Quick Check: water surface temperature

The check: Daily average water surface temperature is usually well correlated with the lagging three day average of the daily maximum air temperature. This is robust enough that it can be used as the basis for an interpolation model, and outperforms a lag of the average air temperature.

Yes, this summary does skip a lot of analytical details: the tables of correlations between site observations for multiple sites, the various linear models fit and tested, and the analysis of the overlap of gaps in data. This method was found to be a particularly robust method, since daily high/low temperatures are frequently available from multiple sources at a research site. A perfect model for filling in missing data does not help if the data it needs is missing at the same time!

Supposition:
Both daytime temperature and water warming seem likely to covary with daytime insolation - a major source of heating.  Water cools off a lot by evaporative heat loss, which will drop when the air is cool. The air primarily cools off by radiative cooling, which also happens to water but perhaps not at the rate that evaporative cooling happens.

Aside:
I have measured clear blue sky with my little non-contact thermometer as being 4F (-14C) in mid-afternoon in Colorado on an 80F day. There's a lot of potential in radative cooling.