During the past few weeks, several of you have asked essentially the same question:
" It's been completely dry but the weather radar last night showed lots of echos, suggesting it rained all night. What is going on ?"
Well, I can give the you the answer: Birds!
Weather radar can see more than raindrops. It can see the mountains, but that signal is generally removed successfully (terrain clutter) since the mountains generally don't move. Weather radar can see other objects in the air and the amount of return generally increases by the sixth power of the object's diameter. (Doubling the size of an object increases the amount of the radar signal scattered back increases by 64 times). Since a bird is much bigger than a raindrop, you can imagine that it would provide a good return.
Another point. The weather radar used by the National Weather Service has two modes: clear-air and precipitation. Clear air mode is much more sensitive and is used when it is not precipitating to get some information on the winds. In this mode, discontinuities in the atmosphere (e.g., where density changes rapidly) and bugs (which get blown about by the wind) can show on the radar (within tens of km of the radar site) to provide some useful information.
But now we get to the birds. During dry periods the radar is on the hypersensitive clear air mode and during the night (particularly during migration periods in spring and fall) a whole lot of birds are up there. According to my birder friends and a few articles I have read on the subject, a number of birds (like songbirds) like to migrate at night, typically flying into the bottom 10,000 feet of the atmosphere. This time of the year they are flying north and in the autumn to the south. The amazing thing is this migration is really tied to the clock...after sunset, the radar is filled with echo, remains all night, and like magic disappears after sunrise. (Keep these numbers in mind: on May 15th, sunset was at 8:40 PM (3:40 UTC/GMT), sunrise was at 5:32 AM (12:32 UTC).
OK, you want to see if for yourself? Here is the radar (Camano Island) at 8:57 PM on May 14th, right after sunset. Not much echo, just ground clutter from terrain and some close in returns near the radar.
Here is the radar image less than an hour later, when it is starting to get dark 9:36 PM). Echo is starting to fill the domain. And it is NOT raining.
And here is the situation at 11:04 PM. Much of the domain visible by the radar has echo. The birds are everywhere!
At 5:06 AM the next day, there is lots of echo still:
But at 6:04 AM (remember sunrise was at 5:32 AM), the echo (and the birds) are mostly gone!
And at 8:02 AM, much less still:
Now want to see something neat? The NWS radar is a Doppler radar and thus can measure the speed of the targets towards or away from the radar. We can check out the velocities of the birds! Here is is:
Greens indicate velocities toward the radar (in knots) and yellows/oranges away. The radar location is in the middle. The birds are migrating north, roughly at 20 kts! (a relief...it would have been a problem if it had been the other way).
Ornithologists know about the value of the weather radar and use this technology to track migrations. Occasionally, there are weekend weather people on TV that make the mistake of misidentifying birds for precipitation...but you won't.