Typically resistors are rated for a temperature of 70C, above this resistor starts to derate. This means that above this temperature the resistor can only utilize a reduced power level. This is illustrated by a derating curve. The most important factors are detailed below. Enclosures The rate of heat loss is limited by installing the resistor in an enclosure. The enclosure limits air flow and therefore the removal of heat by convection. Special flameproof resistors are available, that cause a circuit brake before the temperature reaches a dangerous state. Power rating definition The power rating of a resistor defines the maximum energy a resistor can (safely) dissipate. The heat radiation of one resistor will be received by the next resistor and therefore have an extra increase in temperature for a given power consumption. Altitude Resistors loose heat by convection and radiation. This can have several consequences. The resistance value can shift permanently, the lifetime can significantly be reduced or the component is completely damaged resulting in an open circuit. In extreme cases the excessive power can even cause a fire.
The temperature is not equal across the resistor. The resistor body is slightly hotter than the terminals, with the highest temperature at the center of the body. The higher the rate of heat dissipation to the environment, the lower the temperature rise will be.