Analysis of significant level radiosonde data from a network of Arctic stations reveals a systematic reduction in midwinter surface-based inversion depths over the past few decades, accompanied by a rise in surface temperature. Similar trends are observed over a wide sector, from 62¿W to 162¿W and from 70¿N to 83¿N. Possible causes for these changes include increases in warm air advection, cloud cover, ice crystals, aerosols, and greenhouse gases, but the specific reasons are difficult to identify, due to strong interactions between many potentially important factors. Nevertheless, the changes are significant for studies of Arctic haze, since the midwinter stable boundary layer has been decreasing in depth over time. ¿ American Geophysical Union 1993