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Though it's primarily a weather/time app, Living Earth HD is a great app to keep open next to you on your iPad while doing other work. There's something considerably calming to me about the zoom-able 3D simulation of Earth, which allows you to see other parts of the world from a satellite's-eye view. Simplistic, beautiful, and relaxing.
App Store Link: Living Earth HD ($0.99)
Do you ever wish you could create the perfect thunderstorm? Well, with Storm Sim, the only thing you'll lack is real, actual rain, thunder, and lightning! Working from pre-designed templates or creating your own, you control the specific combination and volume of samples you want to play. A great app for lovers of thunderstorms!
App Store Link: Storm Sim ($0.99 + optional in-app purchases)
Ambiance is like many of the other soundscape creation apps I've listed in this gallery, but with over 2000 free sounds that are exclusive to Ambiance, there are plenty of unique soundscapes awaiting your creation. Play just one sound or layer multiple ones -- all with individual volume selectors. This was the first app of its kind that I ever downloaded, and despite others I've downloaded since, the exclusive sounds belonging to this app are great to return to again and again.
App Store Link: Ambiance ($2.99 + optional in-app purchases)