|
|
|
|
|||||
|
||||||
Subject: veering and backing, again.
From: George Huxtable (george@XXX.XXX)
Date: Tue Mar 04 2003 - 06:40:59 EST
Veering and backing, again.
A thread arose, then fizzled out, some weeks ago about the meaning of the
terms veering and backing of the wind, particularly as to how these terms
apply in the Southern hemisphere. I was away at the time, but have done a
bit of book-study since. I no longer have records of that correspondence,
so can't recall the precise title of that thread, nor much about the
postings. So if I repeat here what others have said, I'm sorry.
Anyway, here's my ha'porth.
American readers will no doubt regard what Bowditch says as gospel. On page
906 of vol 1 (1977), in the section on tropical cyclones, is the statement-
"Within the cyclonic circulation, a VEERING wind (one changing direction to
the right in the Northern Hemisphere and to the left in the Southern
Hemisphere) indicates the vessel is probably in the dangerous semicircle,
and a BACKING wind (one changing direction opposite to a veering wind)
indicates the vessel is probably in the navigable semicircle."
Here, we are not considering Bowditch's impeccable advice about storm
tactics, just his definition of veering and backing.
Bowditch is supported, to some extent, by the usually-dependable Peter Kemp
(ed) in the Oxford Companion to ships and the sea.
Kemp states- "The wind is said to back when it changes contrary from its
normal pattern. In the northern hemisphere, north of the trade wind belt,
the wind usually changes clockwise- from north through east, south, and
west. When the change is anticlockwise, the wind is backing. In the
southern hemisphere, the reverse is the general pattern of the winds. When
the wind backs in either hemisphere it is generally taken as a sign that it
will freshen."
About backing, Bowditch and Kemp agree. But a question arises about what
Kemp thinks of as the "normal" pattern of the winds. When a depression
passes westward across the norther hemisphere, mariners to the north of its
track will see the wind direction changing in an anticlockwise direction,
those south of its track will see the reverse. At least, that's what my own
thumbnail sketches suggest. There could be just as many mariners in each
category. So why should Kemp say that in the northern hemisphere "the wind
usually changes clockwise". It seems against commonsense, and contrary to
my own experience at sea. Can others comment?
However, Kemp defines "veer" as- "The operation of the wind when it changes
direction in a clockwise direction. A wind which veers is frequently a sign
of settled weather in the northern hemisphere, of unsettled weather in the
southern.". It's clear from that last sentence that he really does expect
veer to mean a clockwise change, worldwide. In that respect it is NOT the
converse of his definition for "back" (which seems odd), and is quite
contrary to Bowditch.
It's all rather unsatisfactory. How would the "Bowditch definition" work
out in practice? Remember that in a broad band around the equator, Coriolis
forces are negligible anyway, so this business about the law of storms just
doesn't apply. Is the Bowditch definition of "veer" intended to suddenly
reverse as the equator is crossed? Put yourself in the position of a
watch-officer in low latitudes having to report a 2-point change in the
wind direction to the old-man. Would you have to think-out which side of
the equator you happened to be before you reported it as a veering or a
backing? And would the old-man need to agree about which hemisphere, and
which definition, before he understood? It seems a crazy arrangement that
would not stand up at sea: not in a windship, anyway. Perhaps the Bowditch
definition is by, and for, meteorologists.
I will cite another authority who disagrees with Bowditch completely (in
respect of Southern waters). In "A Glossary of Sea Terms" (Cassell, 1954),
Gershom Bradford is quite specific. He states- "The wind backs when it
changes against the hands of a watch, but veers if it changes with them.",
and "When the wind changes direction to the right with the hands of a
watch- for instance, from west to north, it is said to veer; otherwise, it
backs. This holds in both hemispheres, North and South."
However, he goes on to complicate matters by adding- "If the wind is abeam
and changes forward, it is said to haul, and if it changes aft it veers. It
is, however, often spoken of as hauling aft." Here is a completly different
application of the word "veer", now with respect to the direction of the
ship's bow. According to modern usage these words haul and veer would
correspond to today's heading and lifting of the wind. Harland , in
"Seamanship in the Age of Sail", refers (only) to this understanding of
"veer".
In an attempt to get a definitive answer (in terms of 18th century
practice) I have examined the Beaglehole edition of Cook's journal of his
first circumnavigation (1768 - 1770) in Endeavour, skimming through for
references to backing and veering. Most of that voyage was in Southern
latitudes.
I have not found a single mention by Cook of "backing" of the wind.
However, he used "veering" 13 times, all when he was well into the Southern
hemisphere. In 2 of those references, it wasn't possible to be sure, from
the context, whether this corresponded to a clockwise or anticlockwise
change in the wind direction. On 7 occasions this was clearly an
anticlockwise change, and on 4 it was clearly clockwise. It appears that
Cook took the word "veer" to imply no more than a change in wind direction,
and didn't care which way the change occurred.
Anothr word Cook occasionally used for changes in the wind direction was
"shifting", but I haven't been able to conclude whether this corresponded
to a particular direction of change.
After all that, my conclusion, about the use of "backing" and "veering" in
the Southern hemisphere, is that there is no conclusion to be drawn. The
words can take either meaning. At least, in Northern waters, there's no
disagreement about their usage.
George Huxtable.
==============================================================
contact George Huxtable by email at george@XXX.XXX
or by phone at 01865 820222 (from outside UK, +44 1865 820222)
or by mail at 1 Sandy Lane, Southmoor, Abingdon, Oxon, UK.
==============================================================
|