Grade scale
- A range
- Broken mid/high cloud, sunlit from below across most of the sky; unobstructed solar path. High-end color potential.
- B range
- Good color on part of the sky. Canvas is patchy or the solar path is partially shaded.
- C range
- Some color. Sparse cloud or an overcast deck that only lights at its edge.
- D range
- Clear-sky horizon glow only, or color muted by haze or a shaded path.
- F
- Low overcast, fog, or precipitation at the viewer. No direct light reaches the clouds.
Factors
- Canvas
- Lit cloud fraction overhead: HRRR mid + high cover, each weighted by whether the sun can reach it. Peaks near 50%; thin overcast cirrus (low ice/liquid water path) still scores well.
- Light path
- Transmission along the solar azimuth out to 520 km. Low and mid cloud near the tangent point (~250 km for a 5 km deck, ~340 km for cirrus) shades the canvas.
- View
- Local low cloud, precipitation rate, and surface visibility at the observer.
- Clarity
- HRRR-Smoke near-surface concentration and 2 m RH. Light aerosol enhances reds; dense smoke and humid haze mute them.
Scores are valid at each point's own sunrise − 8 min or sunset + 8 min, interpolated between hourly HRRR output. Releases: sunrise outlook at 7 PM CT, then the sunset outlook at 10 AM CT and the GOES-corrected sunset update at 4 PM CT.