When the surface of the earth summer, what time of the year at a depth of, for example, three meters beneath the surface? Do you think there summer? Wrong! The seasons on the earth's surface and in the soil are not the same as you might think. The soil is extremely poorly conducts heat. In Leningrad water pipes at a depth of 2 m will not freeze in the most severe frosts. Temperature fluctuations occurring on the earth's surface, extend deep into the soil very slowly and reach different layers of its very late. Direct measurements, for example, in Slutsk (Leningrad region) showed that at a depth of three meters, the warmest time of the year comes late in 76 days and the coldest late in 108 days. This means that if the hottest day on earth was, say 25 July, at a depth of three meters he will come only on 9 October! If the coldest day was January 15, at the depth indicated he will come in may! For deeper soil layers of delay will be even greater.
With the deepening of the soil temperature fluctuations not only late, but also weaken, and in some depth disappear completely: all the year round, during the entire centuries, invariably there is the same constant temperature, the average annual temperature of this place. In the cellars of the Paris Observatory, at a depth of 28 m, a century and a half is stored thermometer placed here Lavoisier, and for fifty years he didn't even flinch, always showing the same temperature (+11,7° Celsius).
So, in the soil, we shall be trodden down by the feet, never the same time of year, what stands on its surface. When above ground winter, at a depth of three meters further fall - though not the autumn, which previously existed on the earth's surface, but with a more moderate temperature decreases; when above ground summer, in a depth reaching the faint echoes of winter frosts.
It is important to keep in mind whenever we talk about the living conditions of underground animals (for example, larvae of the may beetle) and underground parts of plants. We should not be surprised, for example, the fact that the roots of our trees multiplication of cells is performed exactly in the cold half of the year and that the activities of the so-called cambial tissue freezes almost the entire warm season, just back than in the trunk, above the ground.