Which weather phenomenon can cause significant turbulence while flying within a cloud layer?

The correct answer is convective clouds, which are typically associated with vertical development and strong updrafts and downdrafts. These clouds form in unstable atmospheric conditions and can lead to significant turbulence when an aircraft flies through them. The turbulent air movement results from the rising and descending air within these clouds, making them particularly hazardous to flight operations.

When pilots encounter convective clouds, particularly cumulonimbus clouds, they may experience severe turbulence, along with other hazards such as lightning, hail, and intense precipitation. This is in stark contrast to stratus and cirrus clouds, which generally indicate more stable conditions and do not have the same vertical motion associated with turbulence. While cumulus clouds can also have some degree of turbulence due to their developing nature, it is the extensive and intense vertical mixing in convective clouds that primarily causes significant turbulence, especially in a cloud layer.