The largest on weight of small components of hydrosphere waters are lakes, the total weight of which is estimated in 2,8*1020 g, though there are also other estimations: 7,5*1020, 5*1020 and even 1,76*1020 g. But even most overestimated of these estimations make insignificant shares of percentage from general weight of hydrosphere. It is almost impossible to count up weight of lake water because of a set of small lake reservoirs, which were never measured, and of the fact that the sizes of the lakes continuously vary and very frequently tend to vary in one party. Despite being named the small components of the hydrosphere, there are such among them, which can really be called the seas. This largest lake of the world is the Caspian sea, the area of which is 371 thousand km2. It is followed by the Top lake in Northern America - 82,1 thousand km2, lake Victoria in Africa - 69,0 thousand km2 and Aral sea - 51,0 thousand km2. .
Lakes are the small models of ocean on land from the point of view of dynamics of waters. The larger the lake is and the more its depth is, the closer it is to the ocean by the qualitative dynamic characteristics, and that is why Caspian sea is a real sea. As well as in the ocean, water in the lakes in summer is frequently divided into a layer of hashing at a surface, layer of temperature jump and more cold deep water. But much is defined by the depth of the lake and geographical situation. The deeper the lake is, the better stratification is seen. If the lake is superficial and small, the layer of hashing achieves the bed and temperature of water is homogeneous everywhere in the lake. Such condition is named homothermia. In many lakes homothermia exists in spring and summer.
Lake water has various dissolved "microatmospheres", the difference of which is connected to geographical situation, mineralisation of water, seasonal climatic phenomena, biogeochemical processes in water. Lake water can be compared to underground water by the set of the gases, but in whole weight of the gases, dissolved of lakes, is insignificant.
The lakes are usually younger than the forms of the relief, containing them. The cases of formation of lakes in historical time are well known. In our century the large lake named Saresian Lake was formed on Pamir. It has arisen during the earthquake in 1911 in a valley of the river Murgab as a result of a huge landslide, that had divided the river. The area of lake is only 86,5 km2, but the depth is up to 505 m.
The lakes can die very quickly, in one cases it takes tens years for them to disappear, and in other cases they die practically instantly as a result of the catastrophic spontaneous phenomena. The life of the lakes in mountains is especially short, where they are drained at break of natural dams, and then appear again.
The data on the amount of the lakes on our planet does not exist, but it is also very hard to take all of them into account because of the large variability. In some countries there are a lot of lakes, for example Finland quite often is named " the country of thousand lakes ", though actually there are more than one thousand of them there. On the territory of former Soviet Union there are 2,85 million of them. Lakes have different size, but there are 2,8 million with the area of a surface from 1 km2 and less,131 lakes with the area of a surface from 100 km2,50 thousand with the area of a surface from 1 up to 100 km2, 27 lakes with the area of a surface 1000 km2
