1 middle, in-between, medial, midway, halfway, transitional, intervening, intermediary:
Middle schools cater for the age-range intermediate between primary and secondary schooling. In sublimation a substance changes from a gas to a solid (or vice versa ), skipping the intermediate liquid state.
irregular, discontinuous, disconnected, sporadic, occasional, random, spasmodic, fitful, broken, periodic, alternating, cyclic(al), rhythmic(al), pulsating, seasonal, on-and-off, on-again-off-again, stop-and-go, stop-go:
Tomorrow, intermittent showers in the morning will yield to sunshine.