Not even halfway into the 2017 hurricane season, and before Irma makes landfall in Florida, tropical mega-storms in the Atlantic basin have already broken several records, and challenged others, experts say.

A few that stand out, so far:

- So strong for so long -

As it swept across the Caribbean, Hurricane Irma generated winds averaging just over 295 kilometres per hour (185 miles per hour) for more than 33 hours, longer than any super-storm of comparable power ever recorded.

"Such an intensity, for such a long period, has never been observed in the satellite era" that began in the early 1970s, Etienne Kapikian, a forecaster at Meteo France, said.

The runner up is Typhoon Haiyan, which left more than 7,000 people dead or missing in the Philippines and packed winds of the same speed for 24 hours in 2013.

- Atlantic surge -

Irma was the first hurricane on record to reach Category 5 status -- the highest intensity level -- while still in the Atlantic Ocean, before entering the balmy waters of the Caribbean Sea, according to the US National Hurricane Center.

Tropical storms draw strength from surface waters warmer than 26 degrees Celsius (79 degrees Fahrenheit).

The fact that the swirling mass of clouds and water was able to turbo-charge over the Atlantic -- whose waters are cooler than the Caribbean but warmer than a few decades ago -- is consistent with global warming, scientists say.

Category 5 tropical storms produce sustained winds of at least 252 km/h for at least a minute at a time. Irma has since dropped down to Category 4.

- $10 billion cost -

Hurricane Irma has so far caused more than $10 billion (8.3 billion euros) in economic losses across the Caribbean, making it the costliest storm ever for the region's island nations and territories, according to the Center for Disaster Management and Risk Reduction Technology, based in Karlsruhe, Germany.

The tally is sure to rise as the storm hits the Bahamas on its way to Florida, but it has already surpassed the damage record set by Hurricanes Ike in 2008, and Hugo in 1989, at $9.4 billion each in today's dollars.

Hardest hit by Irma were Sint Maarten ($2.5 billion) and the US Virgin Islands ($2.45 billion), followed by Saint Martin ($1.55 billion) and the British Virgin Islands ($1.4 billion), according to the estimate.

- Rain, rain, rain -

Tropical storm Harvey -- which made landfall in Texas as a Category 4 hurricane on August 25 -- dumped more rain in places than any cyclone ever measured on the continental United States.

In one area southeast of Houston, Harvey unloaded more than 125 centimetres of water (nearly 50 inches), breaking the previous record (122 cm) set by cyclone Amelia.

- Second-fastest hurricane -

The highest sustained wind speed ever registered for an Atlantic basin storm was 305 km/h (190 mph), for Hurricane Allen, which caused several hundred deaths in Haiti and over a billion dollars in damage.

With consistent winds of 295 km/h, Irma shares the title of second-fastest hurricane with Wilma (2005), Gilbert (1988) and the notorious "Labour Day" storm that devastated southern Florida in 1935.

- Three's a crowd -

Along with Irma, the Caribbean Sea and Gulf of Mexico is host to two other hurricanes: the Category 4 Jose, projected to leave inhabited islands largely untouched on its northwest trajectory, and Category 2 Katia, due to make landfall in the Mexican state of Veracruz.

Three-at-once is not unprecedented, but it is rare -- it last occurred in 2010. Those storms, however, spun harmlessly in the Atlantic, while this time, two of them are hitting land.

The event of four active hurricanes hitting at one time has happened twice -- in 1893 and 1998 when Hurricanes Georges, Ivan, Jeanne and Karl all raged simultaneously.