An experiment to determine the convection coefficient associated with airflow over the surface of a thick stainless steel casting involves the insertion of thermocouples into the casting at distances of 15 and \(30 \mathrm{~mm}\) from the surface along a hypothetical line normal to the surface. The steel has a thermal conductivity of \(15 \mathrm{~W} / \mathrm{m} \cdot \mathrm{K}\). If the thermocouples measure temperatures of 60 and \(50^{\circ} \mathrm{C}\) in the steel when the air temperature is \(100^{\circ} \mathrm{C}\), what is the convection coefficient?