# Thermal resistance Junction - Maximum Power dissipation

I want to understand the real meaning of the parameter 'Thermal Resistance Junction to Air (°C/W) to calculate the power dissipation. So for example if the value of this parameter is 250°C/W, should I take this value as a constant if my ambient temperature is 50°C and then calculate my power dissipation with Pd= (Tj(max)-Ta)/R(°C/W)? Or I have to determine the Thermal Resistance Junction to air as a variable parameter and calculate it considering the ambient temperature and for my case with 50°C (Ambiant temperature).

I thank you for your help.