Nouvelle Méthode de Prévision du Temps

Abstract

MONSIEUR GABRIEL GUILBERT, the winner of the prize offered in 1905 by the Société belge d'Astronomie, de Météorologie et de Physique du Globe, for the most successful short-period forecasts of weather, has published in book-form a detailed exposition of the principles underlying his method. He introduces two new principles into the art of weather forecasting, which, so far as we are aware, have not been stated explicitly by any other writer on this subject. First, he invites us to compare the force of the wind at the surface as observed at the various stations contributing to, our daily weather reports with the barometric gradient at sea-level. If in any region the observed wind forces are markedly in excess of the normal for the prevailing gradient, a surge of high pressure in the direction of the gradient may be looked for, and vice versa. His definition of the word “normal” is entirely conventional. It is based on comparisons made by Clement Ley, Sprung, Koppen, and others, and is that the number expressing the wind force on the Beaufort scale shall be twice that expressing the gradient in millimetres of mercury per degree (111 km.). It follows from this general principle that a depression which is surrounded on all sides by winds in excess of the normal will fill up, whereas a depression surrounded by winds in defect will grow deeper. If the defect is great, a depression of small intensity will develop into a violent storm centre. A depression round which the distribution of wind force as compared with the prevailing gradient is unsymmetrical will move towards the region of “least resistance,” i.e. the region where the winds are most conspicuously in defect. In identifying the region of least resistance the second principle is also used. It is based on the conception of “divergent” winds. Any wind which has a component directed away from a centre of low pressure is divergent for that centre, and as such marks a region of low resistance to its advance. Generally speaking, the greater the “divergence” the less the “resistance.” Strong northerly or north-westerly winds to the eastward of a depression are looked upon as an extreme case of divergence, and as a sure sign of a rapid advance of the depression.

Nouvelle Méthode de Prévision du Temps.

By Gabriel Guilbert. Pp. xxxviii + 343. (Paris: Gauthier-Villars, 1909.)