Water temp above ground vs in ground

Over night temperatures will have a greater impact on an AGP vs IGP. The ground tends to hold heat, while the air temp can swing dramatically, which to an exposed AGP means temp drops that are much more noticeable.

Is this really true though? Isnt the ground temp in the summer cooler than the air and in the winter warmer than the air in general? Down below the freeze line I dont think the temp varies much. Probably down in the 40s. I would think an inground pool would always be battling the ground temp.

I would think the inground must have a heater.
 
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