Tropical cyclones need warm ocean water (about 26 °C / 79 °F or warmer) to a good depth, plenty of moisture, and low vertical wind shear so the storm can stack vertically and organize. Warm water is the fuel: it evaporates, rises, condenses and releases heat that drives the storm’s winds.
Storms climb a naming ladder by wind speed. A tropical depression has sustained winds up to 38 mph; a tropical storm (which earns a name) runs 39–73 mph; at 74 mph it becomes a hurricane/typhoon. The same threshold applies worldwide even though the name changes by basin.
Rapid intensification — a jump of at least 35 mph in 24 hours — can turn a modest storm into a major hurricane with little warning when conditions are ideal (very warm water, moist air, low shear). It’s one of the hardest things to forecast and one of the most dangerous.
Storms weaken when they move over cooler water, hit dry air, encounter strong shear, or make landfall and lose their warm-water fuel — though a large storm’s rain and surge can remain deadly well after the wind eases.