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Discussion of Challenge Question #7
Dr. David Gibo University of Toronto
Dr. Gibo is a zoologist--and a gliding pilot. Here he is pictured
relaxing beside his Grob 103 glider after a two hour, cross-country flight in which he compared the soaring performance
of glider and monarch butterflies. Having soared like a butterfly himself, here's how he interprets the sightings:
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Tactics and Vectors |
At irregular intervals, occasional monarch butterflies and numerous dragonflies would suddenly fly in off the Atlantic Ocean and make their way inland. On one memorable occasion, a single pulse of thousands of large dragonflies and a much smaller number of monarch butterflies flew in from the ocean for about 20 minutes.
I believe that they were all from one huge thermal that had formed well inland and drifted towards the ocean, picking up soaring dragonflies and monarch butterflies (probably dispersing individuals rather than migrants) all along its path and lifting them thousands of feet above the ground. The thermal apparently crossed both the sound and the outer banks in the vicinity of Nags Head and was headed out to the Atlantic before its insect passengers bailed out, descended to the surface, and began to fight their way ashore.
The probable source of the monarchs becomes clear when we
consider that:(1) migrating (have a preferred direction) and dispersing (no preferred
direction) monarch butterflies both engage in soaring flight, (2) high pressure systems are often ideal air masses for generating thermals, (3) thermals always drift downwind, and (4) when a thermal that has formed over warm land drifts out over a cooler body of water, the supply of warm air feeding into the thermal is cut off and the thermal dissipates.
Fascinating details of weather conditions Gibo believes responsible for the sightings are provide below. Here are his conclusions:
Q. So what is the most likely source of the monarch butterflies seen coming ashore in Florida?
A. The region of Florida immediately inland of where the sightings occurred.
Q. Were the butterflies some of the migrants from Mexico?
A. Perhaps. But they may also have been locals.
Q. Were the butterflies exhibiting a complex and sophisticated flight tactic?
A. Unlikely.
Q. Were the butterflies simply unlucky individuals that happen to have started too close to the coast when they picked up their first thermal of the day?
A. Probably.
Q. Were these sightings unusual?
A. Given the weather conditions, not at all.
Detailed Discussion
A check of the weather maps on Journey North, and other weather maps for March 15 shows that a high pressure area was to the west of the observation site during the time of the observations. The clockwise wind around the center of the high would have produced an offshore (North) wind, as reported. At some time around midday, inland areas became warm enough
relative to the surface temperature of adjacent Gulf waters to trigger the sea breeze, a onshore breeze that can penetrate up to 50 miles inland. At this point, the surface wind would swing around nearly 180 degrees and become southerly, as reported.
The probable source of the monarchs becomes clear when we consider that:
(1) migrating (have a preferred direction) and dispersing (no preferred direction) monarch butterflies both engage in soaring flight, (2) high pressure systems are often ideal air masses for generating thermals, (3) thermals always drift downwind, and (4) when a thermal that has formed over warm land drifts out over a cooler body of water, the supply of warm air feeding into the thermal is cut off and the thermal dissipates.
Assumptions: (1) Plume (column shaped) thermals were being generated inland and drifting towards the coast. (2) Migrating (or dispersing) monarch butterflies that encountered the thermals would begin to soar upward in the rising air. (3) The temperature at inland locations in the morning was at least 75F (24C), the same as the temperature reported near the shore (it was probably warmer inland). (4) The butterflies stop climbing (by partially folding their wings) when the air temperature drops to 14C. (5) A
monarch butterfly of average mass and wing loading descends at about 1 m/s.
Because the observer did not mention clouds we will assume a clear sky and use the dry adiabatic lapse rate (rate of cooling with altitude) of about 3C per 1000 ft to calculate the maximum flight altitude for the butterflies. Assuming that the butterflies cool by 10C during the climb, then they can soar upward in thermals (10/3) x (1000) = (3.3) x (1000) = 3,300 ft above the ground before the air around them cooled from 24C to 14C. Because 3,300 ft is about 1000m, a butterfly that began a gliding descend of 1 m/s as soon as it detected that it was being carried out over
the sea would require 1000/60 = 16.7 minutes to get to the surface.
How far out to sea would it drift before it reached the surface? It depends upon the wind. The surface winds for March 15 are indicated to anywhere from force 2 (3 - 7 kts) to force 3 (8 - 12 kts). Because the observer said that the butterflies were bucking a strong headwind, lets assume 10 kts, orabout 11.5 mph (18.5 km/hr). During a 16.7 minute descent, butterflies that either spiraled down, or simply flew parallel to the coast, until they
reached the surface would be carried (16.7/60) x (11.5 mph) = 3.2 miles (5.2 km) out to sea. However, if the butterflies began to glide towards land, as soon as it crossed over the beach, it would reach the surface closer to shore. With a gliding speed of about 3 m/s, or 6.7 mph (10.8 km/hr), monarch butterflies would be carried out to sea at a net speed of 11.5 -6.7 = 4.8 mph. During a 16.7 minute descent, the butterflies would loose ground and end up 16.7/60 = 1.3 miles at sea. Still a considerable improvement. It could then fight its way back to land by staying close to the surface and avoiding the full force of the wind. Of course, if the butterflies take longer to react to being drifted out to sea while riding thermals, perhaps not responding to the deteriorating situation until long after the thermals have died, the butterflies may be carried so far off shore that they have to fly an hour or more to make it back. If the offshore wind picks up, they may never make it back. On the other hand, when the sea breeze starts, the struggling butterflies may suddenly find themselves riding a tailwind, climb up into the faster moving wind, and be swept back to shore, once again zooming over the beach high above the
ground, but in the opposite direction.
Are the butterflies engaging in a complicated flight tactic by soaring in thermals near the coast, even at the risk of being drifted out to sea? Perhaps. The sea breeze forms a reasonable dependable source of lift called the sea breeze front. The sea breeze front is a band of lift paralleling the coastline that is formed when the sea breeze encounters an opposing wind, like the situation on March 15, forms at the front of the advancing
mass of cool air moving inland. This system forms most days from spring through fall, is often marked by a line of cumulus clouds, is well known to glider pilots in costal areas, and can be used as a highway by soaring animals to travel long distances in relatively straight lines.
Monarch butterflies may be adapted to exploit this dependable source of lift, or they may simply be exhibiting their normal soaring behavior and are swept up in the lift from the front as soon as it forms. Butterflies that were soaring in thermals further inland would have been carried S by the N wind and neared the coast just as sea breeze began to move inland. The butterflies would have been swept up by the front could have flown in
lift for the rest of the day. Butterflies that started further to the south may have been drifted over the sea and had to fight their way back, but once the sea breeze developed, they too, would have been swept into the front and been able to soar in lift for the rest of the day. Is this an adaptation, or simply their normal soaring program being played out in a favorable environment. I don't know. I suspect that it is about as much sophistication as we can expect from an insect unless it has senses that we
don't know about.
David Gibo
Dept. of Zoology, University of Toronto
dgibo@credit.erin.utoronto.ca
http://www.erin.utoronto.ca/~w3gibo/