In the statements, It rains; it is cold; it is growing dark; it feels chilly, the subject it cannot be clearly defined.
The meaning of the sentences is clear. We mean that rain is falling, that the weather is cold, that darkness is coming on. We have become so accustomed to use it in such cases as the above that the meaning is always clear, yet we cannot find a word that it stands for. The antecedent of it cannot be determined.
Verbs used with it in such cases are called impersonal verbs.
An impersonal verb is one that has no determinate subject.
Sometimes the subject is not represented at all; as, Methinks, meaning, it seems to me; lists, meaning, it pleases me.
Say may be called an impersonal verb in such expressions as,
There were, say, a hundred persons present.
The meaning is that about a hundred persons were present.