Permafrost
Where subsoil temperatures remain below zero for at least 2 consecutive years, permafrost will occur. It’s estimated that permafrost cover around 25% of the earth’s surface. When summer temperatures rise above freezing, the surface layer thaws from the surface downwards to create the active layer, the thickness of which depends on local conditions, but may extend to 4m. As the ice in this layer melts, large volumes of water are released, which is unable to drain through the permafrost layer and the surface becomes very wet (it’s too cold for evaporation to occur). On slopes are gentle as 2*, this saturation of the active layer encourages soil movement downslope, which is known as solifluction.
Continuous Permafrost
Found in the coldest regions, reaching deep into the surface layers
In Siberia, it is estimated that the permafrost can reach down over 1500m
In the coldest areas, there’s hardly any melting of the active layer
Discontinuous Permafrost
Found in slightly warmer regions, where the ground isn’t frozen to such great depths
On average, it will reach 20-30m below surface – although it can reach 45m
There are gaps in the permafrost under rivers, lakes and near the sea because water is a good conductor of heat and therefore warms the ground
Sporadic Permafrost
Found where mean annual temperatures are around or just below freezing point
It only occurs in isolated spots where the local climate is cold enough to prevent the complete thawing of the soil during summer

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