What phenomenon is referred to as blue thermals?

Question

Quiz SPL - Sailplane Pilot License - Meteorology - What phenomenon is referred to as blue thermals?

QuizVDS · Accepted Answer

Thermals without formation of Cu clouds

QuizVDS · Answer

Thermals with less than 4/8 Cu coverage

QuizVDS · Answer

Descending air between Cumulus clouds

QuizVDS · Answer

Turbulence in the vicinity of Cumulonimbus clouds

SPL: What phenomenon is referred to as "blue thermals"?