Extremes of Heat and Cold
Deserts are regions of aridity with rainfall of less than 20 cm per year and the soil, though L, fertile, is too porous to retain any water. In summer there is great diurnal variation in temperature from 40°C during the day to below 15°C the same night. The light intensity reaches higher peak due to lack of cloud cover. The true deserts are Sahara in Africa and Great desert in Australia where the annual rainfall is less than 2 cm. Plants and animals which survive under such conditions have special features that allow them to withstand high temperature and lack of water.
The other extreme of temperature is seen in the arctic and alpine tundras. The word tundra means bare mountain tops in Finnish. Tundras are kozen most of the year round and are very similar to deserts as the rainfall here is also about 20 cm per year. But during the short summers, water is plentiful as the topmost layer of ice melts. The ground gets covered with short grass and patches of moss and lichens. Beneath the thawed layer of 3-5 cm, the soil called permafrost remains permanently frozen. Since the roots cannot penetrate deep in the soil trees do not grow here.