A new climatology of conditions that are favourable for development of polar lows in reverse-shear flow is presented. In such flow, the wind at the low-level steering level is in the opposite direction of the thermal wind in the adjoining layers. A framework for identifying such conditions along with weak lower-level static stability from any gridded data is developed by defining simple dynamical constraints on standard atmospheric fields, applied here to 40 yr of ERA-40 data. The relevance of the constraints is directly demonstrated using satellite images. There are several areas where such conditions occur with high frequency: the Norwegian Sea (>15% of the time during NDJFM), the region to the south of the Denmark Strait (>10%) and the Bering Sea and the Sea of Okhotsk (>5%). In the Nordic Seas region, the polar low season is longer than in the Pacific because the air temperature stays low throughout March. There are primary peaks in December and January and a secondary peak in March, preceded by a distinct nadir in February. A link between the NAO and reverse-shear conditions is suggested.