Permafrost by permafrost. Of this area slightly more than half is underlain by continuous permafrost, around 20 percent by discontinuous permafrost, and a little less than 30 percent by sporadic permafrost. Most of this area is found in Siberia, northern Canada, Alaska and Greenland. Beneath the active layer annual temperature swings of permafrost become smaller with depth. The deepest depth of permafrost occurs where geothermal heat maintains a temperature above freezing. Above that bottom limit there may be permafrost, whose temperature doesn't change annually—"isothermal permafrost". Permafrost typically forms in any climate where the mean annual air temperature is less than the
Permafrost down to . Discontinuous permafrost is often further divided into "extensive discontinuous permafrost", where permafrost covers between 50 and 90 percent of the landscape and is usually found in areas with mean annual temperatures between , and "sporadic permafrost", where permafrost cover is less than 50 percent of the landscape and typically occurs at mean annual temperatures between . In soil science, the sporadic permafrost zone is abbreviated SPZ and the extensive discontinuous permafrost zone DPZ. Exceptions occur in "un-glaciated" Siberia and Alaska where the present depth of permafrost is a relic of climatic conditions during glacial ages where winters