Abstract
The objective forecasting technique described consists of a composite diagram from which the forecast value of the predictand can be read directly, given the values of the predictors. Each section of the diagram combines a new predictor with an estimate of the predictand obtained from the previous sections. The isopleths in the diagrams are obtained by fitting a curved surface (involving powers and cross‐product terms of up to the fifth order) to the basic data by a ‘least squares’ procedure. Only terms which are significant at the 5 per cent level are retained in the regression formulae so produced. At each stage the predictor to be selected is that which contributes most to the combination so far chosen.
The method was used to forecast visibility (as one of 32 code figures) at London Airport three and six hours ahead. When it was tested on two winters' independent data, correlation coefficients of 0·89 and 0·83 were obtained for the 3‐hr and 6‐hr forecasts, respectively. During the same period the figures for the normal subjective forecasts made at London Airport were 0·87 and 0·74.