I'm a big fan of writing letters, and I liked seeing this project, entitled Skymail, by G. C. Haymes in the 1970's.
Haymes wrote to various artists and asked them to "describe the sky" on an enclosed postcard. The responses range from visual to textual descriptions, and it's interesting to see the variety of responses. I always like seeing how one project can lead to so many different interpretations.
(Letter Writer's Alliance)