Foghorn: Havent ya heard the little saying “a, e, i, o, u, and sometimes y” :)? Uniform is pronounced with a palatal approximant: “y” (in IPA: /j/). Meaning, that sound isn’t a true vowel sound, it’s closer to a consonant. For example, you don’t say “an young person”, neither would you say “an uniform”, since the sounds in both words arent really vowels. So, the rule is still uniform, vowel sounds use “an”, and consonant sounds use “a” (and so do words beginning with the sound of “y” (as in “young”).