Star Index combines cloud cover at multiple altitude layers with humidity, wind speed, moon phase, and twilight timing into a single 0-100 score. Seeing (atmospheric turbulence) and transparency are estimated on separate scales, since planet viewing and faint deep-sky objects need different conditions.

Yes, the core app is free, including today's Star Index, hourly and weekly forecasts for one location, and meteor shower notifications. A one-time Premium purchase (not a subscription) unlocks multiple saved locations, the full Messier catalog, shooting windows, and 7-day forecasts.

No. Star Index does not collect personal data, does not show ads, and does not require an account. Your location is used only to calculate the forecast for where you are and is sent anonymously to the Open-Meteo weather service to fetch conditions. It is not stored on any server or linked to you.

Premium is a one-time purchase that permanently unlocks multiple saved locations, the full Messier deep-sky catalog, shooting window predictions, and 7-day forecasts. It is not a subscription, so you pay once and keep it forever.