Light southeasterly winds early last night enabled a small amount of migration to occur, but after midnight winds switched back around to the northeast and shut the movement down. The radar is a bit deceiving though; because of the clear warm days and cool nights, we’ve been experiencing a strong temperature inversion that gives us a false signal after sunset. This false signal is referred to as Anomalous Propagation, looks similar to bird migration, but is characterized by having a zero velocity reading. If you look closely at both the base reflectivity and velocity output around the Delaware River you’ll see an area that appears to be showing migration in the radar, but has a gray value in the velocity image, indicating zero velocity. This is anomalous propagation. Only in the areas where you see velocity data corresponding to the base reflectivity data should you consider migration. Using the velocity value scale to the left of the image, you can determine how fast the objects in the radar are moving. Birds tend to migrate at speeds >20kts faster than the prevailing wind (assuming a tail wind), so anything moving at the same speed as the prevailing wind is probably not birds but rather could be a pollutants, insects, pollen grains, etc. Last night the winds were from the south/southeast at <5kts, whereas last night’s small migration across the radar was moving 20-25kts. We can assume that some of this movement across the radar was indeed birds, albeit a small amount.