Subject: Re: Jet Streams
Date: Jul 6 17:55:15 1995
From: Alvaro Jaramillo - alvaro at quake.net


>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.

Great to have someone who knows something about this stuff step in! OK, so
the jet stream can go from Alaska to California about 4 times a year. My
interpretation would be that this is really a rare occurrence rather than
common. Does this tend to happen most commonly in the late summer/fall? If
so then they may be common enough for birds to rely on them?

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 don't think we really disagree. I am convinced that birds use favourable
winds while migrating. This has been made clear to me from countless of
hours hawkwatching. However, I am not convinced (not that I don't believe
its not possible) that birds use the jet streams. Now its much more
conceivable that the birds are using the strong winds at a lower level than
the jet streams, but I don't know much about the average heights birds
travel at during migration. I know that Cranes, geese, hawks etc. will go
quite high while migrating, but how about a peep or a warbler? From
listening to nocturnal flight calls during migration I know that a good
number of migrants, especially thrushes and sparrows fly quite low, however
I do not know what proportion are flying low versus high. Anybody know about
what the radar data has to offer on this point?

Alvaro Jaramillo
Half Moon Bay, CA

alvaro at quake.net