I'm going to use a power resistor which needs to dissipate around 2.5W. I will use a resistor rated at 3W. I have a thermal impedance of 88°C/W. If the ambient is at 40°C, I assume that the resistor will go over 2.5W*88°C/W + 40°C = 286°C
Opening the datasheet, it allow an ambient temperature range -55 to 200°C. Then there is a graph showing the dissipation over the allowed temperature rise, that goes over 300°C.
If the resistor will go over 200°C, is it still ok? Can the resistor handle the temperature? (assuming that the tin will not melt). I should be able, by observing the next graph, which is the derating curve. In other words, how one should use the Temp Rise VS Power graph? Here an example datasheet: http://www.farnell.com/datasheets/1449996.pdf
EDIT: Searching around and viewing other datasheets, I've understood that one should see only the derating curve. It is considered to use resistor with its stated thermal resistance, while the temperature rise can be used to understand if the temperature achieved is suitable for you. In fact, the derating curve is shaded over the 200°C (max ambient) up to 315°C, because it is not guranteed a well behaviour of the materials. All resistor's datasheet are explained like this.