The twice-yearly migration of millions of birds from warmer climates to colder northern regions has always fascinated me. It would be like half the USA getting up and walking a few hundred miles twice a year.
And it shows up on weather surveillance radar quite well at times. We notice it here on the Portland radar from early-mid spring and then again in early-mid Fall. You can see the echoes suddenly appear right after sunset, rise in height, and peak out around 8,000′ or so.
Tonight was a great example, the last of the daytime showers were fading away at sunset, moving from NORTH to SOUTH. Then right after sunset the radar screen fills in with “targets” moving from SOUTH TO NORTH. Here is the VAD wind display from the Portland radar around 7pm:
The wind arrows are all northerly up to around 12,000′, then no echos to get a speed/direction from above that level.
Then the same display around 11pm:
The arrows are all southerly or southeasterly, probably the birds flying straight north contaminated a bit by the northerly flow of the atmosphere? The southerly flow seems to top out around 7,000′ too. And how do we know the actual wind direction didn’t suddenly shift through thousands of feet of atmosphere in just a few hours? All models show the northerly flow continuing through another 12+ hours.
Most birds prefer to migrate at night due to smoother airflow (no daytime heating for strong up/down motions), and they need to eat during the day down on the ground.
It doesn’t take that many birds to make a radar show “rain” (what it looks like to you). Think of the surface area of a flock of geese or ducks compared to raindrops. Or dozens of flocks of smaller birds scattered around the region. Several studies have been done showing 20-30 dBZ echoes equal 200-1800 birds per CUBIC kilometer. That’s a cube 1 kilometer wide/high. Lots of birds!
You can read more about it here: http://virtual.clemson.edu/groups/birdrad/index.htm
By the way, there is a movie called WINGED MIGRATION (a documentary) produced about 10 years ago using hand-raised birds. They follow the birds (usually geese) while flying and migrating using some sort of very light aircraft. Or, actually I suppose the birds follow the aircraft since they imprinted on humans at hatching. That means they think humans are part of their family. Now, believe me, this is no “party movie”, actually it’s REALLY slow, but great visuals for little kids to watch. Filming was done all over the planet. Here’s a clip for a little mid-day relaxation:
I like how they have short conversations with each other while flying
Chief Meteorologist Mark Nelsen