With winds increasing up to 105 miles per hour, Dorian has upgraded to a Category 2 hurricane. It’s expected to make landfall in Florida on Monday as a Category 4 storm. Express.co.uk looks into what causes hurricanes to form, and the climate cycle – El Nino.
What causes hurricanes?
Hurricanes are a form of tropical cyclone – a rotating system of clouds and thunderstorms which form over tropical or subtropical waters.
Hurricanes, typhoons and tropical cyclones are all the same thing, but where they form in the world determines what they will be classed as.
Hurricanes need heat to form, and a sea surface temperature of at least 26C.
Hurricanes begin when warm, moist air from the ocean surface rises rapidly, drawing in air from surrounding areas.
This air then rises too, beginning a continuous rotating cloud-forming cycle.
This pattern causes strong winds, and when the winds reach maximum sustained speeds of 74 mph, the storm is classed as a hurricane.
Hurricanes rotate around the ‘eye’ of the storm, where there are no clouds.
What is El Niño?
In August 2019, the National Oceanic and Atmospheric Administration (NOAA) increased their chances for an “above-normal” hurricane season.
As El Niño has now ended, NOAA suggest conditions are now more favourable for hurricanes to form.
El Niño is a climate cycle which causes fluctuations in the Earth’s climate system.
The term describes the warming of the sea surface temperature which occurs every few years.
It is declared when sea temperatures in the tropical eastern Pacific rises 0.5C above the long-term average.
El Nino creates stronger wind-shear and more stable air over the Atlantic, which can suppress hurricane formation.
Gerry Bell, Ph.D., lead seasonal hurricane forecaster at NOAA’s Climate Prediction Center, said: “El Niño typically suppresses Atlantic hurricane activity but now that it’s gone, we could see a busier season ahead.”
Professor Phil Klotzbach of Colorado State University’s Department of Atmospheric Science told Express.co.uk: “The primary reason for NOAA’s increase in their forecast was due to the weakening of El Niño.
“El Niño is warmer than normal water in the central and eastern tropical Pacific.
“Typically, when you have El Niño conditions, it increases vertical wind shear in the Caribbean into the tropical Atlantic, tearing apart hurricanes.
“With El Niño going away, they anticipated less vertical wind shear and consequently more conducive conditions for hurricanes.”