opasti

1. (of amount, level) to drop; to decline; to fall
   Temperatura je noćas opala ispod nule. — The temperature dropped below zero last night.
   Cene stanova su malo opale. — Apartment prices have fallen slightly.
2. (of leaves, hair) to fall off; to fall out
   Sve lišće je već opalo. — All the leaves have already fallen.
   Posle bolesti mu je opala kosa. — His hair fell out after the illness.

Don't confuse with opasati "to gird" and opaliti "to fire; to hit".