What "ambient temperature" means for an astronomical sensor
Standard weather stations define ambient temperature as the air temperature measured in the shade, away from direct sunlight or radiated heat. Professional setups use dedicated radiation shields to achieve accurate readings in direct sun. For astronomical purposes—cloud detection and dew point calculation—the accuracy requirements are modest, but the source of the measurement still matters.
How original CloudWatcher units measured temperature
Early CloudWatcher units used the internal thermometer of the infrared (IR) sensor to estimate ambient temperature. Because the sensor is inside the unit and the unit itself absorbs some heat, readings were typically a few degrees higher than true ambient, but this proved sufficient for cloud detection calculations, which compare sky temperature against ambient, not against an absolute reference.
The updated design—external humidity and pressure sensors
Current CloudWatcher units include an external relative humidity sensor and an atmospheric pressure sensor, each with their own integrated temperature measurement. These sensors are housed in a ventilated, shielded enclosure fitted with an aluminium heatsink, mounted below the main unit where airflow is better.
This arrangement gives significantly better ambient temperature accuracy than the internal IR thermometer because the external sensors are less affected by heat from the electronics. The result isn't as good as a big, dedicated radiation-shield enclosure—but it's much smaller and more convenient, and still quite accurate.
Choosing the right temperature source in the software
The CloudWatcher software lets you select which sensor is used as the ambient temperature reference. The available options depend on your hardware version.
| Source | Where it reads from | Accuracy |
|---|---|---|
| IR sensor (internal) | Thermometer inside the infrared sensor | Fair |
| Humidity sensor | External humidity sensor with integrated temperature | Good |
| Pressure sensor | External atmospheric pressure sensor | Good |
| Lowest value | Takes the lowest reading from all available sensors | Best |
Recommended: select "Lowest value"
The software documentation recommends using the Lowest value option. Since heat from electronics tends to push sensor readings slightly high, the lowest reading among all available sensors is statistically the closest to true ambient temperature.
Important: the ambient temperature source you select—including any calibration offset applied—feeds into calculations for both relative humidity and atmospheric pressure. Choosing a more accurate source improves the accuracy of these derived values too.
Applying a calibration offset
If you have a reference temperature sensor nearby (such as a professional weather station) and notice a consistent offset, you can apply a calibration offset in the software settings. This adjusts the displayed value without changing the underlying sensor reading.