J.M. Ortiz de Zarate

Depto de Fisica Aplicada I, Universidad Complutense, E28040 Madrid,
Spain

J.V. Sengers, Institute for Physical Science and Technology

University of Maryland, College Park, MD 20742, U.S.A.

Starting from the linearized fluctuating Boussinnesq equations we derive
an expression for the structure

factor of fluids in stationary convection-free thermal nonequilibrium
states, taking into account both

gravity and finite-size effects. It is demonstrated how the combined
effects of gravity and finite size

cause the structure factor to go through a maximum value as a function
of the wave number q. The appearance

of this maximum is associated with a crossover from a q**-4 dependence
for larger q to a q**2 dependence

for small q. The relevance of this theoretical result for the interpretation
of light scattering and

shadowgraph experiments is elucidated. The relationship with studies
of various aspects of the problem by

other investigators will be discussed. The paper thus provides a unified
treatment for dealing with

fluctuations in fluid layers subjected to a stationary temperature
gradient regardless of the sign of the

Rayleigh number R, provided that R is smaller than its critical value
associated with the appearance of

Rayleigh-Benard convection.