Sir Napier Shaw, who has written much on the weather of the British
Isles, may be quoted here.
"Anyone who is interested in the weather is always on the lookout
for 'lows' and is very keen to know whether he is going to be on
the south of the centre or the north of it. He is, of course,
interested in the anticyclone too, because as long as an
anticyclone is there, there cannot be a depression; but it is the
depression which has the life and movement about it, giving it a
claim to the attention of everybody who wants to know what the
weather and its changes are going to be.
"This has been recognized from the very earliest days of weather
maps with isobars. The depressions which pass over our shores
(Great Britain) mostly come from the west. Some of them come all
the way from America; one or two have been traced from the west
coast of Africa and so have crossed the Atlantic twice, first to
the westward and then to the eastward. Some have come all the way
from a sort of parent 'low' in the North Pacific Ocean. So general
is the tendency for 'lows' to go eastward that it was thought at
one time, particularly by the 'New York Herald,' that their
departure from the American Coast and subsequent arrival on our own
shores could be notified by cable, and we (the British) might thus
be forewarned of their approach, some three or four days in
advance. The attempt was made by the 'New York Herald' acting in
co-operation with the Meteorological Offices of the United Kingdom
and France. But a depression keeps to no beaten track; it has as
many paths for its centre as there are lines in a bundle of hay.
Though groups can be picked out there are many strays, and,
moreover, the depression changes its shape and intensity while it
travels, so that if you lose sight of it for a day you cannot be at
all sure of its identity."
[Illustration: FIG. 6. NOTOS--THE SOUTH WIND]