Scale-related variability is when the variablity, often measured by standard deviation or variance) is larger (or smaller) when the underlying value is larger. For example, if you are measuring times, such the evaporation time of different liquids, it is likely that the variablity of those with long evaporation times is greater than those with fast evaporation. Often {{log transforms} are used with time-measurements such as these to make the variance more uniform. This is a form of pre-whitening.
Used on Chap. 14: page 334