Two questions come to mind when considering daily weather mapping in the historical instrumental period. How similar are the methods employed to those used in the plotting and analysis of current synoptic charts? And what differences in the methods arise from the historical nature of the observations? A discussion of the actual procedure adopted in the preparation of the daily weather maps for the 1780s will highlight these similarities and differences.

In present-day synoptic weather mapping a number of charts are plotted and analysed at forecast offices every day, with those for the main synoptic hours 0001, 0600, 1200 and 1800 GMT being the most widely used. At the commencement of this project it was decided that although observations in the 1780s were made at many of the stations two or three times a day, only one chart a day would normally be drawn, with 1400 h being selected as the best time, for a number of reasons.

It is known from meteorological theory that if the effect of surface friction is ignored, the geostrophic wind, which approximates to the actual wind at a height of about one kilometre (3300 feet), can be considered to blow parallel to the isobars on mean sea-level pressure charts. Lowest pressure is on the left, and the speed of the wind is inversely proportional to the isobar spacing, i.e. close isobars, strong wind; widely-spaced isobars, light wind.

Over land, the surface wind is most-closely related to the geostrophic wind and hence mean sea-level pressure patterns at about 1400 h to 1500 h, the time of maximum heating, when vertical mixing in the friction layer is at its strongest.