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