RISER RISER A reservoir in a mold that supplies molten material for part contraction during solidification. RISK An expression of the possibility of a mishap in terms of haz- ard severity and hazard probability. ROOM TEMPERATURE AMBIENT (RTA) (1) An environmen- tal condition of 73 ± 5°F (23 ± 3°C) at ambient laboratory relative humidity. (2) A material condition where, immediately following consolidation/cure, the material is stored at 73 ± 5°F and at a max- imum relative humidity of 60%. ROOT That surface of the thread that joins the flanks of adjacent thread forms and is immediately adjacent to the cylinder or cone from which the thread projects. ROOT DIAMETER The diameter of an imaginary cylinder or cone bounding the bottom of the roots of a screw thread. Root diameter is a nonpreferred term for the minor diameter of an exter- nal thread or the major diameter of an internal thread. ROOT FACE That portion of the groove face adjacent to the root of the joint.