who is raining?
'S raining /sre'nIng/ is one way of many that we can pronounce It's raining. The beginning of a sentence, especially if it's predictable and meaning-free, is a frequent place for sound loss in pronunciation. Others of this type include: By the way, you're probably not right to identify 's raining with Is raining. That's an /s/, not a /z/; that is, you wouldn't say /zre'nIng/. But is has a /z/, not an /s/, and it shows up in fast speech rules when it really means is. For instance, if you meant to say He's reading and you were in a hurry, you'd say /zri'dIng/, not /sri'dIng/. So the contraction is just that -- a shortening of a predictable sentence part.