Minecraft wiki entry for Rain_Forest, associated text: Rainforests are wet biomes with many trees, which have a 1 in 3 chance of being large trees, instead of 1 in 10 like all other biomes. They are similar to forests but have a large amount of tall grass and ferns and lush green grass due to temperature and rainfall requirements. A biome is classified as a rainforest if the temperature is greater than 97% and the rainfall is more than 90%. As a result of this, the rainforest can often contain a lot of cliffs and hills because the world generator reduces height variation at lower rainfall values and vice versa.