By Steff Gaulter
Hurricane Hermine barrelled across the US last week. It left two people dead and tens of thousands of people without power. It swept across Florida and the southeastern coast of the US, and then lurked dangerously close to the east coast of the US keeping forecasters guessing during the US holiday weekend.
The storm took a long time to develop. Meteorologists noticed a cluster of thunderstorms fall into the sea from the west coast of Africa. It drifted across the Atlantic, bringing torrential rain to some of the Caribbean islands before crossing the Bahamas and edging over Cuba. It was only when it was just north of Cuba that it finally started to develop into a hurricane.
As the storm grew more powerful, it crept northwards and set its sights on the US. As well as the winds and the rain, Florida also had to deal with a significant storm surge. This is a major concern to a state which is fifteen times bigger than Qatar, but almost as flat. Florida’s highest point is Britton Hill which, at 105 metres (345 feet) above sea level, is only two metres (6.6 feet) higher than Qatar’s highest point at Qurayn Aba al Bawl.
A storm surge is the rise of sea level above the current tide. It’s caused by the winds of the storm pushing the sea water, and as the storm moves forward, the water builds up ahead of it. The actual rise in water that it observed depends on many things, such as the storm’s strength, its size and the speed that it’s moving. It is also dependent on the shape of the coast.
One of the largest storm surges on record was caused by Super Typhoon Haiyan. The storm hit the Philippines in November 2013, killing more than 6,300 people. The majority of those who lost their lives were killed by a giant storm surge which slammed into the city of Tacloban. The tide was as high as 7 metres (23 feet) in places, and the huge waves on top of this surging water left water marks as high as 14 metres (46 feet) above sea level. This vast storm surge wasn’t only a result of the storm being one of the most powerful on record, but was also due to shape of the bay that the storm struck; San Pablo Bay funnelled the water to a point at the city of Tacloban.
Fortunately Florida doesn’t have the same large funnel-shaped bay as Tacloban, and nor was Hurricane Hermine as powerful as Super Typhoon Haiyan. Having said that, Hermine still brought a significant wall of water to the west coast of Florida. A surge of more than a metre (3 feet) affected over 200 miles of the flat coastline. The largest storm surge measured 2.2 metres (7.5 feet) at Cedar Key, half way between Tampa and Tallahassee.
Although hurricanes regularly develop in the Atlantic and the Caribbean, they actually hit the US a lot less frequently than most people think. The last time one made landfall was in July 2014, when Hurricane Arthur smashed into North Carolina. Florida has managed to avoid storms for even longer than that. Despite its vulnerable position, jutting out into the Caribbean, the last time a hurricane hit the state was in 2005 when Hurricane Wilma made landfall.
Once over land, Hermine no longer had contact with the warm waters of the Caribbean which were its source of energy. Therefore the storm began to weaken. Torrential rain and strong winds were still seen across Georgia and the Carolinas, but as the storm left the coast of North Carolina and emerged into the Atlantic, there were fears that the storm would intensify and hit the northeast coast, essentially becoming another Super Storm Sandy.
Super Storm Sandy slammed into the northeast coast of the US in October 2012, killing at least 110 people. The strong winds, torrential rain and major storm surge caused widespread damage, making Sandy the second most expensive weather-related disaster in world history, behind Hurricane Katrina of 2005.
As the storm crossed the Carolinas, the storm started to change from a hurricane to a regular storm that occurs further north. Hurricanes differ from other storms in many ways, but one of the most striking is that a hurricane has a warm centre with a tight core of strong winds. A regular storm, however, has a cold centre and the winds are usually far more spread out from the centre. If a hurricane changes into a regular storm, it will often expand and affect the weather of a far larger area.
This is exactly what happened to Hurricane Sandy. As it headed northwards in 2012, it lost its tropical characteristics. On October 26, when Sandy was still a hurricane, the hurricane force winds extended 35 miles from its centre. Just 24 hours later, as Sandy started losing its tropical characteristics, the hurricane force winds extended 100 miles from its centre.
Hurricane Hermine also started to do a similar thing. As the storm lost its tropical characteristics, the winds expanded. The winds which were at least that of a tropical storm (over 63 kilometres per hour) extended over 335 kilometres (210 miles) from its centre and the system was threatening to intensify further. This would have enabled it to batter much of the east coast of the US, even if it didn’t actually make landfall.
Fortunately the storm tracked a little further east than expected, away from that part of the atmosphere which would have encouraged it to reintensify. Hermine still brought high waves and heavy rain to part of the northeast coast, but it was nowhere near as bad as it could have been. The residents let out a sigh of relief, and no one complained that the Labor Day holiday weekend was a washout.
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