Planned planes around the globe

When aircraft fly inside clouds, they fly under "instrument rules". It doesn't matter whether the visibility is reduced (at night) or totally blocked (in a thick cloud), this mode of flying simply assumes the crew has no external visual reference, they fly solely using indications given by on-board instruments.

What is the most dangerous cloud?
Cumulonimbus
These clouds are capable of producing lightning and other dangerous severe weather, such as tornadoes and hailstones. Cumulonimbus progress from overdeveloped cumulus congestus clouds and may further develop as part of a supercell. Cumulonimbus is abbreviated Cb.

What are 5 major types of clouds?
The list of cloud types groups the main genera as high (cirrus, cirro-), middle (alto-), multi-level (nimbostratus, cumulus, cumulonimbus), and low (stratus, strato-) according to the altitude level or levels at which each cloud is normally found