cloud_cover

Downloads total cloud cover (CLCT) predictions from the DWD open-data server 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

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

cd cloud_cover_rs
cargo build --release

The compiled binary ends up at target/release/cloud_cover.