Thermal Equivalent Circuit: Heat transfer is modeled analogously to Ohm's Law where Temperature $\Delta T = P \cdot R_{\theta}$.
$T_j = T_a + P_d \cdot (R_{\theta JC} + R_{\theta CS} + R_{\theta SA})$
Convection Approximation: $R_{\theta SA}$ is estimated using empirical correlations. For natural convection, heat transfer coefficient $h \approx 5 - 10 \text{ W/m}^2\text{K}$. For forced convection, $h$ increases proportionally to $\sqrt{v}$.
$R_{\theta SA} = \frac{1}{h \cdot A} + R_{cond}$