the sky. However, October is fog season and there will be areas of fog around in those crucial pre-dawn hours. So you have to be a bit lucky. You can always try some of the higher elevations around here, but no guarantee.

If you do get good photos, please submit them to our YouNews service -- we'd love to see them! But we'll also take photos of birds too.