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A hypercane is a hypothetical class of extreme tropical cyclone that could form if sea surface temperatures reached approximately , which is warmer than the warmest ocean temperature ever recorded. Such an increase could be caused by a large asteroid or comet impact, a large supervolcano eruption, a large submarine flood basalt, or "incredible" global warming. There is some speculation that a series of hypercanes resulting from the Impact event of a large asteroid or comet contributed to the demise of the non-avian . The hypothesis was developed by Kerry Emanuel of MIT, who also coined the term.
Hypercanes would have wind speeds of over , potentially gusting to , and would also have a central pressure of less than , giving them an enormous lifespan of at least several weeks. The pressure drop, compared to mean sea level pressure, would be the equivalent of being at almost in elevation, a level sufficient to cause altitude sickness. This extreme low pressure could also support massive storm systems roughly the size of North America. For comparison, the largest and most intense storm on record was 1979's Typhoon Tip, with a 1-minute sustained wind speed of and a minimum central pressure of . Such a storm would be nearly eight times more powerful than Hurricane Patricia, the storm with the highest sustained wind speed recorded, which had 1-minute sustained winds of . However, hypercanes may be as small as in size, and they would lose strength quickly after venturing into colder waters.
The waters after a hypercane could remain hot enough for weeks, allowing more hypercanes to form. A hypercane's clouds would reach into the stratosphere. Such an intense storm would also damage the Earth's ozone layer, potentially having devastating consequences for life on Earth. Water molecules in the stratosphere would react with ozone to accelerate decay into O2 and reduce absorption of ultraviolet light.
In Emanuel's model, if the temperature difference between the sea and the top of the troposphere is too large, there is no solution to the equilibrium equation. As more air is drawn in, the released heat reduces the central pressure further, drawing in more heat in a runaway positive feedback. The actual limit to hypercane intensity depends on other energy dissipation factors that are uncertain: whether inflow ceases to be isothermal, whether shock waves would form in the outflow around the eye, or whether turbulent breakdown of the vortex happens.
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