Weather Archives Explained for Non-Experts
By WeatherRecall editorial project. Date reviewed: . Last updated: 2026-10-06.
A weather archive is a collection with rules about sources, variables, timestamps, and revisions. It may hold original observations, processed daily summaries, reconstructed fields, or forecasts issued in the past. These are not interchangeable. Reading the documentation before downloading prevents a common mistake: choosing an attractive historical chart that answers a different question from the one being researched.
Four different things an archive can contain
An observation archive retains measurements or summaries from instruments. NOAA GHCN-Daily integrates daily land-station climate summaries and provides access to data, format documentation, and station inventories. Its station values describe particular observing locations, with different record lengths and element coverage.
A reanalysis archive stores model-assisted reconstructions informed by historical observations. ERA5 from ECMWF belongs to this category. It can supply coherent spatial coverage where direct station records are sparse. Completeness here means a field is estimated throughout the domain, not that every point was measured.
An archived forecast preserves what a forecasting system predicted at a past issue time. This is needed for questions about forecast performance or what information was available before an event. A later reanalysis has access to information unavailable when that forecast was issued and is not a replacement for the original prediction.
A climatological summary archive contains statistics such as monthly means, normals, or threshold counts. It is useful for background context but may not retain the underlying hourly detail. The WMO climate overview explains why sustained monitoring and comparable information support climate services. A summary should retain its reference period and calculation definition.
Use an inventory before searching by city name
A station identifier is more reliable than a display name. Names can change, several stations can serve one city, and airport observations may be listed under an unfamiliar location. Start with coordinates, elevation, and the required variable. NOAA's metadata and inventories distinguish station listings from element-specific beginning and ending dates.
The period from the earliest to the latest entry does not guarantee uninterrupted coverage. A rainfall station may have no temperature series at all. Count usable entries in the interval of interest after applying the documented quality policy. If metadata are unavailable, say so rather than assuming a station stayed unchanged.
For a gridded archive, retain the product name, requested coordinates, returned grid information, resolution, and elevation treatment. API access is a delivery mechanism, not a distinct type of evidence. Two APIs can serve the same underlying reanalysis while applying different units or aggregation settings.
Illustrative example: decoding before averaging
Illustrative archive only; the codes and values below are fictional, not real observed data or a description of a particular provider's exact format. Suppose a teaching file stores temperature in tenths of a degree Celsius, uses −9999 for missing entries, and marks one value with a fictional suspect flag.
| Raw field | Flag | Decoded interpretation |
|---|---|---|
| 184 | None | 18.4 °C |
| 201 | None | 20.1 °C |
| −9999 | Missing | No usable value |
| 350 | Suspect | 35.0 °C, requires review |
Under a declared rule excluding missing and suspect entries, the usable mean is (18.4 + 20.1) ÷ 2 = 19.25 °C. Coverage is two usable entries out of four expected entries, or 50%. Averaging the raw fields would mix encoded units and a missing sentinel into a meaningless result.
Exclusion is not the only possible quality policy, but it must be consistent and recorded. Keep the original suspect entry and its flag so another reader can reproduce a different policy. For real files, use that archive's own unit scaling, missing codes, and flag meanings; do not copy this fictional scheme.
Read timestamps as part of the data
An hourly temperature may be an instantaneous value, while an hourly precipitation amount may summarize the preceding hour. A daily station record may use a morning observation boundary. UTC and local dates can differ. Those distinctions determine which event a row describes and whether two archives can be joined safely.
The Open-Meteo archive documentation lists timezone settings, daily aggregation definitions, and preceding-hour variables. Read the requested and returned units as well. A valid download in inches cannot be compared directly with a millimetre threshold without conversion.
Do not infer brief extremes from coarse summaries. Daily rainfall cannot locate a twenty-minute downpour; a monthly mean cannot identify the hottest hour. Choose the finest necessary interval that the source genuinely provides, while recognizing that finer timestamps do not eliminate spatial uncertainty.
An archive can change after publication
Late reports, corrected transcription, revised quality checks, and product updates can alter downloads. Preserve the retrieval date and, where available, release or version identifier. Save the untouched source separately from cleaned or aggregated tables. A small manifest listing parameters, files, and transformations makes a later discrepancy explainable.
Also read licensing and attribution requirements. Public access does not automatically permit every commercial redistribution. Follow the source's current terms and cite the underlying dataset, not only the website through which it was downloaded.
Choose according to the research question
Use station observations for evidence about a monitored site, reanalysis for broader spatial context, forecast archives for prediction evaluation, and normals for typical seasonal conditions. If the available archive cannot resolve your question, narrow the conclusion instead of inventing missing detail.
The date search and city tool are entry points for exploration. Read methodology before treating a WeatherRecall result as a specific archive type. Preserve provenance whenever you carry a displayed number into another report.
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References
About the author
WeatherRecall editorial project compiles educational summaries from listed public documentation. It is not a credentialed meteorologist and does not claim original field research or custody of the source archives. The invented decoding exercise is a teaching aid, not a real archive extract.