Via Emily Menon Bender on Facebook, this 6/7/12 Rhymes with Orange cartoon:
The dialectal feature illustrated here is known as “Canadian raising”; it’s a stereotype of Canadian speech.
Canadian raising refers to either of two similar sound changes that occur in a number of North American varieties of the English language, in which certain diphthongs are “raised” before voiceless consonants (e.g., /p/, /t/, /k/, /s/, /f/). The first variant, “classic” Canadian raising, occurs largely in Canadian English and in certain nearby areas of the northern United States, and affects both /aɪ/ and /aʊ/. This results in the stereotypical Canadian pronunciation of about as “aboat” [or “aboot”]. A second variant with a much larger distribution across many parts of the United States affects only /aɪ/, and results in differing pronunciations of the first vowel in the words rider and writer.
The raised variant of /aɪ/ typically becomes [ʌɪ] or [ɐɪ], while the raised variant of /aʊ/ varies by dialect, with [ʌu] more common in the west and a fronted variant [ɛʉ] commonly heard in Central Canada. In any case, the [a]-component of the diphthong changes from a low vowel to a mid-low vowel ([ʌ], [ɐ] or [ɛ]).