How we score a night
Every number on this site is derived from public forecast data and astronomical geometry. Here is exactly how, so you can decide how much to trust it.
The inputs
Cloud cover (split into low, medium and high layers), fog, relative humidity, dew point, temperature, wind speed and precipitation come from the MET Norway Locationforecast model, requested for your exact coordinates and elevation. Sun and moon positions, twilight boundaries and lunar phase are computed from standard astronomical algorithms on our own servers — no third-party ephemeris involved.
The five factors
Cloud
Layers are combined as independent attenuators rather than summed. Low cloud is treated as opaque; high cirrus is weighted at roughly half, because thin cirrus dims a sky without ending the session. Fog is applied on top.
Transparency
How much haze and water vapour dim faint objects, estimated from relative humidity and reduced further when precipitation is forecast.
Seeing (estimated)
Atmospheric steadiness limits usable magnification. Real seeing is driven by upper-air turbulence that public point forecasts do not publish, so we use surface wind as a proxy. This is the weakest of the five factors and we label it as an estimate everywhere it appears.
Moonlight
Scaled by illuminated fraction and the moon's altitude, so a bright moon low on the horizon costs you far less than one overhead.
Darkness
Ramps from the end of civil twilight to full astronomical night at 18° below the horizon, computed for your coordinates rather than your time zone.
Putting it together
Cloud gates everything — a clouded sky cannot be rescued by good seeing. Transparency and seeing then modulate how good that clear sky actually is, and moonlight and darkness scale the result. The output is a 0–100 score per hour:
The "best window" is the longest unbroken run of hours scoring 35 or better inside astronomical darkness.
What we do not model yet
Light pollution at your site, upper-air jet stream (real seeing), aurora activity, and local terrain shadowing. Treat the score as a forecast of the sky above you, not a rating of the site itself — a 90 from a city centre and a 90 from a dark site are very different nights.