Tuesday, April 24, 2012

Ground Temperature

I'm in Florida, and the floor was warm this morning to my bare feet..  Not surprising you may say.  But usually a concrete floor appears relatively cool to us, since it takes away some of the body heat.  The rest of the house is raised up with a crawl space, and we had a pretty cool night.  So the wooden floors were cooler, but when I went down into the part that used to be a garage and is slab on grade, that's when the floor seemed relatively warmer.  It was warmer because the weather has been hotter and then turned cooler, and the ground and the floor is slow to change; but ti made me think about the whole subject of thermal mass and the ground temperature
We sometimes see blanket statements like "the ground temperature is at  60 degrees", which is approximately true for much of the United States - but in fact it depends where you are talking about.  Specifically, the ground temperature more than six feet from the surface is constant, and is the same as the average annual temperature for that location.  Down here in South Florida we have an average temperature of around 75 degrees, and that is the deep ground temperature.  You can see the whole range of annual temperatures from the picture, which is the daily temperature graph for West Palm Beach in 2011.

These excellent sources of climate data are now available free from http://www.ncdc.noaa.gov/oa/mpp/ , it's a great tool for checking heating and cooling strategies for a location.  From this graph, for example, we can see that for much of the Summer not much relief is available from nightime cooling,  since temperatures do not go below 70 deg.