The final thermal impedance required is “sink-to-ambient” denoted by the symbol R_{θ S-A}. This is a measure of how easily heat can be transferred from the base of the heat sink to the ambient air. Heat sink vendors like Same Sky will typically provide graphs like the one shown here or data points to illustrate how easily heat can be transferred from the heat sink to the ambient air under various airflow conditions and loads. For this example, it is assumed that the application is operating under natural convection conditions without any airflow. The graph above can be used to calculate the final thermal impedance (sink-to-ambient) for this particular heat sink. The surface temperature rise above ambient divided by the heat dissipated provides thermal impedance at that specific operating condition. In this example the heat dissipated is 2.78W, which results in a surface temperature rise above ambient of 53°C. Dividing 53°C by 2.78W produces a sink-to-ambient thermal impedance of 19.1°C/W (calculated as 53°C divided by 2.78W).