Hurricane Irma, which has caused havoc in the Caribbean and Florida, has come quickly after Hurricane Harvey flooded Texas.
But what are hurricanes? How do they form? How do they get their names? And why are some much more destructive than others? We answer your hurricane questions below.
What is a hurricane?
A hurricane is a tropical cyclone. These are large, swirling storms which have violent winds, intense rains and are usually accompanied by a large “wall of water” known as a storm surge.
Image:Florida shuts down as the outer bands of Hurricane Irma start to reach the state
How do they form?
Several factors are needed for a tropical cyclone to form.
A warm ocean is necessary. So is a pre-existing weather disturbance, moisture and light winds.
Despite all this scientists do not know precisely why some become hurricanes and others don’t. The strongest storms are often “Cape Verde” style hurricanes. They are named because they form from tropical disturbances near the Cape Verde Islands – off the west coast of Africa.
How are they measured?
The strength of Atlantic hurricanes is measured using the Saffir-Simpson hurricane wind scale (SSHWS).
This places hurricanes in five categories – one to five – depending on the sustained intensity of the wind.
The scale does not measure how much rain a hurricane brings or the level of the storm surge – even though these can sometimes be even more destructive than the wind.
What are the categories and what do they mean?
The US National Hurricane Centre has different description for tropical storms and each of the five categories of hurricane.
Tap or click the images below to find out more about each type of storm and to see some examples of each.
Tropical storm: Winds of 39mph-73mph
Hurricanes start their lives as tropical depressions before becoming a tropical storm.
Although they are often much less powerful than a hurricane, tropical storms can still be deadly.
More than 30 people died in Dominica and Haiti when Tropical Storm Erika hit in 2015, bringing strong winds and intense rain which caused mudslides.
Category 1 hurricane: Winds of 74mph-95mph
These storms have very dangerous winds which produce some damage.
Well-constructed frame homes could have damage to roofs, shingles and gutters. Large branches of trees will snap and shallowly rooted trees may be toppled.
Extensive damage to power lines and poles is likely which will result in power outages that could last a few to several days.
Hurricane Stan was a Category 1 storm when it hit Mexico in October 2005. The hurricane caused flash flooding in mountainous areas, causing more than 1,600 deaths.
Category 2 hurricane: Winds of 96mph-110mph
These storms have extremely dangerous winds will cause extensive damage.
Well-constructed frame homes could sustain major roof damage. Many shallowly rooted trees will be snapped or uprooted and block numerous roads.
Near-total power loss is expected with outages that could last from several days to weeks.
The Category 2 Hurricane Fifi remains one of the deadliest Atlantic hurricanes on record.
The storm brought flooding which killed up to 10,000 people in Honduras and other parts of central American in 1974.
After being downgraded to a tropical storm, Fifi moved in to the Pacific and re-formed as Hurricane Orlene.
Category 3 hurricane: Winds of 111mph-129mph
This is considered a major hurricane which will cause devastating damage.
Well-built framed homes will be damaged. Many trees will be snapped or uprooted, blocking numerous roads.
Electricity and water will be unavailable for several days to weeks after the storm passes.
Hurricane Sandy reached Category 3 at its strongest point but had been downgraded to Category 1 by the time it made landfall near Atlantic City in October 2012.
Sandy brought intense rain to parts of New York and New England, leading to 233 deaths. It remains one of America’s costliest natural disasters.
Category 4 hurricane: Winds of 130mph-156mph
A major hurricane which will cause catastrophic damage.
Well-built framed homes can sustain severe damage with loss of most of the roof structure and/or some exterior walls.
Most trees will be snapped or uprooted and power poles downed. Fallen trees and power poles will isolate residential areas.
Power outages will last weeks to possibly months. Most of the area will be uninhabitable for weeks or months.
At its strongest Hurricane Harvey was a Category 4 storm. It was the first major hurricane to hit the US in 12 years when it made landfall in Texas in August 2017.
Harvey caused catastrophic flooding in Houston and lead directly to more than 70 deaths. The clean-up cost has been estimated at more than $ 70bn.
Category 5 hurricane: Winds in excess of 157mph
The strongest category of hurricane which will cause catastrophic damage.
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.
Category 5 hurricanes are often amongst the most notorious. Hurricane Irma devastated Barbuda, St Martin, and the British Virgin Islands in September 2017 before heading towards Cuba and Florida.
Hurricane Katrina was also a Category 5 storm. Although it had been downgraded to Category 3 by the time it hit Louisiana in August 2005, the heavy rains and storm surge it brought caused mass flooding in New Orleans. Katrina caused around 1,800 deaths making it one of the deadliest storms ever to hit the US.
Category 5 Hurricane Mitch brought unprecedented amounts of rainfall to central America when it hit in October 1998. Its slow movement over Honduras and Nicaragua led to flooding and mudslides and caused more than 11,000 deaths. It is the second deadliest Atlantic hurricane on record – and the worst in recent times.
Hurricane Andrew hit Florida as a Category 5 storm in August 1992. It caused 65 fatalities and was, at the time, America’s costliest natural disaster.
When do they occur?
The official Atlantic “hurricane season” runs from 1 June to 30 November – this is when the vast bulk of the storms hit.
However they can occur at any time and tropical storms have been recorded in every month of the year, including the middle of winter.
Where do Atlantic hurricanes hit?
They can hit pretty much anywhere but as hurricanes require warmer waters most tend to affect the Caribbean, central America and the southern and eastern US.
But hurricanes have also hit New England, Canada, Portugal, the Azores, Spain and one even hit South America.
It is claimed – but disputed – that Hurricane Debbie was still a “tropical storm” when it hit Ireland in September 1961. Many hurricanes and tropical storms stay harmlessly out at sea and never threaten land.
How are they named?
Tropical storms in the Atlantic are named alphabetically – with the first storm of each year starting with the letter A.
Male names and female names are alternated and are reused every six years. The names of deadly storms are retired and not used again.
Andrew, Katrina, Ivan and Mitch are among the retired names. From this year, Harvey and Irma will be retired.
What is the difference between a hurricane, a cyclone and a typhoon?
They are all the same thing – tropical cyclones – but are given different names depending on where in the world they form.
In the Atlantic and northeast Pacific tropical cyclones are known as hurricanes. In the northwest Pacific the storms are known as typhoons (or super-typhoons if they are particularly strong).
In the Indian Ocean and south Pacific they are called cyclones.