Cloud comes from each of the forecasts we average, hour by hour. We count a forecast as clear when it shows 30% cloud or less, and the night's rating is based on its best two-hour spell.
Darkness and the moon are worked out from the positions of the sun and moon for this place. We only count hours after the end of twilight, and a bright moon high in the sky lowers the rating because it washes out fainter stars.
Light pollution is an estimate from the size and distance of every town within 350 km, using a standard rule for how sky glow fades with distance (Walker's law), checked against places whose night sky is well measured. Expect it to be within about one step on the scale. Local lights, like a floodlit car park next door, aren't included. Town populations from GeoNames (CC BY 4.0).