Ruach is a wonderfully physical word that means, all at once, spirit, wind, and breath. The same word describes the Spirit of God moving upon the waters at creation, the breath God breathed into Adam's nostrils, and the wind that drives the sea. To the Hebrew mind these were not separate ideas but one reality — the invisible, powerful, life-giving movement of God.
This is why Ezekiel's valley of dry bones comes alive when the ruach breathes into them, and why Jesus tells Nicodemus that the wind (in Greek, pneuma, which also means spirit) blows where it wills — so is every one that is born of the Spirit. The Holy Spirit is God's own ruach: unseen like the wind, life-giving like breath, and utterly beyond our control.
Ruach in Scripture
And the earth was without form, and void; and darkness was upon the face of the deep. And the Spirit of God moved upon the face of the waters.