Halliday & Matthiessen (2014: 311):
As far as meaning is concerned, we used the expression ‘circumstances associated with’ or ‘attendant on the process’, referring to examples such as the location of an event in time or space, its manner, or its cause; and these notions of ‘when, where, how and why’ the thing happens provided the traditional explanation, by linking circumstances to the four WH– forms that were adverbs rather than nouns.