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Soil surface temperature – This is the temperature of its upper layer (a few millimeters thick) free of vegetation, well loosened and not shaded from the sun, and in winter, when there is snow cover, the temperature of the snow surface. It is measured by a thermometer lying open on the surface of the soil and snow cover, while the thermometer tank is immersed in the soil (snow cover). Measuring the temperature of the soil surface is very difficult due to the inability to obscure the thermometer from the effects of radiation and due to the difference in the radiation properties of the reservoir and the soil (snow).

The soil temperature at the depth of the tillering node of winter crops is measured at the time of observation, and between the time of observation the minimum and maximum temperature in the soil layer at a depth of 2.5-3.5 cm from the ground surface (° C) is measured by special maximum-minimum thermometers.

The temperature of the soil and soil at the depths (soil horizons) is the temperature determined by the readings of thermometers and other sensors installed at certain depths. At meteorological stations, the soil temperature at depths of 5, 10, 15, 20 cm in the treated areas without vegetation is determined in the warm season with the use of TM-5 Savinov thermometers; at depths of 20, 40, 80, 120, 160, 240 and 320 cm – station exhaust soil-depth thermometers under natural cover.

Soil temperature has a significant impact on the formation of the thermal regime of the atmosphere. Data on soil temperature is needed to solve many applied problems: they are used in agriculture, in construction, in the operation of roads and underground utilities, and. etc.

The thermal regime of the soil is determined by the influx of heat and depends on the mineralogical composition of the soil, porosity and humidity, which determine its heat capacity, thermal conductivity, and also depends on microrelief, slope exposure, vegetation, etc.

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The main source of heat entering the soil is the radiant energy of the sun, which is absorbed by the surface layer. This heat is transferred to the underlying layers, and is also spent on air heating and water evaporation.

The soil layer in which daily and annual fluctuations in temperature are detected depending on the influx of solar radiation is called the active or active layer.

Patterns of heat distribution in the soil

Daily and annual fluctuations in the temperature of the soil surface due to thermal conductivity are transferred to its deeper layers. The spread of temperature fluctuations into the soil (with a homogeneous soil composition) occurs in accordance with the following Fourier laws:

  • The oscillation period does not change with depth,

The soil layer, the temperature in which does not change during the day, is called the daily temperature layer. In middle latitudes, this layer starts from a depth of 70-100 cm. A layer of constant annual temperature in middle latitudes lies deeper than 15-20 m.

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  • Maximum and minimum temperatures at depths occur later than on the soil surface. This lag is directly proportional to the depth. Daily maximums and minimums are delayed for every 10 cm depth on average by 2.5-3.5 hours, and annual for every meter of depth are delayed by 20-30 days.

According to the theoretical calculations of Fourier, the depth to which the annual variation in soil temperature is manifested should be approximately 19 times greater than the depth of manifestation of daily fluctuations. In fact, there are significant deviations from theoretical calculations, and in many cases the depth of penetration of annual fluctuations turns out to be more than estimated. This is due to the difference in soil moisture in depth and in time, the change in thermal diffusivity of the soil with depth and other causes.

In northern latitudes, the depth of penetration of the annual course of soil temperature is on average 25 m, in middle latitudes – 15–20 m, in southern ones – about 10 m.

Thermal Isoplates

Materials of long-term observations of soil temperature at various depths can be represented graphically.

Fig. 3 – Isoplet soil temperature for St. Petersburg.

On such a chart, soil temperature, depth and time are related. For plotting, the depths are laid on the vertical axis, and time (usually months) on the horizontal axis. The average monthly temperature of the soil at different depths is plotted on the chart. Then the points with the same temperature are connected by smooth lines, which are called thermal isoplets.

Thermal isoplets provide a visual representation of the temperature of the active layer of soil at any depth in each month. Such graphs are used, for example, to determine the depth of penetration of critical temperatures that damage the root system of fruit trees. These charts are also used in the public utilities, in industrial and road construction, and during amelioration.

The thickness of the frozen layer is necessarily taken into account when laying drains (pipe or underground channel for drainage of groundwater) in reclaimed areas.

In the Leningrad Region, the agrometeorological stations and posts at which the temperature of the soil surface is monitored are located in Sosnovo, Tikhvin, Volosovo, Belogorka, Nikolayevsk, Lyuban, Kolpino, Kipeni and Osmino.

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