ИСТИНА

The ‘underground urban heat island’ (UUHI) is steady positive thermal anomaly in the upper soil layer below a city [1]. Its intensity is a difference between soil temperature TS at an urban station and a rural one. Meteorological observatory of Moscow University (in the city) and ‘Podmoskovnaya’ station (in the western suburbs) were chosen as two reference stations having the most reliable data. Evidently, any changes of the measurements conditions such as replacement of a station, change of a sensor type, etc., may strongly influence on the results. So, only the period of homogeneous data when measurements of TS were carried out at the same places by the same instrumentation was taken into account: from late 1960s to recent years. As a result, the UUHI intensity below Moscow city at the soil depth up to 3.2 m was found as nearly the same during the last half a century. Unlike classic UHI in the air temperature field, UUHI does not demonstrate its strengthening. Moreover, the UUHI intensity surprisingly became even a bit less during the last 50 years. On average for the period 1967-2016 TS increased at both stations, but the soil warming on the 160 cm depth at rural station was a bit faster (+0.03 ºC/year) than at the University (+0.02 ºC/year). Thus, the linear trend of the UUHI intensity was a bit negative, although its absolute value is insignificant. For the deepest available depth (320 cm) the UUHI intensity linear trend was the same (–0.01 ºC/year) whereas for the closest to the ground 20 cm depth it is zero with an accuracy of ±0.001 ºC/year. Specific factors which affects TS (e.g., change of the underground water level, etc) should be taken into account.