Abstract: Stochastic and determined processes and their intertransformation are the sources for the complexity of the variations in the world. According to the statistical theory of Brownian motions, it is issued that turbulences in the unstable fluid motions are caused by the molecule thermodynamic motions and produce the stochastic motions in various macroscales, which are the nature of the atmospheric motions. Observations have shown that the solar radiation, the main factor determining the atmospheric motions and changes, varies in random, so it is a stochastic external forcing on the atmosphere and has dominant impact on the climate change. Airland interaction, a time-varying coupling process with nonlinear feedback, forms the complex random external forcing on the atmosphere in the lower boundary, but the stochastic coupling dynamic model of the exchange in the interface is not set up yet. Owing to the chaotic nature of the atmospheric motions and all the stochastic external forcing factors, the determining prediction of the atmospheric motions is only valid in limited time range, which is determined by the uncertainty in the predicted subject and its temporal and spatial scale, and the uncertainty in the atmospheric motions. Therefore, unpredictability is objective in the atmospheric motions.