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.