Is the Outside Air Temperature a Lagging Indicator?

i've got quite obsessed, as you know, with cold water swimming. There's a time series of the temperatures in the ponds going back ten years. It must be possible to know thatt the outside air temperature over those ten years. Could some one plot the relationship between the two of them? That means that the temper ef the pond temperature, is going macely. The the water temperature, i think, is a lagging indicator. It lags behind the air temperature. Ah, it doesn't just immediately cind of changebu but i don't quite fully understand the relationship. So september's often a good month for water temperature,. if you like it warm