Atmosphere is the most changeable part of the climate system.
In this time interval it is possible to make a weather forecast based on search of hidden internal regularities of variations in atmospheric processes and separating out determinal elements in the structure of hydrometeorological series. A formal forecast of mean air temperature for the coming month is based on extrapolation of development of atmospheric processes immediately preceding the forecast. With increase of a forecast lead time an error grows since in methods being employed an error of calculated values is rapidly accumulated because of inaccuracy of initial conditions and inconsistency between a model and real development of atmospheric processed. These are precisely the methods to which conclusions about limitation of predictability to two weeks are applied. The method is based on numerous conclusions stating that oscillatory rhythmical phenomena appear to be a fundamental peculiarity of nature.