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A generic saddle-node bifurcation is proposed to modelize fast transitions of finite amplitude arising in geophysical (and perhaps other) contexts, when they result from the intrinsic dynamics of the system. The fast transition is generically preceded by a precursor phase which is less rapid, that we characterize. In this model, if an external source of noise exist, the correlation length of the fluctuations increases before the transition, and its spectrum tends to drift towards lower frequencies. This change in the fluctuations could be a way of detecting catastrophic events before they happen., This paper contains 7p. and 4 figures. It is also submitted at the Paris Conference intitled "Science du Nonlineaire", 16-18 Mars 2011, and will be published in "Comptes Rendus du Nonmlineaire, Non-lin\'eaire publications Paris France, 03 2011

International audience; Catastrophes of all kinds can be roughly defined as short-duration, large-amplitude events following and followed by long periods of "ripening." Major earthquakes surely belong to the class of "catastrophic" events. Because of the space-time scales involved, an experimental approach is often difficult, not to say impossible, however desirable it could be. Described in this article is a "laboratory" setup that yields data of a type that is amenable to theoretical methods of prediction. Observations are made of a critical slowing down in the noisy signal of a solder wire creeping under constant stress. This effect is shown to be a fair signal of the forthcoming catastrophe in two separate dynamical models. The first is an "abstract" model in which a time-dependent quantity drifts slowly but makes quick jumps from time to time. The second is a realistic physical model for the collective motion of dislocations (the Ananthakrishna set of equations for unstable creep). Hope thus exists that similar changes in the response to noise could forewarn catastrophes in other situations, where such precursor effects should manifest early enough.

We derive scaling laws for the steady spectrum of wind excited waves, assuming two inviscid fluids (air and water) and no surface tension, an approximation valid at large speeds. In this limit there exists an unique (small) dimensionless parameter $\epsilon$, the ratio of the mass densities of the two fluids, air and water, independently of the wind speed. The smallness of $\epsilon$ allows to derive some important average properties of the wave system. The average square slope of the waves is of order $|ln (\epsilon^2)|^{-1}$, a small but not very small quantity. This supports the often used assumption of small nonlinearity in the wave-wave interaction. We introduce an equation to be satisfied by the two-point correlation of the height fluctuations., Comment: 6 pages, 0 figure

In the framework of the lubrication approximation, we derive a set of equations describing the steady bottom profile of Leidenfrost drops coupled with the vapor pressure. This allows to derive scaling laws for the geometry of the concave bubble encapsulated between the drop and the hot plate under it. The results agree with experimental observations in the case of droplets with radii smaller than the capillary length Rc as well as in the case of puddles with radii larger than Rc., special issue in honor of Paul Clavin