What is the sign that appears in heaven and what are the clouds of heaven?

Matt 24:30: “Then the sign of the Son of Man will appear in heaven, and then all the tribes of the earth will mourn, and they will see the Son of Man coming on the clouds of heaven with power and great glory.

The Bible speaks of Jesus return with both literal and symbolic language. From the Old Testament it often describes God’s physical appearing in glory as a fire and cloud (as in the Exodus journey).

In Jesus’ ministry we see when He was transformed – glory came from within him outwardly and God’s glory rested upon him as a cloud. Matthew 17:5-6: “While he was still speaking, behold, a bright cloud overshadowed them; and suddenly a voice came out of the cloud, saying, "This is My beloved Son, in whom I am well pleased. Hear Him!" And when the disciples heard it, they fell on their faces and were greatly afraid.”

A healthy fear is the right reaction when experiencing God’s glory, John does the same when he is confronted with the glorified Jesus in the book of Revelation of Jesus Christ. The glory also came upon them as a luminescent cloud.

Also in Heb. 11:39-12:1—39: “And all these, having obtained a good testimony through faith, did not receive the promise,40 God having provided something better for us, that they should not be made perfect apart from us. 12:1 Therefore we also, since we are surrounded by so great a cloud of witnesses, let us lay aside every weight, and the sin which so easily ensnares us, and let us run with endurance the race that is set before us,

Here those who have gone before us that are in heaven are described as a cloud (being in glory).

When Jesus ascended in his own rapture-described in Acts 1:9: “Now when He had spoken these things, while they watched, He was taken up, and a cloud received Him out of their sight.”

1 Tim. 3:16 tells us He was “ received up in glory.”

When He comes again He will be seen by all, as he will be glorified and coming in a visible glory- this is the sign of the Son of Man seen in heaven in Matthew 24:30.

Mark 13:26: "Then they will see the Son of Man coming in the clouds with great power and glory.”

Luke 21:27 "Then they will see the Son of Man coming in a cloud with power and great glory.”

Rev 1:7: “Behold, He is coming with clouds, and every eye will see Him, even they who pierced Him”

It says of the same event He is coming in the clouds, with clouds, and on the clouds; and the world will see this event as Hs glory will accompany His return to earth.

II Thess. 1:7-10 explains how the Lord comes and punished the wicked by the glory of His power on that day.