The above diagram is taken from p303 of David MacKay’s book Sustainable Energy – without the hot air. It shows how temperature in the ground varies with depth and time of year if the surface temperature varies as a sine function with a minimum of 3°C and maximum of 20ºC. You can see the surface variation by reading off the temperatures from the horizontal slice through the diagram at depth 0. Further into the ground, at depth 2 for example, you can see that the temperature is colder, varies in the range 10-13ºC, and the minimum temperature lags behind the surface temperature by about 5 months. More rapid temperature variations, within a day and from day to day, are ignored, but these would be very small at almost any depth. The correspondence between depths 0, 1, 2, 3 and depths in meters depends on the ground material; examples from MacKay are given in the table below.
|Granite||Damp sandy soils
|Dry or peaty soils|
Of course, this would be a good predictor of temperature variations in your wine only if you had a few bottle buried outside, and perhaps in some commercial cellars. Domestic cellars are usually directly below houses, and thus have surface temperatures that are both higher and more constant than those assumed in the diagram, and there are not metres of earth or rock between the cellar and the house. But even then, the diagram does perhaps give some idea of the cooling effect from cellar walls. If you are keener than me you could try doing calculations more relevant to wine cellars – MacKay gives you some equations you could use at a starting point. Or you could just put a thermometer in your cellar.
Another wine application of these calculations would be to predict the temperature of vine roots at various depths. I have no idea how important root temperature is to the vine, but if it is you now know where to get the equations.
Finally, a big thank you to David MacKay for allowing his material to be used under a Creative Commons licence. See the website for details, but amongst other things it allows you to download the book free of charge. I admit I have by no means read it all, but it looks well written and very interesting.