Sources and limits

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

Official 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 appear as the agency issued them, unadjusted here, and unit conversions are labelled. The Hong Kong wind speeds on the case pages are ten-minute means measured by the Observatory at the time — also official figures. 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. 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).

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.

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, seven-day forecast, sunrise and sunset 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); as the licence requires, a link appears where that data is shown. This site uses the free, non-commercial tier. 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 on the aviation pages come from the Airport Authority Hong Kong (the hongkongairport.com flight-information interface); the Authority agreed to this use on condition that the data’s update time is shown, so every board carries the Authority’s own update time. The airport weather reports (METAR), forecasts (TAF) and automatic terminal information (ATIS) on the aviation pages come from the NOAA Aviation Weather Center; that material is a work of the US federal government and carries no copyright restriction. All of the above appears as issued by its source and is not for flight operations or any safety decision.

Go to the source

Terms of use

Some data on this site is taken from the Hong Kong Observatory website, published by the Observatory. The Government and relevant organisations own the intellectual property rights in that data. The Observatory’s intellectual property notice and terms of use are on its website; the disclaimer required by clause 5 of those terms follows:

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.