ROHM defines the ambient temperature as the temperature surrounding a an isolated (unconnected) resistor based on a number of factors. For reference, we list the following excerpts from JIS (Japanese Industrial Standards) defining ambient temperature (similar to IEC 68-3-1 and 68-3-1A). [Source: JIS Handbook ‘Electronic Test Methods: JIS C 0010 Environmental Test, Part 1 ? General and Guidance (IEC60068-1:88)].
4.6 Ambient temperature : Temperature of the air defined for the two following cases. Note : In applying these definitions, guidance should be sought from JIS C 0000/IEC 68-3-1 and its supplement 68-3-1A. 4.6.1 Non-heat-dissipating specimens : Temperature of the air surrounding the specimen. 4.6.2 Heat-dissipating specimens : Temperature of the air in free air conditions at such is neglibible. Note : In practice, the ambient temperature is taken as the average of temperature measured at a number of points in a horizontal plane situated between 0mm and 50mm below the specimen at half the distance between the specimen and the wall of the chamber or at 1m distance from the specimen, whichever is less. Suitable precautions should be taken to avoide heat radiation affecting these measurements. : unquote JIS C 0095 Background Information, Section One ? Cold and Dry Heat Tests 1.4 Ambient temperature Users of components and equipments, particularly equipments, require to know the maximum and minimum values of ambient temperature between which the item will operate and these should be specific for the purpose of testing. Certain difficulties arise here due to the fact that heat transfer is connected with temperature gradients and that therefore the temperature of the medium surrouding device is necessarily varying in space. Consequently, the "ambient temperature" of the surrouding atmosphere shall be specially defined. : unquote