A cold-air comma cloud crossed southern Britain early on 18 February 1997 bringing strong gusts and convective precipitation. Development of the comma cloud occurred in association with a positive upper tropospheric potential vorticity (PV) anomaly. A mesoscale analysis of the comma cloud was carried out using special three-hourly soundings from UK radiosonde stations together with data from rainfall radar, satellite, NWP model output and a wind-profiling radar. The analysis reveals that the comma cloud tail was of ana-form whilst the polar cold front (behind which the comma cloud developed) was kata-form. Further analysis indicates that the polar cold ?w front and the comma cloud front were tending to become one entity at the surface and can thus be considered an example of a kata-cold front undergoing a partial transition to more of an ana-form locally in the region of an upper tropospheric positive PV anomaly.