AbstractThe role of the Conditional Symmetric Instability (CSI) in the formation of convective systems in the cold airmass located at the rear of a secondary cold‐frontal system is evaluated. This study is based on observations made on 4 June 1984 during the French experiment LANDES‐FRONTS 84. In particular, the mesoscale three‐dimensional circulation deduced from Doppler‐radar data within one of these convective systems is studied in detail and compared with that predicted by the CSI theory. In spite of the fact that, on the one hand, notable convective‐scale motion masks the mesoscale circulation, and that, on the other hand, the synoptic‐scale circulation is poorly described, this comparison shows that, among the various theories involved at the mesoscale, only CSI gives a proper account of the observed mesoscale