Subject: Jet Streams
Date: Jul 05 00:43:50 1995
From: Jack Bowling - Jack_Bowling at mindlink.bc.ca


Al Jaramillo said to Michael Price:

>However, I don't think that the reason is that these birds happen to catch a >good wind to ride, particularly since the jet stream largely goes from the >west to the east. Of course, meandering of the jet stream does occur and it >can be shooting air from the north almost directly to the south, but this >would rarely, if ever, happen accross a large latitudinal difference. So its >unlikely that a bird could ever ride a jet stream from Alaska to California. >The jet streams may be much more helpful to birds like Blackpolls, >Grey-cheeked Thrush that have to go southeast in the fall. I don't imagine >that at our latitudes the jet streams could ever be used to a bird's advantage >in the spring. In any case, this is all speculation. I don't think that we can >yet be sure that birds use the jet streams during migration, but I would be >interested in reading any data that shows this is the case.

Al - Let an old weather guy get in here. First of all, it is fairly common
during blocking patterns for a northerly jet stream to develop from Alaska all
the way to California. An average would be about four times a year, so yes,
jet streams do have large latitudinal displacements at times. There will be
one developing later this week as a matter of fact. The classical definition
of jet streams is associated with the core of strongest winds near and just
below the tropopause, the boundary between the troposphere and the
stratosphere, at a height of about 12-14 km this time of year at our
latitudes. This is clearly higher than most birds migrate. However, the winds
below the jet stream core are still fairly brisk at the lower levels between 2
and 7 km which many migrants favour. Therefore it is not much of a stretch to
believe that birds would use favourable winds when possible to assist in their
passage. Many hours of watching Sandhill Cranes pass over Fort Nelson at
heights which you need a scope to see them let alone binoculars lends some
credence to this. So, I suppose we disagree here. I believe that birds
actively seek out favourable winds whereas you do not. Just why do you think
that Sharp-tailed Sandpiper rose to such a great height when it finally took
off? Keep in mind that we humans have a distorted sense of comparative
distance between the vertical and the horizontal: from my boyhood home in
Tsawwassen, Mount Baker was a dominant skyline feature at a distance of 120 km
- an order of magnitude greater than the height of the tropopause.

,Jack




Jack Bowling
Prince George, BC
CANADA
jack_bowling at mindlink.bc.ca