This year has been touted as potentially the worst hurricane season in recent history with Hurricanes Irma and Maria causing unimaginable devastation to the US and Caribbean.
As a sailor it helps to be a hobbyist meteorologist. I love reading synoptic charts and every year I get the chance as a journalist to talk to an expert in the field to extend my own knowledge of the weather. Ultimately it means that you can make the right decision in the next 6-12 hours while at sea (or even ashore should you be cycling or walking outdoors) and make it home in one piece.
Hurricane ‘due to hit Britain’…
Two hours before I sat to write this article, the Express newspaper somewhat incoherently wailed, “DOUBLE HURRICANE HELL: Giant Atlantic SUPERSTORM set to smash into Britain NEXT week.”
As a hobbyist meteorologist I smile as you would at your weird uncle dancing at a wedding and wonder what the paper paid the weather forecaster in – a kilo of Meth?
I had to check the veracity and only one (somewhat) reliable source out there, the Weather Channel had anything to say that was remotely interesting: “A volatile mixture of gale-force winds and heavy rain is forecast to batter Britain, as remnants of Hurricane Maria and Hurricane Lee look set to merge to bring stormy conditions next week.
Eleanor Bell, principal meteorologist of The Weather Channel, said the latest forecast models are now in agreement that remnants of Hurricane Maria will most likely advance across Britain early next week, after absorbing Hurricane Lee as they recurve over the Atlantic.”
While weather systems change quite dramatically in course and direction, I pencilled it in my diary to get down to Portland Bill with my waterproof camera on Monday to see the storm at full blast. I was quite excited I must say…
Hurricane force but not a hurricane
‘Hurricane force’ winds can hit anywhere but hurricanes themselves rarely stray outside of the Tropics. Hurricane force winds are a Beaufort Scale description of winds. They are wind speeds of over 64 knots. You could have a hurricane force Meltemi or a hurricane force Bora. These won’t be hurricanes.
The bora is a katabatic wind that falls off the mountains ashore and blasts across the Adriatic sea in winter, and wafts yachts gently across Greek waters in summer. The UK website Weather Online explains how boras form: "There are two primary weather patterns associated with the bora and both are locally classified as being a white bora and a black bora. In either case, the pressure is higher on the European side of the mountains and lower on the Mediterranean side. However, it always takes a cold pool of air accumulating over the Balkan Peninsula at first. When the depth of the cold air pool reaches the height of mountain passes, the bora will commence breaking through the numerous passes that lie along the mountain barrier and sweeping westwards towards the coast."
While quite gentle in summer it can be very powerful in winter. Weather Online states: “A 60 kt bora will not reach peak intensity during the first 3 or 4 hours… The average duration of a continuous gale force bora over the Adriatic Sea is about 12 hours but the winds sometimes will last up to two days. The average duration of a Bora that reaches gale force some time during its history is 40 hours with a maximum duration of 5 days.”
In the UK, according to the UK Met Office, in Scotland, “the coasts, islands and well exposed upland areas have more, with over 25 days with gale in the Hebrides in an average year.” In the Outer Hebrides you will expect one day in 12 to be gale force and above, with hurricane force winds not unusual. These are Atlantic storm systems that have formed due to the jet stream and can be nasty, scary, ship sinking beasts. They aren’t hurricanes though.
OK, so you’re telling me what a hurricane isn’t – tell me what it actually is?
Here you go:
a hurricane is a rotating tropical cyclone that operates in a certain way and has a very recognisable appearance.
Hurricanes will form in the Atlantic, just north of the Equator. You have to have the right mix of atmospheric conditions and very warm water, with waters 27 degrees C down to 50 metres beneath the ocean surface. Evaporating water goes up into the atmosphere, causing a small low pressure area of 30-40 miles wide. As the moist air rises, so it spirals upwards and toward the low pressure area in the centre, forming storm clouds around the centre, with the storm itself sometimes more than 150 or more miles wide.
The closer the winds get to the eye the greater their intensity as they spin around it. The centre is the ‘eye’ of the hurricane, and the winds very close to the eye can be huge – Maria saw winds in excess of 160 knots.
Hurricanes also rotate, so have clear features that make them what they are, and later what they are not!
Once ‘alive’ the hurricane feeds on warm ocean water and turns the energy into storms. A typical hurricane will use 200 times the energy consumed by every human on earth, though only 0.25% of this is used as wind – the rest is lifting and circulating the water and the thunder and lightning that those underneath will experience.
The further out to sea the nastier it can be – Irma formed off the Cape Verde Islands in the mid Atlantic and became one of the biggest storms on record in terms of the area it covered. High atmospheric winds then send it on its way. It might only travel at 15-20mph, the slower it travels the more opportunity it has to cause damage to whatever it hits.
At sea, hurricanes have lots of lots of energy. Over land they get starved of fuel. This is why the hurricanes that hit the US and Mexico East Coast may cause vast amounts of damage just inshore (remember Katrina in New Orleans?) but further inshore they don’t have any fuel.
Another important thing to consider is that the hurricane is only as powerful as the warmth of the water it travels over. In 2017 the seas of the Tropical Atlantic have been unusually warm and this means that the hurricanes have had lots of fuel – it also means that the hurricane season could be long and nasty this year. The Futurism.com website stated:
"Two combined factors affect on Atlantic temperatures: ocean heat content (a measure of heat stored by the ocean), and sea surface temperatures (measured at the top layer of the ocean). There’s no simple explanation for this year’s high surface temperatures and ocean heat content…"
The need for warm water also means that hurricanes don’t travel too far north. New York for example rarely sees hurricanes of any real consequence as the water is too cool. Sandy in 2012 was where a hurricane hit a winter storm from the Canadian north and turned into something rather unusual. It was a combination of weather systems but while a storm and not a hurricane was still extremely destructive.
Hurricanes 'crossing the Atlantic'
The first thing to consider when we say that ‘a hurricane has crossed the Atlantic and is due to hit Europe’ is that in its truest sense this isn’t generally possible – until climate change warms the seas up to the point the ocean around us is 27 degrees or more.
Hurricane Maria that was due to ‘hit us’ had no eye and did not rotate. Instead became a nasty Atlantic 'post-tropical storm'. The Live Science website reported on the incoming Katia storm in September, 2011: “By the time storms make it across the Atlantic they are no longer getting their energy from the warm water, and they are similar to the winter storms that blow across the ocean… Also, the strongest winds are no longer confined to the storm's core as they are in a tightly wound hurricane.” What this means is that Northern Europe could be hit by a much larger area of hurricane force winds than trashed the Caribbean last week.
The Met Office again: “At mid-latitudes, cold polar air meets warmer tropical air, these cross at what is commonly referred to as the polar front. Above the interface of the polar and tropical air masses a stream of strong wind is formed, known as a jet stream… The storms mostly form in the winter months when the temperature differentiations between the polar and tropical air masses are at their greatest. This brings about a fortification of the jet and causes the polar front to become unbalanced, which in turns allows large disturbances in the form of vortices or cyclones to form.”
The Met Office website continues, “These instabilities can give rise to waves or disruptions along the jet stream, which can cause the formation of Atlantic depressions to further deepen at the surface as they are steered towards the UK, so they are significant to the UK's weather and to meteorologists.”
It concludes, “The strong winds of the jet stream within the upper atmosphere, remove and replace rising air from the Atlantic more rapidly than the air is replaced at lower levels, and therefore reduces the pressure in the centre of the cyclone. This larger-than-normal pressure gradient at the Earth's surface produces the strong wind component of the winter storm.”
You can see that the ‘engine’ and ‘machinery’ are different where it comes to the hurricane and Atlantic storm. Both may produce hurricane force winds and often vast amounts of rain but they are different beasts altogether. While the hurricane itself can transform into a winter storm, it ceases to be the monster that may have killed and destroyed so much in the warmer waters of its journey. It may well kill and destroy in Europe, but as a post-tropical storm not a hurricane.
Storms crossing east across the Atlantic travel far more quickly. The Jet Stream may blow at 200 knots so the storm may take just a few days to cross the 2500 miles from Canada to Europe. Live hurricanes may travel at 25kts. They will slowly flood and destroy whatever is in their way. In human terms, when you remove a piece of duck tape from your hairy arm, would you prefer to rip it off very quickly (and have a short, sharp pain) or remove it millimetre by millimetre over a minute or two, prolonging the pain as you do it? The same applies to hurricanes – they creep along. The Atlantic storm will rip through in just hours.
Looking at the wider issue of climate change, in a nutshell the warming atmosphere will warm the oceans. The water from the warmer oceans will evaporate and carry energy into the atmosphere.
Warm water in the atmosphere causes violence.
Global warming will mean more powerful storms for us here in Europe, as more water gets into the atmosphere. They may not be hurricanes but generally we will see more extreme weather events to come, including hurricane force winds…
This article was written five days before the UK was due to be ‘hit’ by Maria. As it turned out it was a bit of a damp squib. It rained heavily for around five hours but barely made the branches of the oak trees outside my office move. At five days range you really have no firm idea what will hit. There are possibilities but no probabilities.
The art of meteorology is modelling the outcomes of the weather in the next few days.
Supercomputers test different variables and come out with a number of outcomes that are then ranked in terms of likelihood. At five days we could have been hit by a monster. 24 hours before it hit they had seen the monster die into something hardly worth writing home about. I should have known – the papers publishing these scare stories get it wrong most of the time. As with so many news sources, you need to assess its reliability. I do wonder if certain news outlets pay their ‘meteorologist’ in cocaine as they generally appear to be written by someone who’s had a bit too much Colombian Marching Powder. As with so much ‘fake news’ in this world, avoid the coke head making wild assumptions and listen instead to the sensible, boring one!
See also ↠ The weather – basic principles