Methodology

1. The number this site publishes

Every figure on CLI.LAX is built around one target: the daily maximum temperature that the National Weather Service reports in its Climatological Report (CLI) for the station. That is the value weather-derivatives markets — including the daily high temperature markets on Kalshi — settle against. Everything the site shows, from the running intraday high to the accuracy scorecard, is measured against that single definition.

This is deliberately narrower than what a consumer weather app shows. An app can average, smooth or re-interpret readings; a settlement page cannot. If the official CLI says the high was 84°F, the site is judged on whether it displayed 84 — not on whether 84 was the most pleasant or most representative number.

2. Data sources

The site reads three distinct data lanes for each station:

• High-frequency ASOS feed. KLAX and KMDW publish an experimental one-minute observation feed (stations KLAX1M and KMDW1M in the Synoptic network) alongside their standard five-minute reports. KNYC (Central Park) reports hourly. This lane gives the fastest possible view of the live temperature, but it transmits temperature in whole degrees Celsius — a limitation that matters for settlement, covered below.
• Official METAR reports. The station's regular aviation reports carry a remark group (the "T-group") that states temperature and dew point to a tenth of a degree Celsius. Hourly reports also carry six-hour maximum and minimum groups. This is the precision lane: slower, but exact.
• NWS climatological products. The CLI product itself (issued after midnight and updated through the day) and, where published, the Daily Summary Message with the station's whole-degree-Fahrenheit record. These are the settlement documents, not live feeds.

3. The climate day

The CLI maximum is not measured over midnight-to-midnight in local clock time. It is measured over the station's Local Standard Time (LST) climate day, which does not shift with daylight saving. In Los Angeles, for example, the climate day ends at midnight Pacific Standard Time, which is 23:00 during PDT. CLI.LAX computes every daily figure on the station's LST day, so the running high and the official settlement always describe the same 24-hour window.

4. Rounding and the coarse-candidate rule

The fast lane codes temperature as a whole degree Celsius. That code is lossy: a transmitted 28°C means the true reading was anywhere in [27.5, 28.5)°C, which converts to a span that can cover two official whole-degree-Fahrenheit values (82°F and 83°F). Treating "28°C" as exactly 82.4°F would invent precision the sensor never reported.

When a day's peak comes only from coarse readings, CLI.LAX publishes the lowest official whole-degree value the code can guarantee — the conservative candidate — and steps up to a higher candidate only when tenth-degree METAR evidence or six-hour maximum groups rule the lower value out. Tenth-degree readings are converted with the same round-half-up convention the NWS applies, so a 27.2°C observation is 80.96°F and displays as 81°F regardless of what the coarse code says.

5. Quality control

Fast-lane readings pass through spike and persistence checks before they can move the running high. An isolated coarse spike that contradicts surrounding tenth-degree evidence is rejected; repeated readings that confirm each other are admitted. When the station stops reporting, the site labels the display "sensor offline", dashes out the current and next-hour values, and marks the day's high as unverified rather than presenting a stale number as live.

6. Projections are forecasts, not observations

The projected high and nowcast band on the dashboard are model estimates anchored to the NWS gridpoint forecast (LOX 149,41 for Los Angeles), adjusted for wind regime — an onshore sea breeze caps the afternoon at KLAX, while an offshore desert flow removes that marine ceiling. Projections can never lower the day's observed high and are always labelled with their source and issue time. The running high itself is built only from observations.

7. Verification

Every settled day is scored: the site's published value is compared against the official CLI maximum (or, where the CLI is delayed, the ASOS one-minute archive maximum). The current scorecard — exact matches, within-one-degree rate, mean absolute error — is published on the dashboard, and the day-by-day record is on the archive page. These are measured results, not targets.