Tundra climate is characterized by harsh winters, low average temperatures, little snow or rainfall, and a short summer season. Tundra, in particular, is influenced by permafrost, a layer of permanently frozen subsoil in the ground. The surface soil, which tends to be rocky, thaws in summer to varying depths. The combination of frozen ground and flat terrain on the tundra impedes the drainage of water.
During the summer the permafrost thaws just enough to let plants grow and reproduce, but because the ground below this is frozen the water cannot sink any lower and so the water forms the lakes and marshes found during the summer months. Although precipitation is light, evaporation is also relatively minimal.