The first group shows two different developments from a long [u] sound:
Middle English sound | ME spelling | ModE spelling | MnE sound |
[u:x] |
bough tough |
bough plow |
[aʊ] |
enough tough |
[ʌf] |
[x] is the voiceless velar fricative as in Scots dialect loch.
The top examples show the loss of the final fricative, and the long vowel becomes a diphthong through the the Great Vowel Shift. When the fricative was retained and changed to [f], the vowel was shortened.