We present aircraft observations of the evolution of the mean and turbulence structure of a well-developed internal boundary layer over the Persian Gulf during a Winter Shamal. The measurements were made by the UK Meteorological Office’s Meteorological Research Flight C-130 Hercules research aircraft during late April 1996 in support of the US Navy SHAREM-115 exercise. Soundings made at Kuwait International Airport show a deep convective boundary layer to exist just inland of the coast. The warm dry air is advected from the desert landmass over the cooler waters of the Gulf, is cooled at the surface and forms a stable internal boundary layer. The layer evolves continuously along wind until at the measurement area, several hundred kilometers downwind from the coast, the original convective boundary layer has been eroded away, and the internal boundary layer replaces it as a shallow marine layer, approximately 300 m deep. Subsidence warming of the lower atmosphere offsets the heat loss to the surface and a near constant downwards flux of sensible heat of 15 W m-2 is maintained for at least 150 km along wind. Upwind an exceptionally large sensible heat flux of approximately 250 W m-2 is sufficient to overcome the downwards sensible heat flux and support weak buoyant convection within the lower 100 m or so of the boundary layer. As the humidity of the layer increases along wind the latent heat flux decreases, so that the surface buoyancy flux is steadily reduced and ultimately reversed. The region above the weak surface convection falls within the local scaling regime (z/LL<1). The velocity variances are found to scale accordingly. The temperature variance, however, does not scale successfully and has values greatly in excess of those reported in previous studies