# Filter 1: Köppen-Geiger class

**Status:** Done (2026-09-14): rule applied, Mixed display built, documentation
updated.

**Data keys:** `koppenZone`, `koppenPrimaryClass`, `koppenSecondaryClass`.
Calculation: [filter-calculations.md](../filter-calculations.md) §1. Display
design and history:
[koppen-mixed-display.md](../koppen-mixed-display.md).

## Findings

**4. The Köppen fallback can create false data.** A county with no valid raster
cells is labeled `Cfa` instead of missing. A null value plus an audit flag
would distinguish missing coverage from a genuine humid-subtropical class.
*Resolved on 2026-09-13: the Köppen builder leaves such counties blank. The
fallback remains only in `build_county_climate_data.py`, whose Köppen
value is replaced by the apply step.*

**21. The documented Köppen labels differ from the app.** `filter-calculations.md`
lists the group as "Climate Classification" and the filter as "Köppen-Geiger
Climate Class"; `app.js` shows "Koppen-Geiger Classification" and
"Koppen-Geiger Climate Class". The app spells Koppen without the umlaut
throughout, while the docs use Köppen. *Resolved on 2026-09-15: every
prose and display use of the name now reads "Köppen", in `app.js`,
`README.md`, `scripts/county_data_sources.md`, script docstrings, help text,
and checker messages (with the matching test expectations), and
`filter-calculations.md` quotes the app's group label, "Köppen-Geiger
Classification". Code identifiers, file names, and data paths such as
`koppenZone`, `normalizeKoppenCode`, and `koppen_geiger_tif/` keep the ASCII
spelling.*

**22. The base build's docstring calls its Köppen value a majority class.**
`build_county_climate_data.py` lists `koppenZone` as the "majority
Koppen-Geiger class", but it assigns the most common class among touched
cells, which need not be a majority. That value is replaced by the Köppen apply
step, and the script is retired in Phase 3. *Resolved on 2026-09-15: the
docstring now reads "largest-share Köppen-Geiger class among touched cells
(replaced by the Köppen apply step)".*

Cross-filter finding 2 (touched-cell aggregation) was resolved for Köppen on
2026-09-13; see [00-cross-filter.md](00-cross-filter.md).

## Decisions

In [decisions.md](../decisions.md): Köppen audit columns, Köppen no-data
fallback, and Applying Köppen to the app CSV (2026-09-12); Metric files and
Mixed climate display (2026-09-13).

## Tasks

Adopted rule: a county is predominantly its top class if and only if that class
covers at least 50% of the county's land and leads the runner-up by at least
5 percentage points; otherwise it is Mixed climate. Expected result for the
50 states and DC: 3,010 predominant, 133 Mixed.

- [x] Investigate low-majority counties and adopt the rule.
- [x] Windowed raster reads and 180th-meridian split.
- [x] Area-weighted class shares (16 × 16 sub-cells per raster cell, scaled by
      cos(latitude)) in `scripts/common/county_zonal_stats.py`.
- [x] Apply the 50% / 5-point rule (`scripts/build_county_koppen_metric.py`;
      counties with no valid cells are left blank).
- [x] Run the builder to write `data/metrics/koppen.csv` and confirm the
      expected 3,010 predominant / 133 Mixed (2026-09-13).
- [x] `koppenZone`-only apply step
      (`scripts/apply_koppen_metric_to_climate_data.py`, with `--dry-run`).
- [x] Apply to `data/climate-data.csv` (2026-09-13; 142 counties changed to
      Mixed, with `koppenPrimaryClass` and `koppenSecondaryClass` added).
- [x] Allow `Mixed` in `check_climate_data.py`.
- [x] Add a Mixed climate category to `app.js`, drawn as stripes of the
      county's top two classes; see
      [koppen-mixed-display.md](../koppen-mixed-display.md).
- [x] Replace the plurality description in `filter-calculations.md` §1 and mark
      review findings 2 and 4 resolved for Köppen (2026-09-14).
- [x] Update the Köppen descriptions and script lists in `README.md` and
      `scripts/county_data_sources.md` (2026-09-14).
- [x] Tests for shares, the rule, boundary cases, and the apply step
      (`tests/test_koppen_metric.py`).