Windy, like many weather forecasting websites and applications, likely uses a combination of historical weather data and numerical weather prediction models to generate forecasts for specific locations. Numerical weather prediction models use complex algorithms to simulate the behavior of the atmosphere based on a set of initial conditions, such as temperature, pressure, and wind speed. These models take into account a wide range of physical processes, such as the movement of air masses, the transfer of heat and moisture, and the effects of topography. The forecasts generated by these models are then adjusted using historical weather data and other observations to improve their accuracy. Some weather apps or websites may also use weather radar and satellite images to provide additional information about current weather conditions and to make predictions about future weather patterns.
To make offline weather forecast, the application will have to download the forecast data for the area of interest and store in the device, so that the user can access them even if the device is not connected to the internet.