For almost a century the principal tool of the weather forecaster has been the synoptic chart or, as it is usually called, the weather map. Drawn every six hours, these maps are based on reports received from about 200 stations over an area extending from the equator to Antarctica, and from western Australia to Pitcairn Island. By international agreement each country maintains a network of weather reporting stations which forward coded reports to a national collecting centre. The reports contain details of cloud, visibility, and weather conditions, wind speed and direction, and atmospheric pressure (at mean sea level) and its change in the previous three hours. Also reported are air temperature and dew-point, rainfall, and the state of the sea (from coastal stations). Ships at sea also forward six-hourly reports to the nearest collecting centre, while hourly reports are contributed by aircraft on trans-ocean flights. Every six hours there is a rapid international exchange of information between collection centres using radio-teletype and facsimile transmissions. The basic observations thus assembled are plotted on the weather map in a kind of meteorological shorthand, and then the forecaster draws the isobars (joining places with equal mean-sea-level pressure) and the fronts (the boundaries along which large air-masses of different temperature converge).

The importance of the weather map arose from an early recognition of the existence of large-scale weather systems closely related to the pattern of the isobars. It was found that over areas of relatively high pressure (anticyclones, ridges), the weather is mainly fine, while the low pressure systems (depressions, troughs, cyclones) are associated with strong winds and unsettled weather. The wind flow follows the direction of the isobars, with low pressure on the right (in the southern hemisphere), and is strongest where the isobars are closest together. Since these pressure systems tend to change rather slowly and to retain their identity for some days, their progress can be followed from map to map. This serves as a rough basis for weather prediction. Serious errors will arise, however, unless changes in movement and intensity of the weather systems can be anticipated.

The above weather maps show the meteorological situations associated with two characteristic New Zealand weather phenomena.

In New Zealand the first weather maps were drawn by Capt. R. A. Edwin who was appointed to the Marine Department in 1874 to provide weather forecasts and storm warnings for shipping. These maps were based on reports from about two dozen stations telegraphed daily (except Sunday) to Wellington. No reports came from outside New Zealand but, following an Inter-Colonial Meteorological Conference in 1879, code numbers were assigned to 24 typical isobaric patterns commonly occurring over New Zealand and 20 over south-eastern Australia. The appropriate code numbers were exchanged daily by cable between the two countries and were also supplied to the main daily newspapers as a guide to the selection of the correct block for printing a daily weather map. Because of such scanty and infrequent reports, and the absence of information from the surrounding oceans, no great accuracy in forecasting was achieved, nor could it be expected. Yet for the next 40 years little change took place, either in the day-to-day collection of reports, or in the technique of forecasting.

In 1909 Rev. D. C. Bates succeeded Edwin as director of the Meteorological Office. At that time radio was in its infancy, but arrangements were soon made to receive weather reports from ships at sea. The intervention of the First World War delayed this project and it was not until 1919 that the flow of ships' reports began in earnest. In the same year the Norwegians introduced the concept of the polar front and the wave theory of cyclonic formation. This was a most important advance, as it offered for the first time a physical explanation for many of the developments which take place in middle latitudes; in particular, the tendency for the cloud and rain accompanying a depression to be arranged in bands rather than uniformly around the low-pressure centre. The Norwegian methods were first applied in New Zealand by Dr E. Kidson, and later extended by C. E. Palmer whose paper “Synoptic Analysis over the Southern Ocean” (1942) became the standard guide for meteorologists in this region for many years. Kidson directed the Meteorological Service from 1927 until his death in 1939.