The route and fields are on the sunrise and sunset reference.
Every event gets two scores from 0 to 100. Both share the categories poor
(below 25), fair (25 to below 50), good (50 to below 75) and great (75
and up).
Colour
The colour score asks whether the low sun will light up cloud from below. It is the product of five factors, times 100, clamped to 0 to 100:
- Canvas. Mid and high cloud overhead for the sun to light. A broken deck (about 30% to 70% cover) is best and scores the full factor. A clear sky still glows a little (0.15 of full); a solid overcast is dull (0.45).
- Local low cloud. A low deck overhead (base below 2 km) hides the show from the observer.
- Light path. The light has to reach the canvas. Low cloud, a near solid mid deck or rain between you and the horizon, sampled 25 to 300 km toward the sun, blocks it. In the lower 48, so can terrain: the ground toward the sun is checked against a ray to the canvas through the colour window, from sunset until the sun is two degrees down.
- Haze. A little aerosol warms the colours and can lift the score slightly; thick smoke, fog or murk mutes them.
- Rain at the observer, from the chance of precipitation.
Clear sun
The clear sun score asks the opposite question: will the disc itself be visible right down to the horizon? A cloudless sky is its best case, which the colour score rates Poor by design. It multiplies four factors:
- Low cloud overhead. Fog or a low deck hides the sun outright.
- The sun line. Cloud at every level along the path toward the sun, with the nearest points weighted most. Thin high cloud counts for part, since it often lets a dimmed disc through.
- Haze. Light haze only reddens the disc; thick smoke or murk hides it.
- Rain at the observer.
Terrain does not count against it: a sun setting behind a ridge is still a clear sun, only earlier.
Categories
Both scores share the same categories.
| score | category |
|---|---|
| below 25 | poor |
| 25 to below 50 | fair |
| 50 to below 75 | good |
| 75 and up | great |