The official forecaster consults a daily weather map and certain
auxiliary maps which show changes in pressure and temperature for twelve
hours or more. He examines closely the contours of pressure as shown on
the map. The synoptic map, as it is called, because it is a glance at
weather conditions over a large area at one and the same moment, is a
map on which are plotted pressure, temperature, wind direction, velocity
and rainfall. The lines of equal pressure or isobars generally curve and
inclose what is known as a cyclonic centre, or depression or LOW. The
arrows point in, but not exactly toward the centre of the depression.
On the map there will probably appear also an area of high pressure
where the surface air flows leisurely outward and away from the place of
highest pressure. Such an area is called an anticyclone, a word first
used by Sir Francis Galton in 1863 to designate not only high pressure,
but general flow of the air in a reversed or opposite direction to that
of the low area or cyclone. The word cyclone was first used by
Piddington in 1843 in describing the flow of the air in the typhoons of
the East Indian Seas. It is from the Greek and literally means the coils
of a serpent. The word cyclone must possess some special merit in the
minds of journalists for it is quite commonly misused for tornado in
descriptions of the smaller and more destructive storm.