Example: Migrations of birds and insects

 

Bird & insect migration on VPR

 

In addition to rain, clouds, airplanes and clear air echoes, radar can observe birds and insects. These observations are most spectacular during the migration period. Birds, especially song birds, migrate at night (it is believed that they prefer to do these long flights when turbulence is minimum because it would be less tiring; and since turbulence at low levels is often caused by solar heating, it reaches a minimum at night). They are also very selective about which night to choose, patiently waiting for one where a good tail wind pushes them in the right direction. Some insects (e.g. butterflies) also migrate, but they prefer the day (supposition: they don't mind the turbulence on which they may float; and they might not want to fly at the same time as birds!).

In this spring-time example, a strong south wind (90 km/hr) wind was blowing at 1.5 km altitude ahead of a storm. This was too good to pass: Insects migrated in the afternoon, while birds started their travel one hour after sunset (see top image). All radars detected their passage so numerous were they. Coverage was uniform on the scanning radar while individual birds could be counted on the VPR image (about 2000 of them during that night, if we assume one bird by echo). Based on this, we can estimate that of the order of 3 to 4 million birds crossed a line between Cornwall (80 km WSW of the radar) and Granby (100 km to the east) that night. Birds can also be observed on profilers where they mess wind measurements because of the addition of their own air speed to that of the wind; in this case the profiler measures strong south "winds" while the wind was somewhat weaker and from the SSW.

Bad winds example due to migration

Bird migrations can also be observed on scanning radars. On this Doppler image taken in fall two years later (3-4 October 1996, 21:20 EST), the north-to-south movement of the birds can clearly be observed (approaching velocities to the north, receding velocities to the south; the gray patches north and south of the radar are stationary echoes from hills, Laurentians to the north, Adirondacks to the south).

While bird migration extend over a couple of months, in only about 3 nights per season will bird echoes be so numerous that they nearly cover the scanning radar display like in this example. Because they favor nights with strong tail winds, they will often fly just before (spring) or just after (fall) the passage of strong weather systems.

Doppler image of migration

 

 

 

 

 

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