Objection 1. It would seem that the judgment will not take place in the valley of Josaphat or in the surrounding locality. For at least it will be necessary for those to be judged to stand on the ground, and those alone to be raised aloft whose business it will be to judge. But the whole land of promise would not be able to contain the multitude of those who are to be judged. Therefore it is impossible for the judgment to take place in the neighborhood of that valley.
Objection 2. Further, to Christ in His human form judgment is given that He may judge justly, since He was judged unjustly in the court of Pilate, and bore the sentence of an unjust judgment on Golgotha. Therefore these places would be more suitably appointed for the judgment.
Objection 3. Further, clouds result from the exhalation of vapors. But then there will be no evaporation or exhalation. Therefore it will be impossible for the just to be "taken up... in the clouds to meet Christ, into the air": and consequently it will be necessary for both good and wicked to be on the earth, so that a much larger place than this valley will be required.
On the contrary, It is written (Joel 3:2): "I will gather together all nations and will bring them down into the valley of Josaphat, and I will plead with them there."
Further, it is written (Acts 1:11): "(This Jesus)... shall so come as you have seen Him going into heaven." Now He ascended into heaven from Mount Olivet which overlooks the valley of Josaphat. Therefore He will come to judge in the neighborhood of that place.
I answer that, We cannot know with any great certainty the manner in which this judgment will take place, nor how men will gather together to the place of judgment; but it may be gathered from Scripture that in all probability He will descend in the neighborhood of Mount Olivet, even as He ascended from there, so as to show that He who descends is the same as He who ascended.
Reply to Objection 1. A great multitude can be enclosed in a small space. And all that is required is that in the neighborhood of that locality there be a space, however great, to contain the multitude of those who are to be judged, provided that Christ can be seen thence since being raised in the air, and shining with exceeding glory, He will be visible from a great distance.
Reply to Objection 2. Although through being sentenced unjustly Christ merited His judiciary power, He will not judge with the appearance of infirmity wherein He was judged unjustly, but under the appearance of glory wherein He ascended to the Father. Hence the place of His ascension is more suitable to the judgment than the place where He was condemned.
Reply to Objection 3. In the opinion of some the name of clouds is here given to certain condensations of the light shining from the bodies of the saints, and not to evaporations from earth and water. Or we may say that those clouds will be produced by Divine power in order to show the parallel between His coming to judge and His ascension; so that He Who ascended in a cloud may come to judgment in a cloud.
Again the cloud on account of its refreshing influence indicates the mercy of the Judge.
Source: Summa Theologica (New Advent)