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
Underground mine ventilation the permafrost heating may not take place to prevent melting the permafrost. "Cold mines" such as Raglan Mine and Nanisivik Mine are designed to operate below 0°C. The wet bulb temperature in any working place does not exceed 33.5°C and where the wet bulb temperature exceeds 30.5°C arrangements are made to ventilate the same with a current of air moving at a speed of not less than one metre per second; Underground mine ventilation Underground mine ventilation provides a flow of air to the underground workings of a mine of sufficient volume to dilute and remove dust and noxious gases