how the score is computed
score = 100 · √(t/tmax) · vis² · √(alt/8°) · sky0.75
Factors are weighted by certainty. The horizon is surveyed
fact, so its visible fraction counts squared — half of totality
behind terrain costs three quarters of the score, fully blocked
is 0 whatever the sky. Below 8° the Sun stands in many air
masses, where haze and horizon murk live: geometry, so it counts
with the certain factors. The sky is a forecast, so its middle
is softened — but under 20 it falls quadratically, and a certain
storm is 0. Duration counts gently: half the maximum is still
most of the show — and the key by the suitability switch takes
it out entirely, leaving the score purely about seeing. For a
Sun under 20° the sky number is the worse of overhead and the
sightline decks toward its azimuth.