Feathery Radar Rings

September 19, 2005


Referred by: Oklahoma Mesonet
Summary author: Deke Arndt

The two radar scans above showing distinct orange-red rings are not weather related at all -- they're birds! These scans were taken from Oklahoma on the morning of August 8, 2005. The top image was acquired at 6:25 a.m. Central Daylight Time (CDT), and the bottom one was acquired at 6:45 a.m. CDT. The oranges and reds signify particles (or in this case feathers) moving away from the radar, while greens and blues are moving toward the radar. So the movement of the birds, or, more properly, the average movement of the population of birds, is indicated by expanding rings (larger rings were observed as the morning progressed).

It appears that these bird-rings were made by large seasonal colonies of purple martins. They amass in huge quantities overnight, then shortly before sunrise, they take off en masse, in a near-symmetric pattern in all directions -- this explains the near-perfect expanding rings.

It seems that a couple of factors cooperated to bring us our ringed, winged radar show: First of all, the refraction of the radar beam allowed us to see nearer the surface than usual. Second, purple martins don't always hang out in giant communal roosts. Most of their time in North America, they live and nest in smaller social units, such as the familiar purple martin houses you see in the suburbs. Only in the weeks just before their annual migration back to South America do they cluster in roosts that can number into the hundreds of thousands (apparently there's some sort of biological group discount on intercontinental travel).

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