When is y considered a vowel in a word?

The letter Y can be regarded as both a vowel and a consonant. In terms of sound, a vowel is 'a speech sound which is produced by comparatively open configuration of the vocal tract, with vibration of the vocal cords but without audible friction...', while a consonant is 'a basic speech sound in which the breath is at least partly obstructed'.wiki.answers.com/Q/When_is_y_considered_a_vowel_in_a_Word&action=edit