How this reading happens

A waterproof temperature probe sits on the filter return line at the Monte Sano Club pool. While the filter pump is running, water from the pool flows past it constantly — so what it measures really is the pool's water temperature, minute by minute. When the pump cycles off, the water sitting in that pipe cools toward the air around it, so the sensor reads low. mswx automatically filters those out so the chart shows actual pool conditions, not pipe conditions.

How the forecast works

A pool is a big slug of water, so it changes temperature slowly — it takes a couple of days for the weather to fully soak in. mswx starts from the latest sensor reading and steps a small physics model forward hour by hour: the water trades heat with the air, the sun warms it during the day, and it loses a little to evaporation and to the night sky.

For the weather it needs — air temperature, sunshine, humidity, wind — a single forecast isn't enough. mswx runs the model against four different global weather models and averages the results. No single model is reliably best a few days out, but the average is steadier than any one of them. For the sunshine specifically it uses each model's own solar-radiation forecast rather than guessing it from cloud cover, which turned out to be the biggest single improvement.

It's most accurate for today and tomorrow, where it leans on the live sensor reading. Further out it's really a forecast of the weather, so the error grows — figure within a degree or so for the next couple of days, and treat the back half of the week as a rough outlook. mswx keeps checking it against what the sensor actually records and tuning it.

For a very detailed explanation, click here.