GWS is a webtool used to serve the current weather conditions at different latitude/longitude coordinates from around the world. Weather conditions are classified according to those used in Pokémon GO.

Weather information is provded by AccuWeather and OpenWeather via their respective APIs. Sattelite imagery is provided by OWM.io/vane:.

The weather information is updated hourly, at 16 minutes past the hour. Wind data is updated upon loading of the map.

GWS queries the weather APIs every hour and saves the data on the server. This allows anyone to access the weather data via the tool without having to perform a new API call. The weather is then sorted into the categories used by Pokémon GO and returned upon request.

The tool (front end and back end) is entrirely written in javascript.

The cities GWS searches are the top 150 cities as reported by AccuWeather. Due to API limitations, this number cannot currently be increased. If you are looking for the weather in a certain city, it's probably better to go to AccuWeather.com or similar and look it up for that particular location.

Available forcast data is currently limited to daily forcasts for the next 5 days due to API limitations. Because the in-game weather is updated hourly, it does not make sense to provide this information at this time.

The specifics of how Niantic determines the in-game weather are currently unknown. It is speculated that level 10 S2 cells are used in determining the in-game weather, as provided by the real weather supplied by AccuWeather. How the weather is averaged into these cells, when the information is updated, and what defintions Niantic uses for the in-game conditions are not available to us. This tool attempts to fill in the gaps based on community-based research; however, the results may be inaccurate. Until we have GO's source code, we'll never know for sure how the weather is determined, and that may never happen.