The annual cycles of soil frost

When the ground temperature drops below freezing, any water contained in the soil freezes. This results in soil frost, or soil that is frozen solid. Soil frost occurs in Finland every winter.

It first affects the surface soil after a week or two of very low temperatures. If sub-zero temperatures continue, the freezing progresses ever deeper. In Lapland, frost depth can reach one metre in late winter, and sometimes even more. In Southwest Finland, the soil may remain completely frost-free in a mild winter.

Factors affecting the occurrence of soil frost include not only freezing temperatures but also plant cover and topography. The most important factor is snow, however. A snow cover of 10 to 15 centimetres is all that it takes to prevent the soil from freezing. This is why in Eastern Finland, where plenty of snow falls most winters, frost depth is usually no more than 10 to 20 centimetres. There are major variations from year to year, however.

Massive soil frost increases soil volume. Capillary action pulls more water into the frozen soil, where it freezes; as a result, the soil surface may go up by dozens of centimetres. This type of frost heaving is typical of compact soil types, including clay soils. Looser soils with a crumb structure are not affected by frost heaving, at least not to the same extent. They develop cavity frost with rod-like crystals.

While there is no actual permafrost in Finland, soil frost that is up to 2,000 years old can be found in the rare palsa mires of Northern Lapland. It is preserved at the core of peat mounds (palsas) throughout the summer and expands year after year, until the peat layer covering the palsa mound finally tears and wears away. The soil frost then melts and the palsa mound collapses.

Vesi.fi: Soil frost situation