It is well known that atmospheric circulation is one of the main factors which determine climate. Consequently, changes of climate are indicative of concomitant fluctuations in the flow of the atmosphere. Given adequate sources of data, mapping methods provide the most effective way to make a study of atmospheric circulation.

In order to obtain a better understanding of present atmospheric behaviour, and to become more aware of the kind of changes which could occur in the future, we need to improve our knowledge of past weather and climate on all time scales, both regionally and globally. A detailed study of weather on a daily basis reveals variations in atmospheric behaviour which are not detectable from investigations on longer time scales.

As far as daily records are concerned, it is possible to examine instrumental meteorological observations made in Europe back to the seventeenth century. Documentary sources allow the search for daily wind and weather data to be extended, albeit with decreasing continuity and reliability, back to the Middle Ages. Up to now, however, apart from a specialised case study of weather conditions during the voyage of the Spanish Armada in 1588 (Douglas et ah, 1978 and 1979), the analysis of daily circulation patterns by means of synoptic charts* has been made only from the latter part of the nineteenth century, that is, from the time when official observations began to be made and plotted on weather charts by the newly-established national meteorological services.