Ghil Michael author Robertson Andrew W. author Columbia University. International Research Institute for Climate and Society Columbia University. International Research Institute for Climate and Society originator text Articles 2002 English Thirty years ago, E. N. Lorenz provided some approximate limits to atmospheric predictability. The details—in space and time—of atmospheric flow fields are lost after about 10 days. Certain gross flow features recur, however, after times of the order of 10–50 days, giving hope for their prediction. Over the last two decades, numerous attempts have been made to predict these recurrent features. The attempts have involved, on the one hand, systematic improvements in numerical weather prediction by increasing the spatial resolution and physical faithfulness in the detailed models used for this prediction. On the other hand, theoretical attempts motivated by the same goal have involved the study of the large-scale atmospheric motions' phase space and the inhomogeneities therein. These "coarse-graining" studies have addressed observed as well as simulated atmospheric data sets. Two distinct approaches have been used in these studies: the episodic or intermittent and the oscillatory or periodic. The intermittency approach describes multiple-flow (or weather) regimes, their persistence and recurrence, and the Markov chain of transitions among them. The periodicity approach studies intraseasonal oscillations, with periods of 15–70 days, and their predictability. We review these two approaches, "particles" vs. "waves," in the quantum physics analogy alluded to in the title of this article, discuss their complementarity, and outline unsolved problems. Atmospheric sciences Meteorology 99 Supplement 1 2493 2500 2002-02-19 http://dx.doi.org/10.1073/pnas.012580899 http://hdl.handle.net/10022/AC:P:14478 NNC NNC 2012-08-22 10:13:57 -0400 2012-08-22 10:21:02 -0400 8498 eng