cloud_cover

Cloud cover forecast maps for astrophotography planning, powered by DWD open data.

What is this?

The German Weather Service (DWD) publishes free, high-resolution numerical weather predictions through its open-data server. The ICON-D2 model covers Germany and surrounding areas with ~2 km grid spacing and produces forecasts up to 48 hours ahead, updated every 3 hours.

This tool downloads the total cloud cover (CLCT) field from ICON-D2 and renders it as a series of map images — one per forecast hour — overlaid on an OpenStreetMap base layer. The result is a quick visual answer to "will the sky be clear tonight?"

Quick start

cargo build --release
./target/release/cloud_cover "2026-03-07T09:00:00Z" 24

Output appears in cache/2026-03-07_09UTC/ — one PNG per forecast hour plus an animated GIF.

Usage

cloud_cover [OPTIONS] <ISO_TIMESTAMP> <HOURS>

Argument	Description
`ISO_TIMESTAMP`	Model run time in ISO 8601, e.g. `2026-03-07T09:00:00Z`
`HOURS`	Number of forecast steps to render (0–48)

The timestamp must match an ICON-D2 model run: every 3 hours starting at 00 UTC (00, 03, 06, 09, 12, 15, 18, 21).

Options

Flag	Default	Description
`--center-lat`	52.56	Map centre latitude (°N)
`--center-lon`	13.08	Map centre longitude (°E)
`--zoom`	10	OSM tile zoom level (higher = more detail, smaller area)
`--no-basemap`	—	Skip OSM tiles; use a plain green background

Examples

# Next 12 hours from the 18 UTC run, default viewport (Falkensee / Berlin)
./target/release/cloud_cover "2026-03-06T18:00:00Z" 12

# Wider area centred on central Germany at zoom 8
./target/release/cloud_cover "2026-03-07T06:00:00Z" 24 \
  --center-lat 51.0 --center-lon 10.5 --zoom 8

# Quick run without map tiles
./target/release/cloud_cover "2026-03-07T09:00:00Z" 6 --no-basemap

Output

Each run produces files in cache/<date>_<hour>UTC/:

clct_000001.png … clct_NNNNNN.png — one 900 × 600 px map per forecast hour, showing cloud cover blended over the OSM base layer with a colour legend
clct_animation.gif — animated loop of all frames (generated when there are multiple steps)
clat.grib2, clon.grib2, clct_NNN.grib2 — cached raw forecast data; re-running the same timestamp skips the download

Image titles show the forecast time converted to CET/CEST (Europe/Berlin). The first frame is labelled "Conditions at …", subsequent frames "Prediction at … (+NNh)".

Cloud cover is rendered as a continuous blend from the base map (clear sky) toward a light blue-white tone (overcast), matching the style familiar from weather apps.

How it works

Download — Fetches bzip2-compressed GRIB2 files from the DWD open-data server: two coordinate grids (clat, clon) describing the ~542,000 points of the ICON-D2 icosahedral grid over Germany, plus one clct file per forecast step. Downloads run in parallel and are cached to disk.
Parse — A built-in minimal GRIB2 decoder extracts the data arrays. Only simple packing (data representation template 0) is implemented, which is the format DWD uses for these fields. Grid points flagged absent by the GRIB2 bitmap are set to NaN.
Interpolate — The irregular grid points are projected into pixel space via an orthographic projection, then connected into a Delaunay triangulation. Each output pixel is interpolated via barycentric coordinates within its enclosing triangle, followed by a NaN-aware Gaussian blur to smooth triangle edges.
Render — The base layer comes from Carto Voyager OSM tiles (fetched and cached separately for the basemap and for labels). Cloud cover is blended on top, then map labels are composited above the clouds. City markers, a title bar, and a colour legend complete the frame.
Animate — All frames are colour-quantised to 256-colour palettes (in parallel) and assembled into a looping GIF.

Dependencies

All dependencies are pure Rust or vendored C 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
`font8x8`	Built-in 8×8 bitmap font for map labels
`clap`	CLI argument parsing
`chrono` + `chrono-tz`	Date/time handling and timezone conversion
`anyhow`	Error propagation
`spade`	Delaunay triangulation for grid interpolation
`rayon`	Parallel downloads, rendering, and GIF quantisation
`image`	PNG decoding (loading frames for GIF assembly)
`gif`	GIF encoding

Building

cargo build --release

The binary is at target/release/cloud_cover.

Future ideas

Configurable display timezone (currently hardcoded to Europe/Berlin)
Configurable city markers / observation sites
Configurable data sources
Vector map tile support for increased rendering resolution
Separation of cache and output files & automatic cache management
Automatic newest prediction fetching

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