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
The K-factor optimisation tool takes a dataset of logged CloudWatcher readings from your site — ideally spanning a mix of clear nights, thin cloud, and overcast conditions — and helps you analyse where the threshold should be set to correctly classify each condition. It presents the data visually, making it easy to identify the K-factor value that gives the cleanest separation between clear and cloudy readings for your specific location.
The tool was designed to eliminate the guesswork from K-factor calibration. Rather than trial-and-error adjustment over many nights, it uses actual data from your site to make the decision evidence-based.
New installations: If you are setting up a new CloudWatcher, collect at least 5–10 nights of data across different conditions before running the optimisation. The more variation in the dataset, the more confidently the tool can identify the right K-factor for your site.
Running the optimisation
- Collect CloudWatcher log data from your site — at least a week of varied conditions is ideal
- Load the log file into the optimisation tool
- The tool plots sky temperature minus ambient temperature (the "corrected sky temperature") against time, annotated with your existing safe/unsafe annotations
- Identify the K-factor value that best separates your clear-sky baseline from cloudy readings
- Apply the new K-factor in the CloudWatcher software settings
For full documentation and download links, see the CloudWatcher software and downloads page.