The bible speaks of a time in the future when, as a form of judgment against the ungodly, God will send massive hailstones weighting in at a talent (an ancient form of measure) each upon those who have willingly served Satan and the AntiChrist. This will immediately follow the pouring out of the 7th bowl, a judgment that will include quite a few different events, including the declaration from Heaven of "It is done!", flashes of lightning, rumblings, peals of thunder, a severe earthquake unlike any in history and numerous other things.
This plague of hail mentioned as part of this judgment will be so fierce and incredible that it will cause those who survive it to curse God even more than they already have by this point in the Tribulation due to the sheer severity of the storm. This will also serve to prove, for one last time, that those who are unsaved and have sided with Satan are unrepentant and will remain so forever, and thus are deserving of their judgments. While the fact that large hailstones are present in this judgment, the actual size and weight of the hailstones is a topic of much debate.
The reasons for this comes from the fact that the generally accepted weight of the classical "Talent", a form of bulk measure used in antiquity, varies widely from as little as 44lbs upwards to 110lbs. The generally accepted "median" weight is 75lbs, while the typically accepted "scholastic" value is 90lbs. But that's the "standard" talent. There are other versions of the talent, such as for silver and gold, that can weight between 125 and 250 pounds! Even so, the most generally anticipated (i.e. the most likely to occur) size for one of these hailstones to be is around 100lbs.
Based on this anticipated value, the size of the hailstone would be approximately 20 inches across and would have an impact velocity at sea level of around 315mph. If these ultimately come from outer space, the likely starting mass of these "hailstones" will be up to well over 200 inches across at the time of planetfall as considerable mass will be lost as they make planetary entry. If they are generated planetside via large storms or unusual upper atmospheric events then their initial mass will be close to 20 inches across. However, it is unlikely that these will be storm generated, as the winds required to create these would be in excess of 600mph, upwards of 1000mph.
As such, the origin of the hailstones is most likely to be celestial or upper atmospheric in nature. This is further verified by the fact that, due to the presence of excessive thermal sheer in Earth's lower atmosphere, it's not possible for storm winds to exceed 350mph, and only then is it possible within a tornado, thus making it impossible for these hailstones to form within storms in the Earth's lower atmosphere. There is also the fact that the hailstorm is global rather than localized meaning that it can't be the result of a single localized storm. Even so, regardless of how God creates them, this hailstorm will be global and it will be extremely devastating and frightening to all who experience it.
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