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![]() ![]() Gliderport near Worcester, Massachusetts; September 16, 1998; Co-ordinates = 42°35'30''N, 71°47'28"W; Altitude = 460 msl; Magnetic Declination = 15.3°W (Subtract 15.3° for True); Magnetic Inclination = 70.40°; Observer: Cary W. Grant |
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Obs. | Species | Time | Flight Behavior | Weather | Field Notes | |||||||||
I | II | III | IV | Va | Vb | Part 1 | Part 2 | Part 3 | ||||||
Obs. Alt.* |
Type of Flt. |
Horiz. Path |
Vert. Path |
Mag. Bear. |
Mag. Head. |
Wind Dir. |
Wind Vel. |
Amb. Temp. |
Thrm. Act. |
Cloud Types |
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1 | Danaus plexippus | after- noon |
1,384 m (4540 ft) |
assumed soaring |
- | - | - | - | 340° | 6.4 m/s | 14.0°C | yes | 50% cumulus |
Both butterflies were flying in a very strong thermal
in the vicinity of a kettle (flock) of 50 hawks.
|
2 | D. plexippus |
after- noon |
1,384 m (4540 ft) |
assumed soaring |
- | - | - | - | 340° | 6.4 m/s | 14.0°C | yes | 50% cumulus |
|
*Altitude above the ground. Altitude above msl = 1524
m (5000 ft) . Additional field notes provided by Cary W. Grant: weather development during flight - The wind was 10 kts (5.0 m/s) from 340° on the runway at takeoff, 10-15 kts (aver. 12.5 kts or 6.4 m/s) from 340° aloft, and from the North at 5-10 kts (aver. 3.5 m/s) on the runway at landing. The temperature was 14.0°C at takeoff and 18°5C at landing. Clouds were scattered cumulus and cloud cover ranged from 60% at takeoff to 30% at landing. Cloudbase ranged from 5,000 ft (1,524 m) to 5,200 ft (1,584 m) msl. Weather development during the day - Sky was overcast from 9:00 to 12:00 with 80% cloud cover. From noon to 6:00 the cloud cover was scattered cumulus, averaging 50% cloud cover and diminishing during the afternoon. Thermals were narrow up to 3,000 ft (900 m) msl, strong up to 5,000 ft (1,500 m) msl, and very strong immediately under the clouds. Comments by Gibo: I assumed that the monarch butterflies were soaring because they were flying in a strong thermal. It is interesting that the butterflies were about 100 km inland and the direction of wind drift was SSE to S, towards Rhode Island Sound and the Atlantic Ocean. See Calculations below. |
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Calculations
1) Time required to reach the Atlantic Ocean:
Because 12. 5 kts = 23.2 km/hr, they could have reached the Coast by simple
wind drift in about 100/23.2 = 4.3 hours, even if they just circled in thermals for the
entire distance. If the butterflies in Massachusetts flying in North winds
have the same behaviour as those in Ontario (See Table VII),
then the Massachusetts butterflies would spend about 50% of their flight time soaring in
circles (and drift downwind), about 25% of the flight soaring straight ahead, and
about 25% flapping straight ahead. Assuming a gliding airspeed of about 3
m/s, and a flapping airspeed of 5 m/s, that the butterflies stay high (See Table V),
and fly directly downwind, they would have a net rate of displacement over the
ground of 0.5 (6.4 m/s) + 0.25 (6.4 m/s + 3.0 m/s) + 0.25 (6.4 m/s +
5.0 m/s) = 3.20 m/s + 2.35 m/s + 2.85 m/s = 8.4 m/s, or 30.2 km/hr. In
this case, they could have travelled from 100 km inland to the coast in a little over 3
hours. On the other hand, if the butterflies also maintain a SW heading during
periods of straight flight, just like migrants in Ontario (See Table IV),
then things are a bit more complicated. Three vectors must be summed to produce a
resultant vector. The vectors to be summed are: (1) the
downwind vector, (2) the vector for straight, gliding flight, and (3) the
vector for straight, flapping flight. In this case, the resultant vector is a
177° bearing and a velocity of about 26.5 km/hr (7.4 m/s). If the
monarch butterflies have the same flight tactics as those in southern Ontario, the
would be flying nearly due South and would reach the coast in about 3.75 hours.
If the butterflies were prevented penetrating inland by West and Northwest
winds, they would probably continue roughly SW, paralleling the coast, pass through
Cape May, New Jersey, and may even be counted by Dick Walton at the Cape May Bird
Observatory.
2) Distance that the butterflies can glide from cloudbase:
The butterflies have a glide ratio of 3.6:1 and cloudbase was 1,384 m
above the ground, the butterflies could have glided 3.6 x 1.384 = 4,982 m (5.0 km)
in still air. Assuming a relatively high sink rate of 1 m/s (Gibo and Pallett, 1979), the butterflies would
require 1, 384 s (23 min.) to reach the ground. The average of the
surface wind (5.0 m/s) and the wind aloft (6.4 m/s) is 5.7 m/s. If the butterflies
glide directly downwind at 3 m/s, they would have an average ground speed of
3.0 + 5 .7 = 8.7 m/s (31.3 km/hr). During the time it takes to glide down from
cloudbase, the butterflies would travel 1,384 x 8.7 = 12, 041 m (12 km) .