The symbols used in the text, the quantities they represent and the appropriate S. I. units are
listed below. Some local variants, which appear only briefly, are defined at the appropriate point in
the text. Mean values of quantities are denoted by a bar over the symbol, in the conventional way.
Most work in the field covered by this book now employs the units of the Systeme Internationale,
which has considerably eased the task of the editors. However, there remain a few differences of
practice. In particular, while many engineers continue to specify water vapour pressure in Torr (mm
of Mercury), the majority of climatologists use the millibar; strictly, neither is an S. I. unit, though
the latter sometimes disguise the millibar as the hecto-Pascal (lo2 Pa). On Professor Gagge's suggestion
the S. I. unit of pressure, the Pascal, was adopted in this text. The multiple of the kilo-Pascal
(kPa) is that used, as it is more convenient numerically. All vapour pressures in the text and figm
have therefore been expressed in kPa, the form most likely to be met in future.