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README.md

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+# cloud_cover
+
+Downloads total cloud cover (CLCT) predictions from the [DWD open-data
+server](https://opendata.dwd.de/weather/nwp/icon-d2/grib/) and renders
+one PNG map per forecast hour, centred on the Brandenburg / Berlin area.
+
+This is a Rust port of `../cloud_cover_prediction.py`.  The output format
+differs: instead of using PyNGL's contour renderer the Rust version does a
+direct point-by-point orthographic projection of the ICON-D2 grid onto a
+bitmap, which avoids every system-library dependency (no CDO, no PyNIO, no
+PyNGL, no libssl, no libbz2 — the binary is fully self-contained).
+
+## Usage
+
+```
+cloud_cover <ISO_TIMESTAMP> <HOURS>
+```
+
+| Argument | Description |
+|---|---|
+| `ISO_TIMESTAMP` | Model run time in ISO 8601, e.g. `2026-03-03T18:00:00Z` |
+| `HOURS` | Number of forecast steps to render (0–48) |
+
+The timestamp must correspond to an ICON-D2 model run, which is issued
+every 3 hours starting at 00 UTC (00, 03, 06, 09, 12, 15, 18, 21).
+
+### Example
+
+```sh
+cargo run --release -- "2026-03-03T18:00:00Z" 12
+```
+
+Output PNGs are written to `cache/<date>_<hour>UTC/clct_000001.png` …
+`clct_NNNNNN.png`.  Downloaded GRIB2 files are cached in the same directory
+so that re-running the same timestamp skips the network requests.
+
+## How it works
+
+1. **Download** — `reqwest` fetches the bzip2-compressed GRIB2 files from
+   the DWD server.  Two coordinate files (`clat`, `clon`) give the latitude
+   and longitude of every point on the ICON-D2 icosahedral grid (~542 000
+   points for Germany).  One `clct` file per forecast step contains the
+   total cloud cover percentage at each point.
+
+2. **Decompress** — `bzip2` decompresses the files in-memory.
+
+3. **Parse** — A minimal built-in GRIB2 decoder extracts the data values.
+   Only simple packing (data representation template 0) is supported, which
+   is what DWD uses.  Grid points that are flagged as absent by the section-6
+   bitmap are set to `NaN` and skipped during rendering.
+
+4. **Project & render** — Each grid point is projected onto the image plane
+   using an orthographic projection centred on Falkensee (52.56 °N,
+   13.08 °E).  Points are painted as 2 × 2 pixel squares, colour-coded by
+   cloud cover percentage.  City markers and labels are drawn on top, and a
+   colour-scale legend is shown on the right.  `plotters` writes the final
+   PNG.
+
+## Cloud cover colour scale
+
+| Cloud cover | Colour |
+|---|---|
+| < 1 % | Land background (clear) |
+| 1–2 % | Very light blue |
+| 2–5 % | Light blue |
+| 5–10 % | Medium blue |
+| 10–20 % | Blue |
+| 20–50 % | Dark blue |
+| 50–100 % | Very dark blue |
+
+## Marked locations
+
+Falkensee · Pausin · Nauen · Hennigsdorf · Ketzin/Brückenkopf · Potsdam ·
+Berlin (Mitte)
+
+## Dependencies
+
+All dependencies are pure Rust or vendored C code compiled into the binary.
+No system shared libraries are required at runtime beyond libc.
+
+| Crate | Role |
+|---|---|
+| `reqwest` (rustls-tls) | HTTP client with pure-Rust TLS |
+| `bzip2` | bzip2 decompression (vendors libbzip2) |
+| `plotters` + `plotters-bitmap` | PNG rendering with built-in bitmap font |
+| `clap` | CLI argument parsing |
+| `chrono` | Date/time handling |
+| `anyhow` | Error propagation |
+
+## Building
+
+```sh
+cd cloud_cover_rs
+cargo build --release
+```
+
+The compiled binary ends up at `target/release/cloud_cover`.