No injuries were reported in the explosions.
After a long dry spell across Lebanon, hailstones as big as walnuts hit the South's villages and struck undiscovered bomblets scattered across the landscape, causing a string of blasts, the state-run National News Agency reported.
Dalya Farran, a spokeswoman for the United Nations Mine Action Coordination Center (MACC), confirmed that "many cluster bombs" exploded Tuesday when they were hit by hailstones in villages near the town of Marjayoun.
More than 30 people have been killed by cluster bombs in southern Lebanon since last year's war between Hezbollah and Israel.
The United Nations and human rights groups accuse Israel of dropping about four million cluster bomblets during last year war, with up to 1 million failed to explode and now endanger civilians in the area.
When cluster bombs, which look like huge canisters, are dropped, they open in flight and eject dozens or hundreds of small bomblets across the ground and vegetation, often across a wide area. Some fail to explode on impact and detonate later when moved.