The app is fully usable offline. All data will be cached locally. If you select a product you will see the last downloaded version first. In the background - if a new version of the product should be available - the app tries to download a newer version. Status is shown on the status bar. As soon as the download was successful, the currently displayed version of the product is refreshed.
If you want to have data available offline, just download the desired products at least once. For instance if you want to have radar images available in the air (offline) just select the desired radar product while on the ground (online). Later when you are offline and select the product, you will see the last version you downloaded.
You can at any time refresh the current view by tapping . To refresh you need to be online.
The image and text cache is stored on a location that is not include in device backup. You can view the current size of the cache by selecting "Settings / Help" and then "Cache / Backup".
You can empty the cache by selecting "Settings / Help", then "Cache / Backup". Then tap on "Clear cache".