The premise of this prophecy is simple. At some point God is going to turn up the heat on the planet to ridiculous and oppressive levels. Given that the verses describe men being "scorched" with great heat, it's likely going to be something never before seen on this planet. It's also been theorized that the first demonstration of what this may look like occurred in Southern Iran when heat indexes reached way up into the 160's for several days straight. While little else is known or understood about this, one possible theory of how this may unfold was presented in the Left Behind series in "The Remnant" and in the Left Behind Kids series #37, "Heat Wave".
Note: Some elements and foretastes of this prophecy will occur prior to the Tribulation, but this will not see its complete fulfillment until well into the Tribulation.
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The Fourth Bowl Judgment