Mercury is known for being an extreme planet. Many people assume that the extreme part of the planet is due to its weather. In a sense it does. However, it is not because Mercury has a lot of storms, but because its weather changes from very hot to not so hot. Some areas of Mercury are icy and cold even though it is the closest planet to the sun.
Another reason for being called an extreme planet has to do with its atmosphere. The atmosphere is almost nothing so it does not protect anything. The atmosphere is so thin because the gases that make up the atmosphere escapes easily and it has to do with the planet having a small mass.