Halliday & Matthiessen (2014: 168):
Within the interrogative, the yes/no type is usually found with tone 2, the ‘uncertain’ rising tone. WH–interrogatives, on the other hand, favour tone 1 [fall], because although they are asking for a missing element, the proposition itself is taken as certain … ‘certainty’ means certainty about the polarity; there is no issue of ‘yes or no?’ with a WH- interrogative clause.