East winds dominated the scene last night, but that was enough to keep the floodgates open across the Northeast and mid-Atlantic. Here’s the radar from 7:00pm last night through 5:00am this morning.

Frames are every 1/2 hour. Click on the thumbnail to view the full-sized animation.

I have been getting a few questions recently about how to interpret the radar. While I would love to answer this question in much more detail, I just don’t have time to spend on a tutorial right now. Therefore I’ve decided to give a quick-and-dirty lesson with this post. Okay, so there are two major products that the NEXRAD produces (there are more, but only two that I use on this website), Base Reflectivity (the blue/green images) and Base Velocity (the red/green) images.

Base reflectivity gives you a measure of “target density”, or the density of the objects in a cubic kilometer (which happens to be the resolution of these images). That density is measured in decibels (dBz), which is shown on the legend to the right of the image. These targets can be weather (which is what the radar is typicall used for), birds, insects, pollen, dust, strafe from millitary bases, and on and on. The radar is pretty sensitive, so filters are often employed to reduce non-biotic clutter (not by my, but by the National Weather Service).

The second product, Base Velocity, shows not the density of the objects, but the overall trajectory and speed of the objects moving across the view of the radar. Cool colors (greens) indicate objects moving toward the radar beam, and warm colors (red) indicate objects moving away from the beam. Picture an imaginary line cutting across the radar at the center and you can get a feel for the overall direction of movement across the circular radar beam.

So, in order to determine whether what we’re seeing is indeed bird migration, I use a few clues. First, does the reflectivity image show an increase in activity just after sunset, and does this increase persist into the early morning hours of the following day? If so, the targets are most likely birds. Why? Because thunderstorms don’t simply appear out of nowhere (although they might seem to when you’re on the ground!), but birds, when they take to the sky after sunset, do “appear” little-by-little in the radar’s view, first close to the origin of the radar, and later further away from the radar, simply due to the shape of the radar beam (think ice-cream cone). Okay, so we see some activity after sunset, but remember, this could be weather, insects, pollen, dust, and in some cases even anomalous propagation (the bending of the radar beam due to temperature inversion after sunset… which happens often when you have really hot days and really cool nights). Well, again the shape can help us, since rain “looks” different than birds, which have a distinctive stipple pattern. But we can also use our second data product to refine our delineation between birds and other biotic and non-biotic targets.

Enter the Base Velocity image. This is our “correction factor” image, because it allows us to screen out any data that doesn’t fit the criteria of migrating birds (within reason… remember, this is on a very gross scale!). Using the velocity image you can determine the speed and direction of the objects you saw on the reflectivity image. All things constant, and when the wind is directly behind the bird (a tailwind), birds will tend to migrate 15-20kts faster than the prevailing winds. Therefore, if you check the current wind during the night, and it says it’s blowing 5-10kts out of the north, but the objects on the radar are clearly moving 25-30kts, you have just confirmed bird migration. Dust, pollen, and insects will be moving in the same direction as the wind, always, and always at or very near the winds speed. Now consider last night, where the winds were easterly. Birds aren’t going to migrate due west under these conditions, but instead they can use the easterly winds as a quasi-tailwind, gaining some benefit from their direction, but also having to compensate. Therefore the use of the velocity image becomes less straightforward, and you must adjust for the difference in wind speed and speed of travel, possibly relying more on the shape of the signal on the reflectivity image, it’s behavior during the night, and some confirmation of direction and speed from the velocity image. At some point, the interpretation goes from a strictly analytical one, to a combination of art and science… of course, with the proper tools, one could really deconstruct these images, extract and quantify the birds migrating across the radar’s view, and calculate true density of migration… but the data you see here is way too course of a scale, and the necessary tools are generally unavailable to the casual observer.

Okay, that’s the quick and dirty… now to the radar. Birds were moving again last night, this time on east winds and therefore in a generally NE->SW direction (as indicated by the velocity image). While the level of migration was less than that of the last two nights (based on the reflectivity image), it was still moderate-to-high considering the lack of northerly wind flow. Inland migrant traps will be the best bets for today given the lack of precipitation across the region. Chimney Rock, the Delaware River floodplain, and the northern Delaware Bay shore should all be good bets.

As always I appreciate your reports from the field as a way to ground-truth the radar observations.

Good Birding

David

P.S. Come check out my migration forecast for the Mid-Atlantic on Birdcapemay.org

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