Here is a chart of the temperatures over the last few days.
Tue 4th March 2025
We have repositioned the shallow sensor a little lower in the water - it was dangling in the air from yesterday afternoon. We have removed the erratic measurements from the chart.
Mon 3rd March 2025
Oops - it looks like the shallow water sensor has, ahem, come out of the water. We hope to fix this tomorrow. Meanwhile, glitches notwithstanding, it's clear that the surface water is getting warmer in the afternoons. Such a shame that the pond closes at 2pm :sigh:. The overnight frosty airs brings the temperature back down of course. We have not yet investigated why the lifeguards are measuring a degree or two lower than our sensors.
Wed 29th Jan 2025
We finally got the temperatures updating automatically; at the moment it's within a minute or two of the temperature being taken; we may slow this down in order to make the pond battery last longer. You may be wondering why the deep water seems to rise when the air and shallow water rise. It turns out that the ground temperature a few meters down is fairly constant, and is basically the average of the air temperatures throughout the year (around 11.5°C). So it seems reasonable to expect that the deep water will be slightly warmed by the ground in the winter when the air is cold. (Presumably "a few meters down" will be deeper under the pond than under the land, so the deep water will be cooler than the ground temperature elsewhere. Remember that above 4°C, warmer water will rise and mix with cooler water above, so the bottom of the pond will not be much warmer than 4°C.)
Tue 14th Jan 2025
We have moved the "shallow" sensor up to the top inch or two of the water, similar where the lifeguards read from. At this level we are reading between 2 & 3°C, a degree or so lower than just a foot or two down. You can see from the graph that the temperature here is more volatile. This top layer of water is going to be what you feel on your face when you swim; but the rest of you is mostly lower down, and the top layer will of course be easily be disrupted by swimming. Our deeper sensor is still getting around 4.5°C. We moved the shallow sensor yesterday (Monday) around 11AM; it didn't make much difference at the time. However it has made more of a difference overnigth when the air temperature dropped.
Fri 10th Jan 2025
We have ice around the south bank. When the temperature drops below 4°C, a thin layer of cooler water "floats" on the surface, and can cool down to freezing, and ice also floats of course. Our "shallow" sensor is about 6/12 inches below the surface, so reads slightly warmer water. I think the lifeguards read from the top inch or so, where the temperature is a bit lower. We will investigate attaching a float to our shallow sensor so we can capture the thin cooler layer of water, although this will not be stable, and is not representative - swimming in it will disrupt it. But we all like to brag to our friends that we swam in ice, don't we? :)
Sun 5th Jan 2025
The shallow and deep temperatures have diverged to nearly a degree apart in the snow. The "deep" one is only 1.5m down, and near the edge; it's confusing that it has risen ahead of the shallow one as the air warmed up today. I suspect it is affected by diffusion and convection currents from deeper water, which may be even warmer. We hope to add more sensors in future to shed more light on quirks like this.
Sat 4th Jan 2025
You can usually see that the water temperature slowly moves towards the air temperature. The warm winds from the south west over New Year warmed the pond by nearly a degree, to 7°C. The last couple of frosty nights have dragged it down to 5°C. The surface temperature has fallen more than the deep one. This may go further if the surface cools below 4°C the temperature at which water is most dense. Below 4°C cooler surface water "floats" like ice; in fact this helps the formation of ice.
Aerators
HMPA is concerned that the aerators are not working this winter. They help to mix the water near the jetty, to keep the ice at bay, at the cost of cooling the deeper water by mixing it with the cooler shallow water.
General points
Where there are gaps or flat sections, it's typically where the battery died or the WiFi failed.
The temperatures in the table are averaged over the day (in contrast to typical weather reports which tend to give min/max; we may try this in future); today's temperatures are averaged over the last half hour or so (but for now it is updated manually one or twice a day so will be a bit out of date). The points in the graph are averaged over 10 minutes.
The lifeguards' thermometer often reads a degree or two lower than our sensors; we use DS18B20 sensors which have a tolerance of +/- 0.5C.
