Question 10 feet below in chennai soil is temperature lower than surface? how much? where to get details for all India
Answer Dear Mr. Eapen
Note 1: Geology of Chennai
Chennai's soil is mostly clay, shale and sandstone. Sandy areas are found along the river banks and coasts, such as Tiruvanmiyur, Adyar, Kottivakkam, Santhome, George Town, Tondiarpet and the rest of coastal Chennai. Here rainwater runoff percolates quickly through the soil. Clay underlies most of the city including T. Nagar, West Mambalam, Anna Nagar, Villivakkam, Perambur and Virugambakkam. Areas of hard rock include Guindy, Perungudi,Velachery, Adambakkam and a part of Saidapet.
Note 2 : Temperature in Chennai.
The hottest part of the year is late May and early June- Maximum temperatures around 38-42 °C (100-107 °F). The coolest part of the year is January, with minimum temperatures around 18-20 °C (64-68 °F). The lowest temperature recorded is 15.8 °C (60.44 °F) and highest 45 °C (113 °F). These temperatures can be normally attributed to the surface temperature of soil.
Note.3: Properties governing subsoil temperature:
Changes in soil temperature are governed by its thermal properties, viz. conductivity, diffusivity, and heat capacity, in addition to solar radiation. Of these properties, heat capacity of soil is the one that controls temperature fluctuations to a larger extent and refers to the amount of heat existing in it. It is estimated from the mass or volume fractions and respective specific heats of different soil constituents such as minerals, organic matter, water content, and air.
Soil thermal conductivity measurements describe the soil properties which govern the flow of heat through the soil. The thermal conductivity is defined as the quantity of heat that flows through a unit area in a unit time under a unit temperature gradient.
Note.4: Important Property of sub-soil
Thermal conductivity values for Sand stone 1.83 - 2.90 (W•m−1•K−1) and that of clay is the thermal conductivity in (W•m−1•K−1) is 0.25 for no soil moisture, about 1.0 for 10% soil moisture (% by volume), 1.25 at 14% 1.67 at 30%, and about 2.0 at 50%.in Chennai area most of the subsurface lithology contains clay with moderate to low thermal conductivity, hence normally in many areas the temperature of the soil just below the topsoil which is normally dry soil is high and it may be slightly lower than that of the surface temperature as mentioned above (may be one or two degrees). However this may not be true when we go still deeper because of presence of highly weathered and weathered material followed by massive rocks where the temperature gradient would be more,
Further the presence of moderate rooted plants that tries to suck water from moderately deeper water table can also cause localized lowering of temperature just below the sub-soil.
This subject is highly interesting and day by day research is going on and you can get very good literature on this topic from : M.K. Ghosal , Centre for Energy Studies, Indian Institute of Technology, Hauz Khas, New Delhi, India, email: M.K. Ghosal, G.N. Tiwari *, N.S.L. Srivastava, M.S. Sodha (gntiwari@ces.iitd.ernet.in)
Thanks,
Manoharan Kothandapani alias Manoharan
