April 29, 2009

There's a bird in my radar!

Weather radar is a great tool for tracking areas of precipitation...but it can see much more. Sometimes it sees birds. In fact, birds are great radar targets, since the amount of radar signal scattered back increases by the sixth power of the diameter of the target...and birds are much bigger than raindrops!

Let me show you an example (see below). Lets go back to April 24th at 8:02 PM (0302 UTC or GMT). The radar is showing little of anything, even though it is in the supersensitive "clear air" mode. A half hour later there is a noticeable increase in the amount of echo (0331 UTC, 8:32 PM PDT), and by 9 PM echoes have really spread and strengthened. This intensification and extension of the radar echoes continued for the next hour.

So did a rainshower move in? Or did a front with rain make landfall? Nope.

The surface observing stations reported dry conditions and generally cloud free skies. It was birds! But why then?

Sunset on that day was 8:12 PM and that is key information. Songbirds like to fly at night (perhaps there are less predators then) and just after sunset they hit the skies for their migration north. And we see this pattern night after night in the spring...and night after night in the fall.

But wait a minute..this is a Doppler radar...we can tell which way they are flying! The Doppler velocity image is shown below for 11 PM. Greens indicate incoming (from the south) and yellows are outgoing (to the north). Thus, with greens south of the radar site (Camano Island) and yellows to the north of the radar, our birds are moving to the north...which makes sense in the spring.

Ornithologists use weather radars for monitoring bird migrations...and now so can you! Imagine when we get a radar on the coast...we will then be able to track the great migrations through Grays Harbor and vicinity. So write or email your Senator and congressman...we need that coastal radar!

Doppler radar below:



    And the colors of the imagery are pretty cool too!

  2. So Cool!! My wife is not much interested in weather science but when I showed her the graphical images of the songbirds migrating, she was truly impressed.
    We took a walk tonight and enjoyed the songs of the woodland birds as the sun began to set. Where we are near Vaughn Bay on the lower Sound, we have a relative abundance of songbirds. However, due to too many outdoor cats that prey on the ground nesters and other vulnerable birds, I'm afraid it is getting to be fewer- like around the cities. But seeing the echoes on the radar of so many birds passing through sure is a sight to behold!
    Thanks for another lesson in the value of the radar technology.

  3. Thank you, very interesting Cliff.

    So one uses the radar in conjunction with surface observations (e.g. airports reporting no precip) to determine that these are, in fact, birds? Per your earlier discussion of the radar, the birds would need to be at some minimum altitude. Anyone know how high that would be, on average?

  4. This set of images is great. Seattle Audubon has had discussion about whether there are bird strikes to buildings in Seattle the way there are "back east." The contention has been that we don't have the waves of migrating birds that also occur back east. These types of images could refute that if the returns are not from masses of starlings, for instance, moving from one place to another. We need to identify what the flocks consist of and hope its not raining!

  5. This is why I am so glad that you're blogging! I had no idea that there was a silent migration every night, witnessed only by meteorologists. Incredible!

  6. A couple of years ago I read a book called "The Big Year" which is when a birder tries to see all 400+ species of birds in North America in one year. That book mentioned that when they put radar in Texas they noticed the reflectivity of the migration of birds across the gulf. Hummingbirds of all things make that flight in of course one long push. They lose 1/3 of their body mass in that flight then land and rest in the trees along that coastline. So if you want to see a lot of species in a short amount of time and a small place, birders hang out in Brown's Texas in the early spring awaiting the annual migration.


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