A Brocken spectre occurs when a photographer stands on an elevated ridge or summit with the sun low behind them and a layer of fog or cloud below. Their shadow is projected onto the cloud surface, appearing enormously magnified and surrounded by colorful concentric rings known as a 'glory' - created by light diffracting around tiny water droplets. Named after the Brocken peak in Germany's Harz Mountains, where the phenomenon is famously frequent due to persistent low cloud and exposed ridgelines.
Inverza checks two things simultaneously: whether the observer's SRTM elevation is above the nearest METAR-reported cloud base (BKN/OVC layer), and whether the sun is in the optimal 2-20 degree altitude band at sunrise or sunset. When METAR data isn't available, the detector falls back to forecast-based indicators - high low-cloud cover, high humidity, and low visibility - combined with significant observer elevation. The upper sky (mid + high clouds) must be clear so the sun shines unobstructed onto the photographer's back.
