Sources and limits

Where every figure comes from, and which ones we compute.

Every source, one row

Source	What it is used for	Licence	Attribution
Hong Kong Observatory	The daily morning tip (nine-day forecast including humidity, local forecast and warnings including fire danger), Current conditions, warnings, the nine-day forecast, the two-hour rainfall nowcast on the Today page (read at the nearest of 11 archived points, or Central and Western without location; it can miss sudden convective rain — on 2 October 2026, 54 mm fell at Tsuen Wan between 16:45 and 17:45, while no nowcast for that hour forecast more than 0.82 mm there in any half-hour), and the Hong Kong station winds on the case pages.	Observatory datasets published on DATA.GOV.HK are governed by that platform's terms and may be used free of charge for commercial and non-commercial purposes, without separate prior written authorisation from the Observatory. Tables 3.5.1 and 3.5.2 of Tropical Cyclones in 2018 may be used as calibration references without reproducing publication content. Station locations and measurement-specific metadata are governed by the CSDI terms. Reproduction of complete historical station data remains to be verified.	Appropriate attribution to the Government, Hong Kong Observatory and DATA.GOV.HK is required; no template is mandated. Station spatial data also credits the CSDI Portal and links its terms. Links to the Tropical Cyclone reports and prominent display of the full disclaimer on a sources page are an acceptable form of attribution.
Japan Meteorological Agency	Tropical cyclone position, intensity and forecast track.	PDL 1.0, which expressly allows commercial use; adaptation is allowed, but edited material must say that it has been edited.	The attribution form is: Based on “…” (Japan Meteorological Agency) (URL).
US National Hurricane Center	Eastern Pacific and Atlantic cyclone bulletins.	US National Weather Service public domain. Adaptation is allowed, but the result must not be presented as official data.	It must be kept visually separate from official bulletin content, and advertising must not sit beside it in a way that implies official endorsement.
NOAA Global Forecast System (GFS)	The global wind field (particle animation, press-to-read).	NWS public domain; adaptation allowed.	A work built mainly on NWS material needs a notice under 17 U.S.C. §403, and advertising must be kept visually separate from official forecast content.
NOAA Aviation Weather Center	Airport weather reports (METAR) and forecasts (TAF).	Treated as NWS public domain — but that is inferred from the NOMADS and Hurricane Center footers pointing at weather.gov/disclaimer. The second licence audit records that no wording on the AWC's own pages has been located. This cell is not closed.	The licence review records no attribution form for this source. The 17 U.S.C. §403 notice applies to work built mainly on NWS material. Access conditions also apply: a 100-requests-per-minute ceiling, a custom User-Agent, and no direct calls from a browser (there is no CORS).
GeoNames / Open-Meteo	Saved-city search, names, coordinates and time zones.	CC BY 4.0; free service is limited to non-commercial use and subject to usage caps.	Appropriate attribution, licence link and notice of changes; available via ⓘ in search.
Open-Meteo	The hourly forecast, apparent temperature and UV-peak time on the Today page, and the current conditions and nine-day forecast on the Current location page, plus US AQI (see the CAMS notice below), using the same two-decimal coordinates.	Two documents govern two different things and must not be merged: the data itself is offered under CC BY 4.0, which allows sharing and adaptation provided any changes are indicated; separately, the free tier's terms of use allow non-commercial use only, and expressly count a website or app that carries subscriptions or advertising as commercial use. The free tier also has rate ceilings of 600 a minute, 5,000 an hour and 10,000 a day.	Appropriate credit, a link to CC BY 4.0, and an indication of whether changes were made. Open-Meteo asks for a link where data is displayed; Aethopia places an ⓘ beside each block showing its data, linking to Open-Meteo.com and the CC BY 4.0 licence, and lists it on this page.
Environmental Protection Department (AQHI)	The Air Quality Health Index (a 1 to 10+ health-risk scale), including the mask reminder in the daily morning tip.	Published through DATA.GOV.HK, whose terms expressly list advertising as a permitted commercial purpose. The terms contain no adaptation clause.	The same three-part DATA.GOV.HK attribution as the Observatory: identify the source of the data, acknowledge the Government and the relevant organisations as owners of the intellectual property, and credit DATA.GOV.HK.
Airport Authority Hong Kong	The Hong Kong International Airport flight boards and flight-change notifications.	The boards come from the Authority's own live endpoint, not from the DATA.GOV.HK open data set (which lists only one resource, a /rest/flights/past URL marked Historical). The site terms expressly forbid modifying, publishing, redistributing or creating derivative works, and require prior approval.	Historical open-data credit: the Government of the Hong Kong Special Administrative Region, Airport Authority Hong Kong and DATA.GOV.HK. This is not a claim of permission for the live endpoint. The terms set out no attribution form, and design/licensing/records/ holds no record of any exchange with the Authority, so the attribution requirement is not closed.
Hong Kong Civil Aviation Department (ATIS)	The ATIS data items shown on the aviation pages (runway, wind, visibility, cloud, temperature), adapted — not the broadcast text.	The CAD terms allow government copyright material to be reproduced and distributed free of charge for personal or internal use within an organisation only, and not for re-sale or commercial distribution; reproducing, adapting, distributing or making such works available to the public requires prior written authorisation from the CAD.	The terms set out no attribution form; what the pages show is described in the “What it is used for” column.
NASA GIBS	Satellite imagery.	Commercial use, including advertising, is expressly allowed, but the material must not explicitly or implicitly convey NASA's endorsement of goods or services. The restriction is about the insignia and the impression of endorsement, not about the imagery itself.	The NASA insignia must not be used (expressly not on sites carrying open advertising), and advertising must not sit against the imagery in a way that implies endorsement. GIBS also asks for an acknowledgement line; its wording is a request rather than a requirement.
NASA Worldview Snapshots	Satellite image snapshots.	Treated together with GIBS for now. The second licence audit records that no separate terms for Worldview Snapshots could be located: the official pages point only at the Earthdata Forum, which carries no text. This cell is not closed.	No citation guidance could be located either; the GIBS acknowledgement is being used for now. This cell is not closed.
OpenFreeMap	Base map tiles.	MIT for the software, ODbL for the data; the official FAQ allows commercial use outright, with no registration and no usage ceiling.	Required, verbatim: OpenFreeMap © OpenMapTiles Data from OpenStreetMap. It has to be added by hand when a static image or a video is produced.
adsb.lol	Aircraft positions.	ODbL 1.0, with nothing anywhere on the site saying non-commercial. The two official statements address different things rather than contradicting each other: the CC0 line on the privacy-license page is about people who send data to feed.adsb.lol waiving their own rights, while the ODbL line in the API spec covers the data adsb.lol publishes. ODbL carries share-alike: publicly using an adapted database means offering that database under ODbL too. The terms also ask that anyone using the API for production purposes make contact first.	The notice required by ODbL clause 4.3: say that the content was obtained from the database and that it is available under that licence.
Flightradar24 API	Flight tracking. Not enabled at present: both an API token and production mode have to be set before this source is used at all.	Commercial use is allowed on every subscription tier, but it needs a paid plan and API credits; the terms expressly forbid using this API to supplement or backfill another provider's live flight data, and raw data may not be redistributed.	Clause 8.2 requires Flightradar24 to be credited explicitly. The Portal carries further legal notices, including cache limits, which are only visible behind a login and have not been checked.
OpenStreetMap	The coastline and land polygons for the eighteen-district rainfall map, and the airport ground geometry on the aviation pages.	ODbL 1.0, which allows commercial use. A derivative database is subject to share-alike and may only be released under the same licence; two further clauses apply — a notice must accompany any produced work that is publicly used, saying the content came from the database and is available under ODbL, and anyone publicly using a derivative database must offer recipients a machine-readable copy.	Attribution must be to OpenStreetMap; © OpenStreetMap contributors is an accepted form, and it must be made clear that the data is available under ODbL, with a link to openstreetmap.org/copyright.
Home Affairs Department, Hong Kong District Boundaries	The district boundaries on the eighteen-district rainfall map on the Today page.	Published through DATA.GOV.HK and the property of the Government of the Hong Kong Special Administrative Region, under the DATA.GOV.HK terms. The coordinates are copied as issued and not simplified, so no adaptation is involved.	The same three-part DATA.GOV.HK attribution as the Observatory: identify the source of the data, acknowledge the Government and the relevant organisations as owners of the intellectual property, and credit DATA.GOV.HK.
OurAirports	Airport codes, English names, coordinates, elevations and runway data; Chinese names are editorial translations.	Recorded in the data catalogue as public domain, allowing use, adaptation and redistribution.	No mandatory attribution for the public-domain data; data source: OurAirports (https://ourairports.com/data/).

Agency data

Current conditions, warnings and the nine-day forecast come from the Hong Kong Observatory; cyclone position, intensity and forecast tracks from the Japan Meteorological Agency and the US National Hurricane Center; the global wind field from the NOAA Global Forecast System. Figures are presented as the agencies issued them, with units converted, values rounded and names translated here, each marked as such; the underlying values are not altered. The Hong Kong wind speeds on the case pages are Observatory observations from the reports for those years, using the averaging periods defined in those reports. Mangkhut calibration uses maximum hourly mean wind on the hour and maximum 60-minute mean wind separately; neither is a ten-minute mean. These observations are official figures; model estimates are labelled separately. The wind, pressure and visibility under “Right now” are readings from individual Observatory automatic weather stations, not territory-wide values: neither wind nor visibility is measured at the Observatory headquarters, and visibility is reported from four stations only — Central, Chek Lap Kok, Sai Wan Ho and Waglan Island. The station shown here: pressure is the Observatory headquarters; wind is Central Pier, falling back to the first station with a reading in the Observatory table when Central Pier has none; visibility is the first station with a reading in the Observatory table, in practice Central, moving down the list when that station has no reading. In the native app, when location is allowed in Hong Kong, temperature, humidity and wind on the Today page each come from the nearest Observatory automatic weather station that measures them; when that station has no reading, temperature and humidity fall back to the Observatory headquarters, and wind to the station named above. The district outlines on the Today page rainfall map are taken from the Hong Kong Administrative Boundaries dataset, released by the Home Affairs Department through data.gov.hk; the dataset is the property of the Government of the Hong Kong Special Administrative Region. The coastline on the same map is a separate dataset: © OpenStreetMap contributors, licensed under the Open Database License 1.0 (ODbL). We extract the Hong Kong coastline from it, clip and simplify it, and ship the result with the site; that geometry is a derivative database of OpenStreetMap, is licensed under ODbL in turn, and is not a proprietary asset of this site. The land on that map is a third dataset, again © OpenStreetMap contributors under the Open Database License 1.0 (ODbL): we take the land polygons published at osmdata.openstreetmap.de, clip them to the Hong Kong area, simplify them and ship the result with the site, where the browser uses them to keep the district rainfall colours on land and leave the sea uncoloured. That geometry is a derivative database of OpenStreetMap, is licensed under ODbL in turn, and is not a proprietary asset of this site. The clipping happens only while the page is drawn; not one coordinate of the district outlines is altered. Aircraft positions and routes on the aviation pages come from ADSB.lol, a network of receiving stations run by volunteers, whose database is licensed under the Open Database License 1.0 (ODbL); what it can show depends on whether a station is within range, so it is never every aircraft in the sky. Satellite imagery comes from the NASA Global Imagery Browse Services (GIBS / Worldview): VIIRS and MODIS true colour on the case pages, and the visible band of Himawari or GOES for the thumbnails on the storm cards. The imagery is public domain, unprocessed, and nothing computed here is drawn on top of it. A visible band sees nothing at local night, so each thumbnail is stamped with the time of the image itself, which can be earlier than the time of the storm bulletin. We acknowledge the use of imagery provided by services from NASA's Global Imagery Browse Services (GIBS), part of NASA's Earth Science Data and Information System (ESDIS).

This content has been edited by Aethopia using Japan Meteorological Agency data; units are converted, values rounded and names translated.

What this site computes

The wind field, signal probabilities and historical replays are computed here with a physical model: research-grade, not a forecast. The engine is a Holland (1980) wind field with the Willoughby et al. (2006) radius-of-maximum-wind fit — not artificial intelligence. The impact on Hong Kong shown on the storm pages is computed here too, from the agency forecast track and gale radius: research-grade, not a forecast. All of it is labelled on screen, and none of it decides whether a signal, school closure or flight change is coming.

Sky: calculations, assets and limits

The index uses local 20:00–02:00 hours with the Sun at least 12° below the horizon: cloud 50 points, moonlight 30 and light pollution 20. Rain probability of 40% caps the score at 34. Missing weather prevents scoring; missing light data rescales 80 points. Moon/planet positions and eclipses use astronomy-engine (MIT). Space-station propagation uses satellite.js (MIT) and CelesTrak orbital elements; elements over seven days old are withheld. A predicted pass is no guarantee of visibility.

Light bands are derived from NASA Earth Observatory Black Marble 2016 global night lights. This dated light-emission proxy is not calibrated measured sky brightness and cannot represent obstructions, tonight’s sky or recent urban growth. NASA does not endorse this product. Star positions are factual XHIP catalogue coordinates distributed by d3-celestial; the 88 constellation figures use Olaf Frohn’s BSD-3-Clause data. Declination is public-domain WMM2025 model output at 2026.82 on a 2° grid, used only for heading correction; polar views use manual mode. Aurora chances use NOAA SWPC three-hour geomagnetic forecasts and a dipole magnetic-latitude approximation, not a prediction of naked-eye brightness. Aethopia compiles the meteor calendar; hourly rates do not predict how many you will see.

Night-light map · Catalogue and figures · BSD-3-Clause · Magnetic model

Known limits

“Cannot estimate” on a storm page means the agency published too little data to compute an impact, not that the storm is harmless.
The intensity colours use the Hong Kong Observatory class names and are defined on ten-minute mean wind, with band edges chosen here. Storms issued by the US National Hurricane Center use a one-minute mean, about ten per cent higher, so their band reads one class too strong. Nothing is converted.
Signal probabilities on the map have not been checked against Hong Kong measurements for the individual storm, so they may be too high or too low.
A storm's page is removed once it dissipates; old links stop working.
No storm surge is modelled; nothing on the map is about water level.
A replay tracks the real storm closely, but that is not forecasting: its steering flow is inverted from the best track after the event, so the answer was known before the run. Fast convergence is hindsight, not skill. These cases calibrate the physics; they do not prove accuracy.
The district name on the Today page is worked out inside your browser, against the official boundaries, and only after you press allow. Your coordinates never leave your device and are never sent to this site or to any third party; without permission, outside Hong Kong, or when no district matches, the line reads “Hong Kong”. The current-location page of the native app is the exception; see the privacy policy. The readings beside it come from several automatic weather stations and represent the territory, not that district.
The aviation page keeps the report's own shorthand: “10 km+” is the ICAO meaning of the coded value 9999 — ten kilometres or more, not exactly ten. Below ten kilometres the actual figure is written out.
In every case, the official bulletin takes precedence.

Other sources

The hourly forecast, apparent temperature and UV-peak time on the Today page, and the current conditions and nine-day forecast on the “Current location” page, come from Open-Meteo (open-meteo.com). The weather data is licensed under Creative Commons Attribution 4.0 (CC BY 4.0). Open-Meteo asks for a link where data is displayed; Aethopia places an ⓘ beside each block showing its data, linking to Open-Meteo.com and the CC BY 4.0 licence, and lists it on this page. Aethopia converts units, rounds values and selects time windows; this is not Open-Meteo's raw output. Aethopia's share images also contain Open-Meteo and CAMS data; this page is their source and licence notice. This site uses the free, non-commercial tier. Away air quality uses the US AQI scale, with the same two-decimal coordinates; see the CAMS notice below. The Air Quality Health Index (AQHI) comes from the Environmental Protection Department, taken from the hourly station values published at aqhi.gov.hk; it is a 1 to 10+ health-risk scale, not an Air Quality Index (AQI), and the two cannot be converted. The Hong Kong International Airport flight boards and flight-change notifications are derived from the Airport Authority Hong Kong’s public flight-information interface (hongkongairport.com). Every board shows the Authority’s own update time. The airport weather reports (METAR) and forecasts (TAF) on the aviation pages come from the NOAA Aviation Weather Center; that material is a work of the US federal government, carries no copyright restriction, and is shown here as issued. The automatic terminal information (ATIS) comes from the Hong Kong Civil Aviation Department (atis.cad.gov.hk). This site does not republish the broadcast text: the pages show only the data items it lists (runways, information letter, wind, visibility, cloud, pressure, temperature and dew point, and the runways for which wind shear is forecast), marked there as adapted rather than original. Airport locations and identifiers come from OurAirports (public domain); airport diagram geometry comes from OpenStreetMap contributors under ODbL 1.0. Like everything above, it is not for flight operations or any safety decision. CAMS data and licence

Air-quality and UV-index data come from Open-Meteo, which derives them from the Copernicus Atmosphere Monitoring Service (CAMS). Contains modified Copernicus Atmosphere Monitoring Service information 2026. Neither the European Commission nor ECMWF is responsible for any use that may be made of the Copernicus information or data it contains.

CC BY 4.0 · doi:10.24381/04a0b097

Where attribution appears

Source names, licence terms and disclaimers for the whole site are gathered on this page. App screens, website pages, push notifications, widgets and share images do not name sources; each keeps an entry point to this page. The privacy policy names the services that receive your data, which is a separate matter. Two exceptions exist where a licence requires attribution at the place the data is shown: weather, air-quality and place-name data supplied under CC BY 4.0 carry an ⓘ beside each block showing it, with the supplier and licence links; aircraft positions and airport ground geometry supplied under ODbL carry the same ⓘ. The ⓘ on a screen is the point-of-display credit the licence asks for; this page holds the full form of every attribution.

Go to the source

Terms of use

Observatory open datasets published on DATA.GOV.HK are governed by its terms and may be used free of charge for commercial and non-commercial purposes, without separate prior written authorisation from the Observatory. Data sources: the Government of the Hong Kong Special Administrative Region, Hong Kong Observatory and DATA.GOV.HK. The Government and relevant organisations own the intellectual property rights. The full Observatory disclaimer is retained below with report links, an attribution form accepted in the Observatory's reply; this does not imply endorsement of Aethopia or its estimates.

This website/publication/product/service contains information that is copied or extracted from the website of the Hong Kong Observatory (the “Observatory”), including https://www.weather.gov.hk and https://www.hko.gov.hk, and the sub-domains of weather.gov.hk and hko.gov.hk. The provision of information copied or extracted from, or links to, the Observatory’s website does not constitute any form of co-operation or affiliation between the Observatory and any person in relation to this website/publication/product/service or any content herein. Nothing herein constitutes any representation, warranty or implication that the Observatory agrees with, endorses, recommends or approves any content of this website/publication/product/service. The Observatory shall not have or accept any liability, obligation or responsibility whatsoever for any loss, destruction or damage (including without limitation consequential loss, destruction or damage) howsoever arising from or in respect of any use or misuse of or reliance on or inability to use any content herein.

The Observatory’s open data is also published via DATA.GOV.HK. Under those terms of use the data may be browsed, downloaded, distributed, reproduced and printed free of charge for commercial and non-commercial purposes; users must clearly identify the source and acknowledge the Government and relevant organisations as the owners of the intellectual property rights, and credit DATA.GOV.HK.

Model calibration refers to the Observatory's Tropical Cyclones in 2018: Table 3.5.1 (maximum hourly mean wind, on the hour) and Table 3.5.2 (maximum 60-minute mean wind). These are different statistics; official observations and Aethopia's own computations are also labelled separately. The Observatory confirmed use of these tables as calibration references without reproducing publication content. Reproduction of complete historical station data remains to be verified. Station locations and measurement-specific metadata come from official open datasets. Credit: the Government of the Hong Kong Special Administrative Region, Hong Kong Observatory and the Common Spatial Data Infrastructure (CSDI) Portal, under the CSDI terms.