# Filter 2: Annual avg temperature

**Status:** In progress: method decided (2026-09-15).

**Data key:** `avgTempF`. Calculation:
[filter-calculations.md](../filter-calculations.md) §2.

## Findings

**1. Annual temperature weights months equally.** February has the same weight
as January or July. If the intended label means an average across all days,
monthly normals should instead be weighted by the number of days in each
month. *Resolved on 2026-09-15: equal weighting is the WMO and NOAA standard
for annual normals and is kept; see "Annual avg temperature month weighting"
in [decisions.md](../decisions.md).*

Cross-filter findings that affect this filter, in
[00-cross-filter.md](00-cross-filter.md): 2 and 11 (county aggregation),
12 (Alaska and Hawaii not covered), and 15 (source text cites the station
Normals).

## Decisions

In [decisions.md](../decisions.md), all 2026-09-15: Annual avg temperature
month weighting, aggregation, completeness, and in Alaska and Hawaii.

## Tasks

Adopted definition: the equally weighted mean of the 12 monthly 1991–2020
normals of nClimGrid-Monthly `tavg`, area-weighted to each county; blank
unless all 12 months are present. The per-cell normals follow NCEI's own
gridded-normals method, "a simple 30-year average of monthly grids"
(Rennie and Palecki, *U.S. Monthly Gridded Precipitation and Temperature
Climate Normals*). Alaska and Hawaii stay blank; see
[decisions.md](../decisions.md).

- [x] Verify the calculation against the source data and the WMO and NOAA
      normals definitions (2026-09-15).
- [x] Decide month weighting, aggregation, completeness, and Alaska/Hawaii
      handling (2026-09-15; see [decisions.md](../decisions.md)).
- [x] Rewrite `filter-calculations.md` §2 with the adopted definition, citing
      WMO-No. 1203 §4.3.3 and NOAA's 1991–2020 Normals methodology. State that
      average temperature is (Tmax + Tmin)/2 and that the grid covers the
      contiguous U.S. only (2026-09-15; the current touched-cell method is kept
      as "Current method" until the new values are applied).
- [ ] Add a continuous-value area-weighted mean to
      `scripts/common/county_zonal_stats.py`. It takes an array and transform,
      because nClimGrid is read from netCDF rather than a rasterio file, and
      reuses `cell_coverage_fractions`, the cos(latitude) scaling, and the
      padded window. NaN cells are excluded.
- [ ] Build `scripts/build_county_avg_temp_metric.py`, writing
      `data/metrics/avg_temp.csv` with `avgTempF` and a valid-month count. The
      1991–2020 climatology step is NOAA-specific, so it stays with the NOAA
      code rather than `common/`.
- [ ] Single-column apply step
      `scripts/apply_avg_temp_metric_to_climate_data.py` with `--dry-run`.
      Move `read_csv_rows` (4 identical copies in `apply_*` scripts) into
      `common/` as part of this step.
- [ ] `check_climate_data.py`: keep the 20–85 °F rule and the Alaska/Hawaii
      blank allowance until that open decision in
      [decisions.md](../decisions.md) is made; confirm no new blanks in the
      contiguous U.S.
- [ ] Tests (`tests/test_avg_temp_metric.py`): partial-cell weights,
      cos(latitude), NaN exclusion, the 12-month rule, equal month weights,
      Celsius-to-Fahrenheit conversion and rounding, and the apply step.
- [ ] Apply to `data/climate-data.csv`, run the checker, and compare: the
      spread of changes, the largest shifts (expected in small, narrow, and
      coastal counties), and no new blanks among the 3,109 contiguous-U.S.
      counties.
- [ ] Mark finding 2 resolved for this metric in
      [00-cross-filter.md](00-cross-filter.md).
- [ ] Correct the `avgTempF` source text in `app.js`, which cites the
      station-based U.S. Climate Normals; describe it as 1991–2020 normals
      computed from nClimGrid-Monthly and link nClimGrid. The "(Normals)"
      label stays.
- [ ] Correct `scripts/county_data_sources.md`: Source 2 says the build reads
      monthly normals files, but it reads the nClimGrid monthly series and
      averages 1991–2020. Update the `avgTempF` definition line to match.
      Also remove the "Run the generator" note that fallback values are
      applied to counties outside NOAA coverage; they are left blank
      (finding 12).
- [ ] Add the area-weighted `avgTempF` to the §7 guardrail on rerunning
      `build_county_climate_data.py` in
      [pipeline-plan.md](../pipeline-plan.md).