Airs are usually slow tunes played by one instrument alone (or with sparse accompaniment) where the player has the freedom to express the tune often varying the speed at appropriate moments.

I like a viola, me. I agree with what people are saying about it being a great instrument to accompany with. But you might be looking for alto parts rather than descant.

But don't violas attract similar jokes to the ones we apply to bodhrans, banjos, and (insert instrument name here)s?