By Carol Cullar
Monarchs are all about flight. They are paired with the air. So, to consider whether monarch butterflies avoid mountains during their migration, it’s necessary for us to look at some of the physical attributes of the atmosphere and mountains—AND how the two factors interact.
Mountains
and More Turbulent Air
In general, a good metaphor for this interaction is to picture mountains
just as if they were rocks and boulders in a bubbling stream. The atmosphere
flows around the hills and mountains as if they were large and small rocks.
Large rocks in the stream part the water and cause turbulence as the flow
splits to pass to either side. Smaller rocks push the water up over their
surfaces, creating additional turbulence on the backside of the flow.
In addition to pushing up waves of air to the sides and above hills, there are some other basic characteristics of atmosphere that man has learned during the last few decades we have been learning about mechanical flight.
Mountains
and Thinner Air
As we ascend in the atmosphere, it becomes much thinner. Actual molecules
are further apart, which means that the fewer the molecules there are
pushing against a wing, the less lift there is for the wing. For an airplane,
this means burning more fuel to get more push from the motor. A butterfly
would find the same required: more energy burned to obtain the same lift
they were getting at lower elevations. Burning more energy to obtain the
same forward distance isn’t efficient. You can imagine the implications
for the monarch.
Did you know that if a pilot in a small plane chooses to fly above 10,000’ above sea level, that there is so little oxygen that she is required to use an oxygen mask and provide oxygen for her passangers because she could make grave errors in judgment —or even pass out— in a matter of minutes?
Mountains
and Colder Air
Another phenomenon as relates to the atmosphere is the phenomenon of adiabatic
lapse rate. In other words, because the molecules are further apart, the
atmosphere retains less heat. The higher we go, the cooler the air is.
Air is not a good conductor of heat. The temperature drop is just over
3.5dF per 1000’ of elevation. So that in normal weather conditions,
at 1000’ above sea level, it will be about 3.5dF cooler than at
ground level (AGL). At 5000’ AGL, what will the temperature difference
be?
Let's see what this means for a monarch: If the temperature on the ground in the spring is 50dF, then the temperature at 3,000 feet would be only 39.5 degrees! Do you know why this cold air would cause problems for a monarch?
In a small plane, the pilot has to adjust the combination of air and fuel to most efficiently fly at a particular elevation. What would the monarch have to do?
If a small plane wants to fly in a southwesterly direction, but the winds on the ground are blowing really hard from the southwest, the pilot might discover that the winds aloft are moving at a different direction, so what does the pilot do?
If the temperature on the ground is 40dF, then what would be the temperature at 10,000’? If there is any moisture present, what could form on the wings?
Carol Cullar
Rio Bravo Nature Center
Foundation, Inc.
Eagle Pass, Texas
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