CloudWatcher K-factor optimisation tool

What is the K-factor?

The CloudWatcher's cloud sensor works by measuring the temperature of the sky in the infrared. On a clear night, the sky is cold: a good radiator of thermal energy. When clouds are present, the sky appears warmer because clouds radiate at a temperature much closer to the ground temperature.

The K-factor is a coefficient that scales the relationship between the measured sky temperature and ambient temperature to produce the "cloud condition" value that the software uses to determine safe/unsafe status. The correct K-factor depends on your climate: humid climates, high altitudes, and different latitudes all affect the baseline sky temperature on clear nights.

The optimisation tool

Getting K-factors right used to be an arduous process—many owners just change their safety limits a couple of times a year instead, which works but takes more effort in the long run. Committed CloudWatcher owner Joerg Klaas built a tool that finds and optimises the perfect K-factor settings for you, by simulating different scenarios and temperature ranges (including humidity climate effects). In his own words, it simulates around 6 million calculations and finds the sweet spot automatically, with visual diagrams showing how results change as you adjust the factors.

We're extremely grateful to Joerg for developing this optimiser and sharing it with the Lunaticoastro community.

Running the optimisation

1. Collect CloudWatcher log data from your site—at least a week of varied conditions is ideal
2. Load the log file into the optimisation tool
3. The tool plots sky temperature minus ambient temperature (the "corrected sky temperature") against time, annotated with your existing safe/unsafe annotations
4. Identify the K-factor value that best separates your clear-sky baseline from cloudy readings
5. Apply the new K-factor in the CloudWatcher software settings

For full documentation and download links, see the CloudWatcher software and downloads page.