a distinction between a deterministic system and a predictable system. Lorenz's original mathematical model (Lorenz 1963 -note its title!), in common with more recent simple models used to illustrate chaotic behaviour, is perfectly deterministic, i.e. its evolution is perfectly determined by its initial conditions. Nevertheless it also exhibits unpredictability, because the smallest error in the initial conditions eventually leads to complete loss of forecast skill. Therefore we cannot conclude that a physical system is not deterministic just because we find it to be unpredictable.Perhaps the most we can conclude about physical systems which exhibit chaotic behaviour is that there is a severe and fundamental limit on our ability to investigate whether and to what extent such systems are deterministic. We certainly cannot conclude, on the basis of these arguments, that "determinism is an untenable proposition".

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The available potential energy of the atmosphere may be defined as the difference between the total potential energy and the minimum total potential energy which could result from any adiabatic redistribution of mass. It vanishes if the density stratification is horizontal and statically stable everywhere, and is positive otherwise. It is measured approximately by a weighted vertical average of the horizontal variance of temperature. In magnitude it is generally about ten times the total kinetic energy, but less than one per cent of the total potential energy. Under adiabatic flow the sum of the available potential energy and the kinetic energy is conserved, but large increases in available potential energy are usually accompanied by increases in kinetic energy, and therefore involve nonadiabatic effects. Available potential energy may be partitioned into zonal and eddy energy by an analysis of variance of the temperature field. The zonal form may be converted into the eddy form by an eddy-transport of sensible heat toward colder latitudes, while each form may be converted into the corresponding form of kinetic energy. The general circulation is characterized by a conversion of zonal available potential energy, which is generated by low-latitude heating and high-latitude cooling, to eddy available potential energy, to eddy kinetic energy, to zonal kinetic energy. I. The concept of available potential energy The strengths of the cyclones, anticyclones, and other systems which form the weather pattern are often measured in terms of the kinetic energy which they possess. Intensifying and weakening systems are then regarded as those which are gaining or losing kinetic energy. When such gains or losses occur, the source or sink of kinetic energy is a matter of importance. Under adiabatic motion, the total energy of the whole atmosphere would remain constant. The only sources or sinks for the

Anticipating the opportunity to make supplementary observations at locations that can depend upon the current weather situation, the question is posed as to what strategy should be adopted to select the locations, if the greatest improvement in analyses and forecasts is to be realized. To seek a preliminary answer, the authors introduce a model consisting of 40 ordinary differential equations, with the dependent variables representing values of some atmospheric quantity at 40 sites spaced equally about a latitude circle. The equations contain quadratic, linear, and constant terms representing advection, dissipation, and external forcing. Numerical integration indicates that small errors (differences between solutions) tend to double in about 2 days. Localized errors tend to spread eastward as they grow, encircling the globe after about 14 days. In the experiments presented, 20 consecutive sites lie over the ocean and 20 over land. A particular solution is chosen as the true weather. Every 6 h observations are made, consisting of the true weather plus small random errors, at every land site, and at one ocean site to be selected by the strategy being considered. An analysis is then made, consisting of observations where observations are made and previously made 6-h forecasts elsewhere. Forecasts are made for each site at ranges from 6 h to 10 days. In all forecasts, a slightly weakened external forcing is used to simulate the model error. This process continues for 5 years, and mean-square forecast errors at each site at each range are accumulated. Strategies that attempt to locate the site where the current analysis, as made without a supplementary observation, is most greatly in error are found to perform better than those that seek the oceanic site to which a chosen land site is most sensitive at a chosen range. Among the former are strategies based on the ''breeding'' method, a variant of singular vectors, and ensembles of ''replicated'' observations; the last of these outperforms the others. The authors speculate as to the applicability of these findings to models with more realistic dynamics or without extensive regions devoid of routine observations, and to the real world.