Hurricane Irma as a Category 5 hurricane Hurricane Irma churns across the Atlantic Ocean. (Photo: National Oceanic and Atmospheric Administration)

Hurricane Harvey, which made landfall in Texas as a Category 4 storm, killed at least 60 people, caused as much as $90 billion in damage by some estimates and rained down four feet of water that was so heavy it dented the Earth's crust and caused dams, levees and reservoirs to overflow.

So imagine the devastation a Category 5 storm might cause.

Hurricane Irma hit parts of the Caribbean as a Category 5 storm but was downgraded to Category 4 storm as it headed for the Bahamas and Florida. For a time, it had max winds of 185 miles per hour that remained that speed for a whopping 37 hours, according to Philip Klotzbach, a meteorologist at Colorado State University. Only one Atlantic storm in recorded history exceeded that level. Hurricane Allen, which hit Mexico and Texas in 1980, had top wind speeds of 190 mph, the Washington Post reports.

Clearly, Irma isn't your average storm. Dangerous Category 5 hurricanes — including Hurricane Katrina in 2005 (the costliest Atlantic hurricane in recorded history), Hurricane Wilma that same year and Hurricane Matthew in 2016 — require a particular set of circumstances to form. Luckily, such a perfect storm doesn't happen often.

(And it's worth noting here: If you see or hear anything about Irma being a Category 6 storm because of its extraordinarily high wind speeds, disregard it. There's no such thing as a Category 6 hurricane.)

How a Category 5 hurricane forms

A view of Hurricane Isabel from the International Space Station in September 2003. A view of Hurricane Isabel, a deadly Category 5 storm in September 2003, from the International Space Station. (Photo: NASA/Wikimedia Commons)

NASA says that during hurricane season between June and November, the Atlantic Ocean becomes a "meteorological mixing bowl, with all of the ingredients necessary to create the recipe for hurricanes."

The first ingredient is warm water of at least 80 degrees Fahrenheit on the surface, which acts as fuel. The second ingredient is wind. According to the National Oceanic and Atmospheric Administration (NOAA), evaporation and condensation of warm ocean water builds cloud columns with the wind circulating around a center like water down a drain. As these clouds gather, a tropical disturbance is created.

As the storm grows higher and larger and winds increase, it becomes a tropical depression. Once the winds hit 39 miles per hour, it becomes a tropical storm and gets a name. When wind speeds reach 74 mph, it becomes a tropical cyclone, the scientific term for a hurricane.

Hurricanes act as heat engines, NASA says, which draw energy up from tropical ocean waters to power the winds and ocean surges. The more energy it can draw and the less friction it encounters, the more the wind and waves can grow. Wired explains more on how a hurricane can become a Category 5:

“Irma had everything going for it,” says Kerry Emanuel, an atmospheric scientist at MIT. “The water was warm, the layer of warm water was deep, and there was almost no wind shear, which tends to be very destructive to hurricanes. It can live up to its potential, if you will.”

Since ocean conditions vary year to year, the exact set of factors needed to allow for a Category 5 hurricane may not be in play. And it's rare for a hurricane to make landfall as a Category 5 storm because it's no longer powered by the warm ocean water it relies on for strength — though it does happen.

What a Category 5 means on land

Hurricane Matthew was a Category 5 storm in 2016. Hurricane Matthew was a Category 5 storm that hit Haiti, Cuba, the Bahamas and the southeastern U.S. in 2016. (Photo: NASA Earth Observatory/Wikimedia Commons)

Category 5 hurricanes are major storms with winds of 157 mph or higher that will cause catastrophic damage, according to the National Hurricane Center. The center says, in no uncertain terms: "A high percentage of framed homes will be destroyed, with total roof failure and wall collapse. Fallen trees and power poles will isolate residential areas. Power outages will last for weeks to possibly months. Most of the area will be uninhabitable for weeks or months."

Given this, it's no surprise that Hurricane Irma has triggered evacuation orders in the Miami area, while residents elsewhere in the path of the storm prepare for it to make landfall.

Editor's note: This story has been updated since it was originally published.

Angela Nelson ( @bostonangela ) is an exhausted mom of two young daughters and two old cats, and a Pulitzer Prize-winning digital editor with more than 15 years of experience delivering news and information to audiences worldwide.