# Healthy Weather

> Healthy Weather is an iOS app that combines a custom Vitamin D Index, UV forecasts, air quality, and pollen tracking. It models how much vitamin D your skin produces from sunlight using a calibrated photochemical model, including a soft saturation cap and cross-session recovery dynamics.

The site documents the app, its science, and its roadmap. It is a static website with no tracking and no cookies. Pollen tracking is limited to Europe; everything else is global.

## Core pages

- [Home](https://healthyweather.app/): App overview and App Store link.
- [The Science](https://healthyweather.app/science/): Full documentation of the vitamin D model — UV-to-IU conversion, calibration anchors (Holick 2007, Webb & Engelsen 2006, McKenzie 2009), skin and exposure factors, the saturation cap, cross-session recovery (~11.5h half-life), and ±15–25% accuracy bounds.
- [FAQ](https://healthyweather.app/faq/): The Vitamin D Index vs. standard UV Index, data sources (Open-Meteo), supported regions, pricing, privacy, and on-device session history.
- [Roadmap](https://healthyweather.app/roadmap/): Personalized D3 tracking, notifications, multi-day outlook.
- [Support](https://healthyweather.app/support/): Bug reports, feature requests, contact.

## Key concepts

- **Vitamin D Index**: A custom index (separate from the WHO UV Index) that accounts for solar altitude and atmospheric UVB filtering. 0 = no vitamin D production; ≥1 = "vitamin D window" open.
- **Saturation**: Skin reaches photochemical equilibrium at roughly 10–20% conversion of 7-dehydrocholesterol. Past this point, more sun produces no more vitamin D — only sunburn risk. The app models a soft per-session cap and cross-session recovery with an exponential decay (~11.5h half-life).
- **Calibration**: The IU/min formula `IU_per_min = C × UVI × skin_factor × exposure_factor` uses C ≈ 290, anchored to McKenzie (experimental), Holick (MED), and Webb & Engelsen (SDD).
- **Accuracy**: ±15% in typical conditions, ±20–25% at edges (low sun, very dark skin, near saturation). Inter-individual biological variation adds another ±20–30% on top.

## Privacy

No cookies, no analytics, no server-side session storage. Session history lives on-device only.

## Optional

- [App Store listing](https://apps.apple.com/app/id6760667940)
- [Privacy policy](https://legal.ilnua.com/#privacy-healthyweather)
- [Terms](https://legal.ilnua.com/#terms-healthyweather)
- [Impressum](https://legal.ilnua.com/#impressum)