{"nature":"This API supplies data organisation and trend description only. It is not a recommendation to act; the decision is yours. Purely descriptive - no judgement, no prediction. Every record carries an official source plus a record_hash (canonical sha256, recomputable offline for self-verification; cross-check with free /gauge/verify).","note":"Free sample: this is the COMPLETE REAL RECORD that the paid /gauge returns - it includes the common ruler (normalized_index, which quantifies how deep the reading is), the statistical ruler (same-season percentile), a self-verifiable record_hash, and provenance (age computed at request time plus the freshness criterion). Source independence is labelled per record in sources[].independent; source_agreement_cv measures cross-source agreement (null when only one independent source exists, a coefficient of variation otherwise). Try before you buy","sample":{"signal_id":"airquality.aqi","entity":"us-chicago","entity_name":"Chicago, IL","snapshot_date":"2026-09-26","observed_at":"2026-09-26T13:00:00.000Z","data_age_hours":0,"current":{"us_aqi":36},"previous":{"us_aqi":51.3},"change":{"period":"vs_previous","us_aqi":{"delta":-15.3,"direction":"falling","direction_reason":null}},"trend_7d":[41.8,32.7,30.8,31.5,37.2,51.3,41.7],"sources":[{"name":"Copernicus CAMS via Open-Meteo","role":"current primary source (model)","as_of":"2026-09-26T13:00","value":36,"weight":0.5},{"name":"AirNow (US EPA ground measurement)","role":"second source (ground measurement)","as_of":"2026-09-26T06:00","value":52,"weight":0.5}],"source_agreement_cv":"0.182","standard_ruler":{"band":"below_moderate","note":"Severity = normalized_index, comparable across sites on the same scale; thresholds are the official values, interpolation is plain arithmetic and never a forecast","unit":"AQI","bands":["moderate","usg","unhealthy","very_unhealthy","hazardous"],"scale":"us_aqi","metric":"us_aqi","source":"U.S. EPA (classification standard) / CAMS (current value)","standard":"U.S. EPA Air Quality Index (AQI) categories (official)","thresholds":{"usg":101,"moderate":51,"hazardous":301,"unhealthy":151,"very_unhealthy":201},"index_basis":"normalized_index: piecewise linear (moderate=0/usg=1/unhealthy=2/very_unhealthy=3/hazardous=4); below the first band = (value - first threshold) / first threshold, clamped to [-1,0); above the top band it is unbounded. pct_to_action = value / first alert threshold (bounded, 1 = at the first alert)","current_value":36,"pct_to_action":0.706,"normalized_index":-0.294,"official_category":"Good","distance_to_action":15},"stat_ruler":{"doy":269,"note":"Purely descriptive statistics: where the current value falls against the same season of the last few years of official data (the seasonal window removes cyclical confounding). Not a forecast and not a risk judgement. Method and sample size are disclosed so you can recompute it yourself.","unit":"AQI","basis":"day-of-year seasonal window","range":{"max":89,"min":24.708},"scale":"empirical_percentile_seasonal","method":"Daily mean CAMS values over the last 3 years, taking a same-season +/-30 day window each year (day-of-year=269) to compute an empirical percentile (linear interpolation); current_percentile = share of same-season historical days not above the current value","metric":"us_aqi","n_days":183,"source":"Copernicus CAMS via Open-Meteo (official model reanalysis; not a ground measurement)","n_total":1096,"percentiles":{"p05":29.258,"p10":31.233,"p25":37.313,"p50":48.167,"p75":56.104,"p90":63.292,"p95":70.608},"source_tier":"T1","window_years":3,"current_value":36,"frequency_label":"common (10-25% or 75-90% band)","current_percentile":0.219,"season_window_days":30},"summary_zh":"Chicago, IL 的 us_aqi 為 36,較前值 falling。統一尺(us_aqi,AQI;官方閾值):moderate 51;深淺 pct_to_action=0.706(0~1,1=達 action)、嚴重度 normalized_index=-0.294(action=0/minor=1/moderate=2/major=3,跨站可比)、band「below_moderate」;現距 action 15 AQI;NWS 官方分類「Good」(官方閾值+純算術內插,非本 API 判斷/預測,深淺可量化)。統計尺(近 3 年·同季節窗 ±30 日,n=183 日;經驗百分位):現值落在同季節第 22 百分位 → 屬「常見(10–25% 或 75–90% 帶)」;同季節區間 24.708–89、中位數 48.167(方法:USGS 官方日均值 day-of-year 季節窗線性內插百分位,可自算複核;純統計描述非預測)","source_ref":"https://open-meteo.com/en/docs/air-quality-api(Copernicus CAMS)","source_tier":"T1","record_hash":"sha256:d132b08846c26bc6a690b28abf15c7c6662cdedef9f4c458ac02ae271bdf0791","hash_core_version":4,"ots_status":"pending","lifecycle":null,"summary_en":"Chicago, IL: 36 AQI us aqi - Good, 15 AQI below the first official alert. seasonal percentile 0.219 (common). falling vs 24h ago.","provenance":{"served_from":"GAUGE stored snapshot (SQL) - not a live upstream API call at request time","data_as_of":"2026-09-26T13:00:00.000Z","oldest_leg_as_of":"2026-09-26T13:00:00.000Z","data_age_hours":0,"age_hours_now":195.5,"freshness":"stale","freshness_basis":"cadence=daily; fresh<=24h; recent<=192h; dead_source>1440h; age_basis=request-time(now minus observed_at)","data_lineage":[{"signal":"airquality.aqi","independent_dimension":"air-quality","traceable_to":"US EPA / Copernicus CAMS","uncertainty":"source_agreement_cv=0.182","observed_at":"2026-09-26T13:00:00.000Z","record_hash":"sha256:d132b08846c26bc6a690b28abf15c7c6662cdedef9f4c458ac02ae271bdf0791"}],"validation_status":"not-yet-backtested (screening-level; framework Ch19 historical baseline still being built)","note":"Non-real-time decisions are served from stored data to avoid upstream API latency; use for decisions within this data cadence. Timestamps are UTC."}}}