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Tactics and Vectors 98/99 |
How monarch butterflies can fly over high mountains. (adapted from tvectors)
David Gibo, List Owner, March 26, 1998: In a recent post (reprinted in part below), about monarch butterflies crossing the Sierra Nevada mountains in California, Paul Cherubini asked about various weather phenomenon that might assist in this process. I think this is a good topic for tvectors. From Paul Cherubini (March 22, 1998): I believe I heard from someone that you grew up in the San Fernando Valley near L.A. If so, then you should remember the infamous California Santa Ana winds that are common in the autumn. I believe these winds are associated with high pressure in the Great Basin. Are thermals present in the clockwise circulation of wind around a high pressure cell? Or during a Santa Ana, (Is this type of)...decending air conducive to soaring? Are Santa Ana's bad news for glider pilots because of the sinking air that is warmed by compression or can you still get lift when the air hits mountain ranges like the east slopes of the nearby Coast Range and San Gabriel Mts? Paul Cherubini's questions address the incredibly complicated interactions that occur among wind, mountains, high pressure systems, low pressure systems, and thermals. First, a short review of how plume thermals form. Sunlight heats the ground and the ground heats the layer of air directly above it. This heating results in an unstable situation because the warmer layer of air near the ground expands and becomes lighter than the layer of air directly above. At some point a the warm (light) air starts to bulge upward, forming a dome. Because rising air expands and cools, what happens next depends largely upon the lapse rate, the rate of decrease in temperature with altitude. If the lapse rate is steep (temperature decreases quickly with increase in altitude), the rising, expanding, and cooling, dome of air will still be warmer (lighter) than the surrounding air and will continue to rise. Warm air near the surface rushes in from all directions, replacing the rising air, and rising in turn. The result is a thermal, a rising column of warm (light) air. The newly formed thermal drifts along with the wind, draining warm air upward. The thermal lasts about as long as it can tap a steady supply of warm air near the ground. The final height of the fully developed thermal is determined by the altitude at which the temperature at the top of the dome has cooled to the temperature of the surrounding air, or by a capping inversion. If the warm air is also moist, a flat-bottomed cumulus cloud will form when at the altitude where the temperature of the rising air drops to the dewpoint. Wind can either help or hinder the formation of thermals. Gentle to moderate winds (up to about 15 mph or 25 km/hr) can help trigger thermal formation by stirring up the warm layer of air near the ground enough that parcels to of warmer air break loose and start rising through the overlying cooler layers. This is the situation on a day with abundant puffy white cumulus clouds. Stronger winds tend to fragment parcels of warming air near the ground and fragment thermals. On days with strong winds, thermals that manage to form near the ground usually have very narrow cores and violent updrafts. This type of thermal is common in desert regions and is often organized into a powerful dust devil. High pressure systems in flatlands are generally associated with good soaring conditions. The air is clear, cool, and dry, and the lapse rate is steep. The clear, dry, air allows abundant sunlight through to heat the ground. Because the entire airmass is cool (dense), just a small amount of heating of the layer of air near the ground trigger convection and thermals begin developing by mid morning. The steep lapse rate allows rising columns of warm (light) air to penetrate thousands of feet above the ground before encountering a layer of air of similar temperature. Conversely, low pressure systems are generally associated with hazy, warm, moist, air, and a moderate lapse rate. Thermals may not develop until after mid-day, or not at all. On the other hand, once convection starts in a warm, moist, airmass, there is often a risk of afternoon thunderstorms. Mountains cause many complications. Interactions between wind and mountainous terrain always results in lift (rising air) on the upwind sides of slopes (ridge lift) and sink (descending air) on the lee sides. There can even be lift on the lee side because the wind moving over the peaks can curl back to form a rotor. In general, the more broken and complicated the terrain, the more broken and complicated the resulting patterns of lift and sink. Finally, under certain conditions specific, if the velocity of the wind passing over a mountain range, or passing over a set of parallel mountain ranges, has the appropriate set of properties, standing, harmonic, waves will develop. These waves can be rather shallow, extending upward only a few thousand feet, or they can be real monsters and extending upward for more than 50,000 ft. The up wind side of the smaller standing waves are possible sources of lift for soaring animals. The up wind side of the monster waves are for attempts at setting world records for altitude reached in a glider. Thermals and mountains work together to produce beautiful soaring conditions. Mountains cool more than adjacent lowlands during the night, but receive as much, or more, sunlight during the day. As a result, thermals are triggered earlier in the day in the mountains. Furthermore, for air moving up though the valleys and passes, cooling caused by expansion is offset a certain amount by heating from the surrounding terrain. The additional heating helps produce a chimney effect and result in the nearly daily development of strong, up slope, winds. Converging up slope winds from both sides of a mountain range will join to form a huge regions of lift paralleling the peaks. This wide, linear, band of lift, often extends for long stretches of the Sierra Nevada mountains, providing a superhighway in the sky for glider pilots. The interaction of high pressure and low pressure systems and mountains and the daily development of up slope winds can be very complicated. If the arrangement of high and low pressure systems are such that up slope winds are suppressed and air is forced to move down slope, like the Santa Anna wind, thermal activity may also be suppressed, largely, I believe, because the strong wind breaks up pools of warm air before thermals can be triggered. On the other hand, if the arrangement of pressure systems is such that air is forced to move up slope, then heating that generates up slope winds will probably be reinforced producing an even more powerful up slope wind. In both cases, ridge lift and, perhaps, wave will occur in the appropriate settings. Finally, when thermals, wind, and wave are all working together, only the best and bravest glider pilots, including, of course, monarch butterflies, should risk leaving the ground to take on the mountain monsters of lift, shear, sink, turbulence, and rotor. Putting it all together, here is one possible scenario on how monarch butterflies
could cross the High Sierra. Assuming a day under the influence of a high pressure system
to the north of the crossing point producing gentle to moderate east winds at the surface.
Butterflies starting on the east side of the Sierra Nevada mountains in mid morning could
soar in thermals and make their way west until they encountered the up slope winds in the
mountains. The butterflies could ride the up slope winds until they cleared the peaks,
then continue soaring west across the wide band of lift formed by the two sets of
converging up slope winds. Because cooling of the up slope winds by expansion has been at
least partially offset by contact and radiative heating from the various sun-warmed
surfaces (rock, soil, vegetation, etc) on the mountain, and because monarch butterflies
regularly soar in temperatures as low as 15 degrees Celsius (and probably as low as 10
degrees Celsius as long as they are in full sunlight), they should be able to clear the
peaks with altitude to spare and still be flying in air at ambient temperatures that are
well within their activity range. Is the cloud base too low? No problem (as long as there
is no precipitation). Monarch butterflies can maintain oriented flight in clouds. After
the monarchs have soared across the band of lift, the east winds would drop back down to
their altitude, having been lifted upward over the peaks by the converging up slope winds,
and carry the butterflies on into the central valley. Since the butterflies are flying in
the weather conditions dominated by a high pressure system, they should continue to
encounter abundant thermals all along the way. Assuming that the east wind increased to 20
miles per hour at altitude, even if the butterflies spend much of their time soaring in
circles, they should still reach the central valley by mid afternoon. Successful crossings
of the Sierra Nevada mountains in late summer should be possible by riding east winds
generated by either the clockwise rotation of high pressure systems located to the north,
or by the counterclockwise rotation of a low pressure systems located to the south. Later
in the season, when average ambient temperatures are lower, successful crossings are
probably associated with east winds generated by a low pressure system located south of
the mountains, perhaps offshore. The counterclockwise rotation of the low pressure system
would sweep warm air from the southern deserts north towards Nevada and west over the
Sierra's and into the central valley. This same scenario applies to other mountain ranges,
such as the Rockies and the Appalachians. |