5/9 Acts 2:2-3
2 And suddenly there came from heaven a sound like a mighty rushing wind, and it filled the entire house where they were sitting. 3 And divided tongues as of fire appeared to them and rested on each one of them.
A visible manifestation of God is referred to as a theophany. The wind was audible and the fire was visible, physical signs of the invisible Holy Spirit. Those signs were also present when Moses was on the mountain with God (Exodus 19-20) and seen by the prophets Isaiah (Isaiah 6:6) and Ezekiel (Ezekiel 1:4, 13). The Jews were celebrating Pentecost, which included gratitude to God for giving the Law to Moses and a reading from Ezekiel's first three chapters. While the images of God manifesting Himself in wind and fire were familiar ones to those attending Pentecost, He caused it to happen again, right before their eyes. That would certainly help them believe the message Peter preached.
An ancient Jewish tradition tells of the angels putting crowns of fire on the heads of the Israelites when God spoke the Law to Israel. In addition, the tradition says that the voice of God divided itself into the seventy languages of the world so that all heard God speak in their own language (The Legends of the Jews, Louis Ginsberg, 1909). While this may or may not have actually taken place, at Pentecost the Jews who knew of this legend saw it actually happening.
In the cases of both Ezekiel and Isaiah, God followed His divine manifestation by the command to deliver His message to the people (Ezekiel 3:4-5). That is exactly what believers in Jesus are to do as well. The Spirit who came upon those prophets now indwells all who believe in Jesus. He is there to empower us to be a witness of Jesus by our lives and our words.
Consider: God's work in Israel in the past was all leading up to the revelation of Jesus, the Lamb of God, who takes away the sins of the world.