In the spring of 1863 the assistant of Le Verrier and head of meteorology section at the Paris Observatory, Hippolyte Marié-Davy, decided to develop a new synoptic method that could foretell the arrival of storms from meteorological charts. Well aware of the increasing interest in storm warnings as part of the surge of Napoleon III’s nationalist spirit of defence and Le Verrier’s reluctance towards forecasting, he noted the lack of progress in this endeavour. The key to storm forecast, he argued in a note read before the Academy of Sciences, lay not in the identification of ondes atmosphériques whose existence at first seemed consistent with the barometric data showed on the charts, and whose search was the practice theretofore followed at the Observatory. 1 Rather, it was a matter of dynamical isobaric geometry, a view far removed from Maury’s statistical wind charts, which sought the most favourable conditions for navigation rather than the origin and course of storms. 2 The method, he announced in his note, consisted of identifying areas of low pressure—or coherent atmospheric patterns, also called bourrasques—from the isobars plotted on the maps. By the end of 1863, he had a successful technique, culminating with the first charts ever published in the Observatory’s Bulletin and the launch of the weather forecasting service (see Figure 3.2 in Chapter 3). 3