The large-scale structure of the atmospheric circulation varies from year to year, but the basic structure remains fairly constant. However, individual weather systems - midlatitude depressions, or tropical convective cells - occur "randomly", and it is accepted that weather cannot be predicted beyond a fairly short limit: perhaps a month in theory, or (currently) about ten days in practice (see Chaos theory and Butterfly effect). Nonetheless, the average of these systems - the climate - is stable over longer periods of time.