The difference between the formation of ordinary clouds and the generation of hurricanes is partly a matter of degree: Both owe their existence to rising bubbles of warm, moist air. For hurricanes, though, an additional factor is the formation of a huge, spinning low-air-pressure cell that continually refuels itself by sucking in more and more warm, moist air.
Being of lower density than cool air, warm air is buoyant and rises like a hot-air balloon. If it's a humid day, the rising warm air hoists large amounts of water vapor into the heavens. As a moist, warm air parcel ascends, the moisture condenses and cools into large water droplets and clouds -- e.g. the fluffy, sheep-like little cumulus clouds that meander innocently across the sky.
But on exceptionally warm, humid days, that process goes into overdrive, sometimes with scary results. On such days, the intense heat and humidity continually pump parcels of warm, moist air skyward.
As the air parcels rise, they continually release latent heat that propels the rising warm air even higher -- as if the warm air were pulling itself up by its own bootstraps, as the saying goes.
Exceptionally warm, moist air can rise so high that it forms extremely tall thunderstorms or hurricanes. The taller they are, the more violent they tend to be.
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