What is the Sign of the son of Man?

Certainly the death on the cross is a sign that occured in the past, but is it really a sign of the future? Surely Matt. 12.39 is positively our salvation but does it mention the sign of the return of Christ? I think the sign in Matt. 24.30 is quite unlike that of 12.39.

"But he answered and said unto them, An evil and adulterous generation seeketh after a sign; and there shall no sign be given to it, but the sign of the prophet Jonas" (v.39).

"And then shall appear the sign of the Son of man in heaven: and then shall all the tribes of the earth mourn, and they shall see the Son of man coming in the clouds of heaven with power and great glory" (24.30).

The word is "generation" here, which means that as long as there are evil and adultery so long will the generation be. At this point three different words from the Greek should be contrasted in their usage:
(1) world (kosmos)—which points to the organized world in the place called earth;
(2) age (aion)—which signifies a period in time of indefinite duration; and
(3) generation (genea)—which strictly refers to a period of a lifetime: say, 30 or 40 years; broadly, though, it refers to a period marked by special characteristics. And it is this Greek word in its broadest sense that is used here in verse 39. "Evil" reveals the nature, while "adulterous" discloses the particular phenomenon which marks this generation.

We must conclude therefore that the sign of the Son of Man is not the same as the Son of Man. Jesus is not the sign for a sign precedes that which it points to.

Matthew 24:29 Immediately after the tribulation of those days shall the sun be darkened, and the moon shall not give her light, and the stars shall fall from heaven, and the powers of the heavens shall be shaken:
30 And then shall appear the sign of the Son of man in heaven: and then shall all the tribes of the earth mourn, and they shall see the Son of man coming in the clouds of heaven with power and great glory.

Mark 13:24 But in those days, after that tribulation, the sun shall be darkened, and the moon shall not give her light,
25 And the stars of heaven shall fall, and the powers that are in heaven shall be shaken.
26 And then shall they see the Son of man coming in the clouds with great power and glory.

There is celestial events as described in all 3 gospels, but no specific mention of a singularly specific supernatural sign, nor even is there mention of a sign in Mark. Obviously this is later revelation when we are in the Tribulation itself getting close to the end of the Tribulation.

Luke 21:25 And there shall be signs in the sun, and in the moon, and in the stars; and upon the earth distress of nations, with perplexity; the sea and the waves roaring;
26 Men's hearts failing them for fear, and for looking after those things which are coming on the earth: for the powers of heaven shall be shaken.
27 And then shall they see the Son of man coming in a cloud with power and great glory.
28 And when these things begin to come to pass, then look up, and lift up your heads; for your redemption draweth nigh.

In Luke we see multiple signs but no indication of what that singular sign is: "And then shall appear the sign of the Son of man in heaven: and then shall all the tribes of the earth mourn, and they shall see the Son of man coming in the clouds of heaven with power and great glory" (Matt. 24.30). We just can't say what this sign is since Jesus has not yet come in the clouds with power to reign with an iron rod with His overcomers. He came on the cross with authority and was seen resurrected for 40 days, but He has not returned, not at 70 AD, nor yet today.