A 29-year daily temperature record at 30 hPa in the northern hemisphere stratosphere has been used to estimate the potential of the atmosphere to support polar stratospheric cloud (PSC) formation. Under certain caveats, type 1 PSC formation is assumed to occur at temperatures of 192 K (195 K) or lower at 30 hPa (50 hPa). At 30 hPa in January at least one point in the northern hemisphere was colder than 192 K on 34% of the days in the record. The PSC ''season'' begins in late November and ends in middle March; the maximum likelihood is attained in January. The PSC formation potential is defined as the fractional area of the northern hemisphere cold enough (on the synoptic scale) to support PSC formation. In the mean it exceeds 1% between late December and early February at 30 hPa and is generally larger at 50 hPa. There is considerable interannual variability connected with the synoptic evolution of the stratospheric winter, so that up to 4% of the northern hemisphere is cold enough for PSC formation for shorter periods in individual years. Temperatures low enough for H2O ice cloud formation can occur in January and February; these are generally associated with adiabatic ascent in amplifying planetary waves. The location of the cold areas relative to the vortex center is examined and a measure introduced to quantify this. ¿ American Geophysical Union 1995