Check here: http://www.greenbuildingtalk.com/Forums/tabid/53/aff/13/aft/31492/afv/topic/Default.aspx http://www.greenbuildingtalk.com/Forums/tabid/53/aff/13/af

Check here: http://www.greenbuildingtalk.com/Forums/tabid/53/aff/13/aft/31492/afv/topic/Default.aspx http://www.greenbuildingtalk.com/Forums/tabid/53/aff/13/aft/... The 3rd post talks about the variation in temperatures. Seems that once you get to around 30 feet down the temperature becomes constant year round. For Houston the chart says 71 degrees. Because it's that single number instead of a range I assume that's the temp you get at 30 feet deep.