Are you using Stage Manager? If so, resize the Weather App window by dragging the handle that you’ll see at bottom-left or bottom-right of the App Window - or, using the three-dot button at top-centre of the App window, select Full Screen.
If you are not using Stage Manager, you may have your Weather App displayed in a slide-over view. Again, using the three-dot button at top-centre of the App window, select Full Screen.
More information about using multi-tasking can be found in this support page:
Use multitasking on your iPad - Apple Support
The iPad User Guide is also a rich source of information about using iPad and its many features. The Guide is available both online using a web browser (such as Safari), or as an Apple Books download:
iPad User Guide - Apple Support
https://books.apple.com/book/id1567104892