Underground urban heat island below Moscow city

• The effect of “Underground urban heat island” was detected in Moscow region.
• Mean difference between urban and rural values of soil temperature is equal to +0.6…+0.8 °C.
• The most probable reason of the existence of this phenomenon is human activity which has resulted in a lot of factors.
• Soil temperature at the depth of 160 cm increased in Moscow city by 1.8–1.9 °C during last 116 years.

Long-term measurements of the soil temperature on different depths in Moscow and its suburbs have been collected since the end of the XIX century. At Moscow University this parameter has been measured simultaneously at two locations since 1954: one below the usual natural cover and one below a special naked area. Following the air warming, in the last 116 years the soil temperature increased in Moscow by 1.8–1.9 °C (at 160 cm depth). During the last half a century the temperature increased much faster at the University below the naked area (+0.04 °C/year) than below the natural cover (meaning the snow cover in winter and grass and sod cover during other seasons): +0.02 °C/year.The spatial distribution of the soil temperature in Moscow region has been discussed using the simultaneous data of ground meteorological network. Mean annual values of the surface temperature (0 cm) and soil temperature at eight depths from 20 to 320 cm have been analyzed. Mean annual soil temperature in the city center is higher by up to 1.0–1.2 °C than at the city periphery and by up to 1.6–1.7 °C than in Moscow region rural zone (at the depths of 120 and 160 cm). At smaller depths these differences are a bit less but still statistically significant. Thus, an underground urban heat island exists at any depth. The evident cause of this phenomenon is human activity which has resulted in a lot of factors. A special index has been suggested for estimation of the average intensity of the underground urban heat island. According this index, the mean difference of the soil temperatures can amount to +0.6…+0.8 °C if we compare the city with the rural zone and from +0.4 to +0.6 °C if we compare only urban periphery (without city center) with the rural zone. Another discovery was that the underground heat island evidently extends below the depth of 320 cm. The annual dynamic of the difference between urban and rural soil temperatures reaches its maximum in winter (+0.9…+1.2 °C depending on the depth) due to strong urban heating and drops to its minimum in summer (−0.5…+0.4 °C at different depths).