Final answer:
The primary mode of heat transfer in a structure fire is radiation, specifically through infrared radiation, while convection and conduction are less significant in transferring heat to observers.
Step-by-step explanation:
The primary mode of heat transfer from one compartment to another in a structure fire is typically through radiation. This is due to the transmission of the infrared radiation which can heat objects without the need for a medium. In the case of a fire, while all three modes of heat transfer—conduction, convection, and radiation—occur, it is infrared radiation that is primarily responsible for the heat felt by observers. Convection occurs as hot air rises and cooler air replaces it, creating a circulation effect, but it tends to transfer heat away from the observers. Conduction, the transfer of heat through direct contact, is usually negligible in this scenario due to the lack of direct contact between the fire and the observers.