The core principle
A clear sky has a lower infrared reading than a cloudy one. Clouds radiate more heat downward than a clear atmosphere does — and the CloudWatcher's IR sensor detects this difference. When the measured temperature rises above a threshold, the sky is classified as cloudy or overcast.
This is reliable in most situations, but the actual numbers depend heavily on local conditions: your altitude, typical humidity, local pollution, and the seasonal atmosphere at your site. This is why K-factors exist — they are site-specific calibration parameters that translate raw IR readings into meaningful safe/unsafe decisions.
What complicates detection
Multiple atmospheric factors can shift IR readings without clouds being present:
- Humidity: High atmospheric water vapour increases IR emission even in clear skies
- Dust and aerosols: Particulates scatter and emit IR, raising readings on clear but hazy nights
- High, thin cloud: Cirrus and high-altitude haze are the hardest to detect — they may raise IR readings only slightly above a clear sky reading
- Nearby heat sources: Buildings, rooftops, and warm surfaces can contaminate the sensor field of view if the CloudWatcher is mounted too close
There is no single formula that accounts for all of these contributions accurately for every site. This is why calibration and local tuning matter.
Two calibration approaches
Frequent manual calibration
Observe the cloud condition graphs (in person or via an all-sky camera) on a clear night and adjust the clear/cloudy/overcast boundaries to match what you see. A 5–10° range between the cloudy and overcast thresholds is a reasonable starting point. Repeat when seasons change significantly.
K-factor determination
The more systematic approach: observe cloud condition graphs during daylight hours on clearly sunny days and determine the optimal K-factors following the procedure in the CloudWatcher online manual. This produces site-specific parameters that remain valid for longer periods.
A community-built automated tool can also run the simulation for you — see the K-factor optimisation tool.
Installation location matters
Mounting the CloudWatcher on a rooftop is convenient but not ideal. Heat radiating from roof tiles, absorption of sunlight, and temperature gradients near structures all affect the sensor's readings. A free-standing position away from heat sources gives cleaner data.
Why a rain sensor is essential
Even a perfectly calibrated cloud sensor cannot reliably detect all rain events. High, thin clouds and rapidly developing convective cells can produce rain before the IR sensor sees them as fully overcast. The rain sensor provides essential supplementary protection — catching what the cloud sensor can miss, particularly for fast-developing summer thunderstorms.
Best practice: Configure the CloudWatcher so that either the cloud sensor or the rain sensor alone is sufficient to trigger an unsafe condition. Never require both to agree simultaneously — if it is raining, the roof should close regardless of what the IR sensor says.