Beating a dead horse as we can often do in regards to tea ...
I actually have been doing some more serious observations of water temps in a yuzamashi (taller version) based upon discussion in two other current topics ... and some cursory observations I had noticed for quite some time while using a digital thermometer (DT) that registersw readings very quickly (some DTs are much slower).
Today (temps in F):
First I placed the digital thermometer probe (DTP) all the way in and it is resting upon the bottom, 150-155*
Raising the DTP even slightly off the bottom instantly raises the temp 10ish* to 160-165*
Slowly raising the DTP upward the reading gradually rises as well.
As I near the surface, maybe an inch deep, the temp is hottest, 175-180*
Then as the DTP gets closer and closer to the surface, the temp decreases slightly.
< 1/4" deep the temp is still around 165-170* (I will try to get as shallow as possible and see if I still get a reading)
Almost all the differences in temp are rather alleviated by swirling the water a little bit with the DTP, but still the bottom surface remains much cooler. Even <1/4" deep gives a very similar reading as plunged deeply into the Yuzamashi. Similarly, raising the Yuzamashi and swirling the entire thing has the same effect. Allowing the Yuzamashi to sit again, the differentiation again occurs, but not as dramatically.