About thermal admittance and difusivity
Hi,I have two questiones about thermal admittance and thermal diffusivity. -why soil with low thermal admittance subject to extreme surface temperature fluctuation? -why does high thermal diffusivity result in large and rapid subsurface temperature response to surface temperature change?
The answers to these 2 questions were covered in detail during the lecture #12, so I'll only briefly answer the 1st question here and you should try to use that explanation for the 2nd question.
Assume that there are 2 soil types (organic and mineral soil) adjacent to each other, receiving the same amount of solar energy, and having the same water content. Solar energy absorbed at the surface is converted to heat energy. In the organic soil, as surface temperature rises, a temperature gradient develops, causing some (but slow) heat conduction downward (due to low thermal conductivity). The heat energy tends to remain near the surface of this organic soil. The increased heat energy content makes soil surface temperature rise very high, because the low thermal capacity (Cv) means that even a little increase in soil heat content will cause a large increase temperature.
In contrast, mineral soil (having high thermal conductivity) will conduct heat rapidly away from the surface and (heaving high thermal capacity Cv) would need a great increase of heat content in the surface if it were to develop a high surface temperature.
Hi Maja/Sandra,
I was wondering if you could elaborate on the second part of this question. All i can gather from that lecture and slides is that higher thermal diffusivity means the more rapid thermal heat change at depth. If we drain said soil it would warm rapidly..... I find the lecture posted does not give enough background or depth to this. Its just scratching the surface.....why does high thermal diffusivity result in large and rapid subsurface temperature response to surface temperature change? a short to the point answer would suffice. Thank you
Steven, thermal diffusivity (slides 21-22 of lecture #12) is driven in part by thermal conductivity (which increases exponentially in mineral soils with a small amount of soil moisture - see slide 15). Consider 2 soils (1 mineral and 1 organic) both of which are subject to a 1 degree increase in soil temperature - the mineral soil will rapidly conduct heat downwards due to it's high thermal conductivity. While it takes more energy to warm a "wet" mineral soil, once the temperature increases, that heat will be conducted down.