Methodology

Every record is computed from NOAA's Global Historical Climatology Network Daily (GHCN-Daily): daily maximum temperature (TMAX), minimum temperature (TMIN), precipitation (PRCP), snowfall (SNOW) and snow depth (SNWD). Any observation with a quality flag is dropped. This build uses 42,764 stations in 181 countries.

Values are stored in tenths of a degree Celsius and tenths of a millimetre. The site shows Celsius and millimetres first, with Fahrenheit and inches alongside. A temperature such as 41.2 C is shown as 41.2 C (106.2 F). A temperature difference converts at 1.8 F per 1 C without the 32 degree offset.

A station is eligible for an element once it has at least 30 years in which more than 182 days have a complete observation for that element. Eligibility is evaluated as of each date, not as of today, so a station only starts producing records once it has qualified. This matches NOAA's approach, so results are comparable.

• Per element: maximum temperature, minimum temperature, precipitation, snowfall and snow depth each have their own test, so a station can be eligible for one and not another.
• As of each date: the count of qualifying years is evaluated with the data available up to that date, so the ledger never applies today's eligibility to the past.
• Complete days: a year counts only when it has more than 182 days with a complete, unflagged observation.

• Types: highest maximum temperature, lowest maximum, highest minimum, lowest minimum, highest daily precipitation, highest daily snowfall and highest snow depth.
• Timescales: daily (that calendar day across all years), monthly (any day in that month) and all-time.
• One event per observation: when a single observation breaks several timescales at once, the public ledger and every count show it once, under the broadest timescale it broke (all-time, else monthly, else daily). So a count of daily records means observations whose broadest broken timescale was daily.
• Ties are stored and labelled. Feb 29 has its own daily record and is tested explicitly.
• Margin is how far the new value is beyond the record it replaced, always shown as a positive amount. Years stood is the age of the replaced record. Period of record is the length of the station's history at the time.

The significance score ranks records from 0 to 100 so that a minor daily record and a national all-time record are not treated alike. It is deterministic and the same rule ranks every list on the site and gates the home page headline and the facts.

• Margin (weight 0.5): the margin in standard deviations of that station's values for the calendar window, capped at 3 deviations.
• Age (weight 0.3): years the old record stood, capped at 50 years.
• Period of record (weight 0.2): length of the station's history, capped at 100 years.
• Timescale multiplier: all-time 1, monthly 0.8, daily 0.6.
• Tie multiplier: 0.5.

Score = 100 x (margin part + age part + period part) x timescale multiplier x tie multiplier, rounded to a whole number. The bands are Exceptional (80 and above), Major (60 and above), Notable (35 and above), Routine (0 and above). A first record with no earlier value scores on period of record only.

Rain and snow records have a floor for ranking. A rain record under 25 mm in a day, a snowfall record under 50 mm, or a snow depth record under 100 mm is still listed and still counted, but its displayed score is capped at 20, so a trace of rain at a dry station does not outrank a flood.

Worked example

Take the record leading the home page, computed when this page was built: record daily precipitation (daily) at Rome on 10 October 2026, 50.8 mm (2.0 in). It beat the previous record by 20.3 mm after 117 years, at a station with 134 years of record.

• Margin part: 2.20 standard deviations / 3 = 0.733. This is the standard deviation implied by the published score; the data stores the margin, not the deviation.
• Age part: 117 years / 50 = 1.000.
• Period part: 134 years / 100 = 1.000.
• Weighted sum: 0.5 x 0.733 + 0.3 x 1.000 + 0.2 x 1.000 = 0.867.
• Multipliers: daily 0.6, not a tie.
• Score: 100 x 0.867 x 0.6 x 1 = 52.0, rounded to 52, which matches the published score of 52 (Notable).

Observations can be corrected after the fact, so records follow a lifecycle.

1. 60 day window. Any record set in the trailing 60 days is provisional. It carries a "provisional" label on every page that shows it, and the fact text says so.
2. Daily reprocess. Each day the pipeline reprocesses the whole window against the latest GHCN-Daily files. A record can stay, change value, or be withdrawn.
3. Corrections log. Every withdrawal, change or addition is written to the corrections log with the before and after values and the reason.
4. Final. Once a record is more than 60 days old the label drops, and the record changes only through a logged correction.
5. Yearly verification. Once a year the full history is replayed from the raw data, and any difference from the published ledger is reported and logged.

• Events (heat waves, cold outbreaks, extreme rain, snow events) are clusters of record events close together in space and time, with minimum station count, duration and area so that noise does not become an event. An event keeps its slug for life. Heat waves are named heat waves; heat dome confirmation from reanalysis data is a later addition.
• Storms come from IBTrACS. Agencies measure wind differently, and pages say which agency's values are shown. Accumulated cyclone energy (ACE) sums the squares of six-hourly wind speeds at or above 34 knots. Active storms show the latest advisory with its source and time, and we make no forecasts.
• Cities join stations into a thread from a curated mapping file, so a city record can outlast a station move. Only mappings in that file are published.