The Rate of Rise (RoR) is defined as the rate of increase of the coffee bean temperature during the roasting process. The RoR is usually measured every 30 seconds.
In North America, the RoR is usually expressed in Fahrenheit; in Europe, Celsius is generally accepted as the measure for temperature.
A prolonged RoR of more than 15 degrees F. (15 degrees per 30 seconds), will accelerate the roasting progress, creating the risk that the operator looses control of the process, which jeopardizes a balanced flavor of the roasted coffee. Roasting too fast can also result in sour, astringent flavor notes.
A RoR of less than 5 degrees F. can slow down the roasting profile, which can possibly cause the roasting process to stall, resulting into bittersweet and leather-like flavor notes.