Goggle lenses come in a variety of tints to suit different weather conditions. It’s a great idea to carry at least one spare lens should the visibility change or you take a bad crash that leaves your first one frozen.
When you buy a new pair of goggles, it will usually come fitted with a moderately dark tint – a lens like this cuts out a fair amount of light thanks to its reflective coating, and can be used in mixed conditions including patchy sun and clouds.
Removing the lens is a simple matter of starting in one corner and pulling out the clips that keep it in place around the frame. Other systems – like Anon’s magnetic lenses – are even easier.