Daylight overwhelms auroral contrast
Aurora emissions can be present above a sunlit location, but the scattered light of daytime normally makes visual observing impractical. Twilight is gradual: the sky darkens as the Sun moves from just below the horizon through civil, nautical and astronomical twilight.
GreenHalos calculates solar altitude locally and treats daylight as a zero darkness factor. That is a viewing calculation, not a space-weather statement. The magnetosphere does not switch off at sunrise.
Cloud is a physical barrier
Cloud sits between an observer and most auroral emission. Thick overcast can block even strong activity; broken cloud may create brief openings. Total cloud fraction is useful, but low cloud, fog and local topography can make conditions differ over short distances.
Unknown cloud data must stay unknown. Substituting a clear-sky value would inflate a combined viewing score and could send an observer out on a false premise. GreenHalos therefore withholds the combined score when cloud context is unavailable.
Moonlight changes contrast, not space weather
A bright Moon raises sky brightness and can wash out faint structure, particularly for dark-adapted eyes. It does not stop charged particles from producing aurora, and a strong display can remain striking under moonlight.
Moon altitude matters: an illuminated Moon below the horizon contributes no direct moonlight. Phase alone is therefore incomplete. Thin cloud can also scatter moonlight, making a local sky brighter than a phase icon suggests.
Light pollution and terrain remain outside the score
Artificial skyglow reduces contrast, while buildings, mountains or trees can block the low poleward horizon. These effects vary at a scale that a global model cannot reliably infer from a coordinate alone.
GreenHalos discloses that light pollution and terrain are not included in the launch Viewing Potential. Users should inspect the site in daylight, respect access restrictions and select an unobstructed direction rather than treating a numeric index as complete.
Read the factors instead of chasing one number
A useful local assessment presents auroral activity, darkness, cloud, moonlight, source freshness and forecast confidence separately. The combined GreenHalos category is a heuristic summary, not an observed probability.
A high activity factor with daylight should still yield no useful visual window. A dark clear sky with low activity may support patient observing but not a guarantee. The factor breakdown explains that trade-off and makes missing data visible.
Evidence register
Primary sources
Sources were checked on . Each record identifies which sections it supports.
- The Atmosphere... After Dark! NASA Science Supports: Daylight overwhelms auroral contrast; Moonlight changes contrast, not space weather
- Locationforecast 2.0 documentation MET Norway Supports: Cloud is a physical barrier
- Aurora Tutorial NOAA Space Weather Prediction Center Supports: Read the factors instead of chasing one number