a2f5c902a3
Maps raw cloud cover to six distinct stepped colour bands (clear green through overcast red) on a dark navy background, with fine gradation below 30% where conditions matter for astrophotography and two coarse bands above. Skips OSM base map and alpha blending entirely. The triangulation now stores raw cover values; scale_cloud_cover() is applied post-blur only in the default blending mode, keeping its behaviour identical. Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
166 lines
6.2 KiB
Markdown
166 lines
6.2 KiB
Markdown
# cloud_cover
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Cloud cover forecast maps for astrophotography planning, powered by DWD open data.
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## What is this?
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The German Weather Service (DWD) publishes free, high-resolution numerical weather
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predictions through its [open-data server](https://opendata.dwd.de/). The
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**ICON-D2** model covers Germany and surrounding areas with ~2 km grid spacing and
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produces forecasts up to 48 hours ahead, updated every 3 hours.
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This tool downloads the **total cloud cover (CLCT)** field from ICON-D2 and renders
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it as a series of map images — one per forecast hour — overlaid on an OpenStreetMap
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base layer. The result is a quick visual answer to "will the sky be clear tonight?"
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## Quick start
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```sh
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cargo build --release
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./target/release/cloud_cover "2026-03-07T09:00:00Z" 24
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```
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Output appears in `cache/2026-03-07_09UTC/` — one PNG per forecast hour plus an
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animated GIF.
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## Usage
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```
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cloud_cover [OPTIONS] <ISO_TIMESTAMP> <HOURS>
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```
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| Argument | Description |
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| `ISO_TIMESTAMP` | Model run time in ISO 8601, e.g. `2026-03-07T09:00:00Z` |
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| `HOURS` | Number of forecast steps to render (0–48) |
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The timestamp must match an ICON-D2 model run: every 3 hours starting at 00 UTC
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(00, 03, 06, 09, 12, 15, 18, 21).
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### Options
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| Flag | Default | Description |
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|---|---|---|
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| `--center-lat` | 52.56 | Map centre latitude (°N) |
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| `--center-lon` | 13.08 | Map centre longitude (°E) |
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| `--zoom` | 10 | OSM tile zoom level (higher = more detail, smaller area) |
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| `--no-basemap` | — | Skip OSM tiles; use a plain green background |
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| `--false-color` | — | False-colour mode: distinct stepped colour bands, no base map (see below) |
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### False-colour mode
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`--false-color` skips the OSM base layer and maps cloud cover directly to
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semantically meaningful colour bands, making go/no-go decisions quick to read
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at a glance — particularly useful for automated or agentic interpretation:
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| Cover | Colour | Meaning |
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|---|---|---|
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| < 1% | Bright green | Clear sky |
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| 1–5% | Light green | Near-clear |
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| 5–15% | Yellow-green | Light cloud |
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| 15–30% | Amber | Marginal |
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| 30–60% | Orange | Cloudy |
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| > 60% | Red | Overcast |
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Areas outside the ICON-D2 data region are shown in dark navy.
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The fine gradation below 30% reflects where cloud cover actually matters for
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astrophotography; above 30% only two coarse bands are used.
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### Examples
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```sh
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# Next 12 hours from the 18 UTC run, default viewport (Falkensee / Berlin)
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./target/release/cloud_cover "2026-03-06T18:00:00Z" 12
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# Wider area centred on central Germany at zoom 8
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./target/release/cloud_cover "2026-03-07T06:00:00Z" 24 \
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--center-lat 51.0 --center-lon 10.5 --zoom 8
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# Quick run without map tiles
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./target/release/cloud_cover "2026-03-07T09:00:00Z" 6 --no-basemap
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# False-colour mode for easy agentic interpretation
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./target/release/cloud_cover "2026-03-07T09:00:00Z" 6 --false-color
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```
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## Output
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Each run produces files in `cache/<date>_<hour>UTC/`:
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- **`clct_000001.png` … `clct_NNNNNN.png`** — one 900 × 600 px map per forecast
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hour, showing cloud cover blended over the OSM base layer with a colour legend
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- **`clct_animation.gif`** — animated loop of all frames (generated when there are
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multiple steps)
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- **`clat.grib2`, `clon.grib2`, `clct_NNN.grib2`** — cached raw forecast data;
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re-running the same timestamp skips the download
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Image titles show the forecast time converted to CET/CEST (Europe/Berlin). The first
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frame is labelled "Conditions at …", subsequent frames "Prediction at … (+NNh)".
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In the default mode, cloud cover is rendered as a continuous blend from the base map
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(clear sky) toward a light blue-white tone (overcast). With `--false-color`, a stepped
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colour scale is used instead (see above), with no base map.
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## How it works
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1. **Download** — Fetches bzip2-compressed GRIB2 files from the DWD open-data
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server: two coordinate grids (`clat`, `clon`) describing the ~542,000 points of
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the ICON-D2 icosahedral grid over Germany, plus one `clct` file per forecast step.
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Downloads run in parallel and are cached to disk.
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2. **Parse** — A built-in minimal GRIB2 decoder extracts the data arrays. Only
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simple packing (data representation template 0) is implemented, which is the
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format DWD uses for these fields. Grid points flagged absent by the GRIB2 bitmap
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are set to NaN.
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3. **Interpolate** — The irregular grid points are projected into pixel space via an
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orthographic projection, then connected into a Delaunay triangulation. Each output
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pixel is interpolated via barycentric coordinates within its enclosing triangle,
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followed by a NaN-aware Gaussian blur to smooth triangle edges.
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4. **Render** — The base layer comes from Carto Voyager OSM tiles (fetched and
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cached separately for the basemap and for labels). Cloud cover is blended on top,
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then map labels are composited above the clouds. City markers, a title bar, and a
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colour legend complete the frame.
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5. **Animate** — All frames are colour-quantised to 256-colour palettes (in parallel)
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and assembled into a looping GIF.
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## Dependencies
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All dependencies are pure Rust or vendored C compiled into the binary — no system
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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 |
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| `font8x8` | Built-in 8×8 bitmap font for map labels |
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| `clap` | CLI argument parsing |
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| `chrono` + `chrono-tz` | Date/time handling and timezone conversion |
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| `anyhow` | Error propagation |
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| `spade` | Delaunay triangulation for grid interpolation |
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| `rayon` | Parallel downloads, rendering, and GIF quantisation |
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| `image` | PNG decoding (loading frames for GIF assembly) |
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| `gif` | GIF encoding |
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## Building
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```sh
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cargo build --release
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```
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The binary is at `target/release/cloud_cover`.
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## Future ideas
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- Configurable display timezone (currently hardcoded to Europe/Berlin)
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- Configurable city markers / observation sites
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- Configurable data sources
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- Vector map tile support for increased rendering resolution
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- Separation of cache and output files & automatic cache management
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- Automatic newest prediction fetching
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