why if we use hour put an hour not a hour

here 'h' is silent ...& it has a vowel sound ,,do we use 'an hour' instead of 'a hour'...bcz vowel sound always keep 'an' ...

Hour is pronounced as the same way as our, h is silent in this word according to pronounce. The rule is to use 'an' before a word beginning with a vowel sound. so..