Ozone Layer
The ozone layer is a part of the earth's atmosphere, consisting of special gas, ozone. It is made up of oxygen atoms, and has very important qualities. However, it can only be found in this part of the atmosphere, where the concentration of molecules is one per some square meters. This is very important, because the ozone is very reactive. It is important to live on earth, because one molecule of ozone may absorb a big quantity of ultraviolet energy, which has destructive effect to the live organisms.
However, when a molecule of ozone absorbs too big amount of energy, it may split into an oxygen molecule, and a free oxygen atom. The most dangerous is that the free atom formed may get to another ozone molecule, and split it into two new oxygen molecules. This forms big holes in the ozone layer, through which the ultraviolet radiation can pass, and harm the animals and plants below it. Such holes form over territories with cold climate, because the ozone in such places able to hold, and reflect less energy. Such wholes are also produced as a result of some human activities.