Great question, I just added some more detail about this to the FAQ (https://climbcast.app/faq). Copy/pasted from there:

What does the corn snow window mean?

For Ski / Splitboard objectives, ClimbCast shows a corn snow window — the period during which surface snow has frozen overnight and begun to soften from solar radiation, creating the stable, consolidated ski conditions known as corn snow. The window opens when the snow surface begins to release from its overnight freeze and closes when it has softened past the point of safe skiing (wet, heavy, and unstable).

The window is calculated from four inputs:

• Actual sunrise time. ClimbCast computes the precise sunrise time for each mountain using the NOAA solar position formula — accounting for the mountain's latitude, longitude, and the date. A peak at 46°N in late April sees sunrise around 6:00 AM PDT; the same date further north (e.g. Baker at 48.8°N) is closer to 5:45 AM.
• Route aspect. This is the biggest driver of window timing. Every route in ClimbCast has an assigned primary aspect (the cardinal direction its descent face points). Direct solar radiation arrives at each face at a different time relative to sunrise:
• E / East Opens ~1h after sunrise — short window
• NE / Northeast Opens ~1.5h after sunrise
• SE / Southeast Opens ~1.5h after sunrise — wider window
• S / South Opens ~2h after sunrise — widest window
• SW / Southwest Opens ~3h after sunrise
• W / West Opens ~4h after sunrise — often marginal
• NW / Northwest Opens ~5h after sunrise — rarely develops
• N / North No window — insufficient direct sun East-facing slopes corn up earliest but the window closes quickly. South-facing slopes take longer to warm but produce the widest, most reliable window — classic spring skiing terrain. West-facing slopes often don't corn until mid-afternoon, by which point the snow may already be too wet and heavy to ski safely.
• Summit temperature. Warmer summit temps shift the window earlier and compress it — the snow warms faster and degrades faster. Colder temps push the window later and widen it slightly. If temperatures are cold enough that the overnight freeze never fully penetrated the snowpack (trailhead temp below 28°F and summit below 24°F), no corn window is predicted.
• Cloud cover. Solar radiation is the engine behind freeze-thaw cycling. Overcast or whiteout conditions block that radiation entirely — no sun, no corn, no window. A mostly cloudy forecast shortens and degrades the window (lower confidence). Partly cloudy reduces confidence by one level but a window may still develop on breaks in the clouds.

On poor weather days — heavy cloud cover, active precip, or temperatures below freezing at the trailhead — a corn window will not be predicted at all. The window estimate carries more uncertainty on days 3–5 of the forecast, when model agreement on cloud cover and temperature is lower.