How to read: w
Same as y, w also special sign, can't form syllable alone. It only expresses u, and appears in some Pin-Yin joinder of standing as an alones. The vowel u can only become one Pin-Yin character in Chinese, when it forms syllable alone should add w before it, becomes [wu]. When [ua],[uai],[uo] etc. Compound vowels form syllable alone, change u to w, are written as [wa],[wai],[wo]. It is similar to 'oo' in the English 'room' .

乌云wū yún:black clouds
乌鸦wū yā:crow