We report an experimental study of a fluid layer of Prandtl number Pr510.3 subject to a horizontal temperature
gradient with the surface open to the air. For certain values of the control parameters ~fluid depth and
temperature difference! the basic flow is destabilized and a pair of so-called oblique hydrothermal waves is
observed. It is found that some main physical features are in agreement with those analyzed by Smith and
Davis @J. Fluid Mech. 132, 119 ~1983!#. Vertical and horizontal temperature profiles were measured, showing
bulk temperature oscillations that propagate in the direction of the temperature gradient. This behavior is
similar to the instability mechanism explained by Smith and Davis for intermediate Pr.