Lead-in to stratus or cumulus

ALTO

This prefix often describes clouds that form at mid-level altitudes, between the high wispy cirrus and low-hanging rain producers.

This clue is looking for a prefix that goes before "stratus" or "cumulus," two types of clouds. "Alto" fits because it forms "altostratus" and "altocumulus," which are mid-level cloud names. The wordplay is straightforward—just a common meteorological term used as a lead-in. Think of it like the "alto" section in music, but for clouds instead of voices.