Take a picture of some clouds outside and robustly derive a cryptographic key from them. Broadcast some information encrypted with this key.
Anyone in the vicinity can take their own picture of the clouds, derive the same key, and decrypt. Time passes, the clouds change shape, and the information becomes impossible to decrypt: it was ephemeral. Attackers far away also cannot see the clouds.
How can one robustly derive the same key from pictures of clouds taken at slightly different points in time from slightly different vantage points? The sky is big: how can the sender and receivers agree on which cloud? Can weather satellites constantly recording every cloud defeat this?
What would this technology be useful for?
Are there other things like clouds which can serve as the basis for this kind of system? We could artificially generate such a signal (previously).