The graph below indicates how to reduce the power dissipated by a potentiometer based on the increase in operating temperature. Three different materials are listed; Carbon Composition, Conductive Plastic, and Cermet. An additional curve for Wire Wound potentiometers is also shown. Cermet material is the best material to use for the widest range in operational temperatures, note the material works out to 125C before any derating is required. However there are a number of different types of packages, and each package will have its own operational characteristics. The graph is a general guide and in many cases a wire-wound device will operate up to 300C.
Percent Rated Power x Maximum Operating Temperature
The power rating of the device must be decreased as the ambient temperature is increased. The decrease in power rating with increasing temperature has to do with a number of factors, and could change with case style changes within the same family of components. The data sheet would need to be reviewed to determine the correct derating for any specific part. The link shows the derating curve for an Adjustable WireWound resistor.
Keep in mind that two temperature rating might be provided; a maximum ambient temperature at rated wattage, and a maximum ambient operating temperature. Of course when using the maximum operating temperature the maximum rated wattage may not be used.
Derating guidelines for components; Guideline for Derating Electronic Components;
How to Derate components as the temperature changes.
