Now, whilst it's likely that the principal purpose of this expedition was to enlighten the Dalai Lama on the importance of using large-eddy simulation to understand the interaction of brake-duct winglets with the spat vortex, it's also possible that Adrian was drawn by the legendary bi-annual migration of the bar-headed goose.
These birds are amongst the highest-flying in the world, and travel across the Himalayas in a single day. William Bryant Logan claims in Air: Restless Shaper of the World (2012), that "the bar-headed goose has been recorded at altitudes of over thirty-three thousand feet. This is the altitude where your pilot remarks that the outside temperature is 40 degrees below zero, where the great fast-flowing rivers of the jet streams set weather systems spinning. The air here contains only one-fifth of the oxygen near sea-level, where the goose winters in lowland India wetlands and marshes. Yet in the space of a few hours the bird can fly from the wetlands to the top of the high peaks and then out onto the world's largest high plateau. There are lower passes through the mountains, but the goose does not take them. It may even preferentially go higher."
However, it seems that some of the claims made for the bar-headed goose lack empirical support. Research led by Bangor University tracked the bar-headed geese with GPS as they migrated over the Himalayas, and reached the following conclusion in 2011:
"Data reveal that they do not normally fly higher than 6,300 m
elevation, flying through the Himalayan passes rather than over the
peaks of the mountains...It has also been long believed that bar-headed geese use jet stream
tail winds to facilitate their flight across the Himalaya.
Surprisingly, latest research has shown that despite the prevalence of
predictable tail winds that blow up the Himalayas (in the same
direction of travel as the geese), bar-headed geese spurn the winds,
waiting for them to die down overnight, when they then undertake the
greatest rates of climbing flight ever recorded for a bird, and sustain
these climbs rates for hours on end."
A more recent iteration of the research, The roller-coaster flight strategy of bar-headed geese conserves energy during Himalayan migration, (Science, 2015), suggests that the geese "opt repeatedly to shed hard-won altitude only subsequently to regain height later in the same flight. An example of this tactic can be seen in a 15.2-hour section of a 17-hour flight in which, after an initial climb to 3200 m, the goose followed an undulating profile involving a total ascent of 6340 m with a total descent of 4950 m for a net altitude gain of only 1390 m. Revealingly, calculations show that steadily ascending in a straight line would have increased the journey cost by around 8%. As even horizontal flapping flight is relatively expensive, the increase in energy consumption due to occasional climbs is not as important as the effect of reducing the general costs of flying by seeking higher-density air at lower altitudes.
"When traversing mountainous areas, a terrain tracking strategy or flying in the cool of the night can reduce the cost of flight in bar-headed geese through exposure to higher air density. Ground-hugging flight may also confer additional advantages including maximizing the potential of any available updrafts of air, reduced exposure to crosswinds and headwinds, greater safety through improved ground visibility, and increased landing opportunities. The atmospheric challenges encountered at the very highest altitudes, coupled with the need for near-maximal physical performance in such conditions, likely explains why bar-headed geese rarely fly close to their altitude ceiling, typically remaining below 6000 m."
No comments:
Post a Comment