Temperature inversions in France – Part A: time variations

Temperature inversions in mainland France are computed using a network of 859 sampling sites spread as evenly as possible across the country. Each site comprises a high station located within 30 km of the low station with which it is paired. We use the daily minimum (tn) and maximum (tx) temperatures from Météo-France weather stations over 10 years (2008–2017). The time variations of three inversion characteristics (frequency, intensity, and duration) are explored. The primary result is the high frequency of inversions. On average, for the 859 sites, tn-inversions occur on 63% of days and tx-inversions on 22.6%. Inversion intensity is usually weak, the mode being 2°C for tn and 1°C for tx. However, high intensities may be reached (maximum 23.6°C for tn and 22.6°C for tx). The longest sequence of isolated tn-inversions (the inversion is destroyed in the daytime but recurs over several days) ran for more than four months (126 days). Such inversions occur most frequently from August to October. The longest sequence of isolated tx-inversions (the inversion is destroyed overnight) ran for 38 days. Such inversions are most common from November to February. The longest persistent inversion (night and day) lasted 88 days. Persistent inversions occur most frequently from November to January. The results as a whole reveal a number of original points: the existence of a large number of tx-inversion sequences, the seasonal pattern very clearly differentiating tn- and tx-inversions, and the significance of atmospheric conditions.