Why Irma got so big, intense, and scary

The strength of a hurricane is determined by three main factors: water temperature, wind shear, and moisture in the atmosphere. Warmer water and atmospheric moisture give the system energy. A low wind shear — i.e., sharp changes in wind directions as you go higher and higher in the atmosphere — keeps a hurricane from dissipating. 
All these conditions have been ideal for Irma. As for the wind shear, Klotzbach says there’s “almost zero.” That’s just bad luck. “If you plot shear averages over the Atlantic over the past 30 years, there’s really no trend,” he says. 
The Atlantic, meanwhile, reaches its hottest temperatures for the year in September (that’s why the peak of hurricane season is September 10). Klotzbach says the Atlantic is a degree or two Fahrenheit hotter than it usually is this time of year, which is providing Irma with some extra fuel. (The surface area of water in the Atlantic topping 82.4 degrees in September has been growing slightly since the 1970s, which also could contribute to Irma’s strength.)
Irma reached a top speed of 185 mph Tuesday, and maintained those winds for 37 hours. That’s “close to the maximum intensity you’d ever get out of an Atlantic hurricane for that long,” Klotzbach says. 
What’s more, Irma only encountered more hot water — approaching 90 degrees Fahrenheit — on its path to Florida. (The Gulf of Mexico, on the other hand, has been about 4 degrees above normal, which helped Harvey intensify so rapidly.) And, according to Klotzbach, “the atmosphere is more saturated than normal too.” So there’s plenty of energy and moisture to keep this system spinning quickly.