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left arrowarrow left1998, Table I a  -  Page 1
Gliderport near Worcester, Massachusetts;  September 16, 1998;
Co-ordinates = 4235'30''N, 7147'28"W;  Altitude = 460 msl;  
Magnetic Declination = 15.3W (Subtract 15.3 for True);  Magnetic Inclination = 70.40;  
Observer:  Cary W. Grant
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 
1 Danaus plexippus after-
noon
1,384 m
(4540 ft)
assumed
soaring
- - - - 340 6.4 m/s 14.0C 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 14.0C 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 14.0C at takeoff and 185C 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) .