Night Sky Pi Moisture Battle, Part 3: Data In, Heat Out

For the entire month of May the sensors from Part 2 ran on the Night Sky Pi, logging conditions inside the dome and outside for comparison. The aim was simple: collect enough data across different weather to reach a confident decision on how to stop the dome fogging.

Preparing the data

Before any analysis the logs needed cleaning. The OpenWeather API occasionally failed to respond, so those missing timestamps were removed from the API series. To keep comparisons fair, the same timestamps were also removed from the dome and external sensor series. After that, all three series were aligned so the graphs compared like with like.

OpenWeather API vs external sensor

The day–night cycle is obvious on the graphs. The external sensor repeatedly shows daytime spikes that the OpenWeather data does not. The sensor itself is fine. The likely culprit is its housing, which traps heat and warms the air around the probe when the sun hits it. Later in the day, the location moves into shade and the sensor cools more aggressively, which shows up clearly each evening and overnight.

External vs dome sensor

The dome sensor shows the same daytime spike pattern, only worse. Heat builds quickly inside the enclosure and then drops fast once direct sun ends. Electronics inside the case add to the heat load. Peaks reached around 70 °C, which is not safe for hardware longevity or consistent performance. That means moisture at night is not the only problem. Heat build-up during the day is a root cause that must be fixed first. Warmer air can hold more moisture and it also cools faster, so starting the evening at a higher temperature makes condensation more likely once conditions drop toward the dew point.

What this means

Two issues are now clear:

1. Heat needs to be ventilated. Circulate air inside the enclosure and likely exchange some of it with outside air to prevent the dome from becoming a hot box during the day. Balancing internal and external conditions should also reduce the evening moisture load.
2. Residual moisture still needs control. Even with better airflow there will be nights where temperature approaches dew point. A simple resistor-ring heater will raise the dome air a few degrees above dew point, switching on only when needed. Combined with circulation, that should keep the dome clear and gradually purge moisture from the enclosure.

The data did its job. First fix the heat, then manage what little moisture remains with dew-point-aware heating.

Thanks for reading. If you have a compact airflow design for small outdoor enclosures or a reliable resistor-ring layout, share it so others can try it too.