https://en.wikipedia.org/wiki/Tundra#Climatic_classification

http://www.worldbiomes.com/biomes_tundra.htm

http://science.opposingviews.com/tundra-rain-5006.html

http://www.eoearth.org/view/article/156735/

http://environment.nationalgeographic.com/environment/habitats/tundra-threats/