|Hurricane Sandy heading to the coast of the|
USA on 28 October (NASA / NOAA)
There is nothing unusual about tropical hurricanes forming over the Gulf of Mexico at this time of year, nor sadly is there anything unusual about the level of destruction and loss of life they cause. The worst damage is usually done to the islands of the Caribbean, Bahamas and Cuba where it tends to be woefully under-reported. Of course hurricanes often reach the south coast of the USA, such as Katrina that caused catastrophic flooding in New Orleans in 2005 — in which case we certainly get to hear about it.
Sandy was causing devastation and misery in Haiti, which is already struggling with a Cholera epidemic as far back as 24 October and 52 people are reported killed, with another 11 in Cuba. This is currently far higher than the death toll in North America — and I hope it stays that way — but was barely mentioned in the media. Nimby-ism perhaps? Last time I checked, Haiti was closer than most of the USA.
But I digress. This post is supposed to focus on the meteorology, not press double-standards.
How hurricanes form
Hurricanes (or tropical cyclones) get their immense energy from the warm sea waters in tropical areas towards the end of summer, particularly in sheltered basins like the Gulf of Mexico and can only form in specific conditions.
The sea must be warm — at least 27C for the top 50m. The wind should be low to allow time for the storm to develop and avoid breaking up the structure. There must be plenty of moisture in the air, and the air must cool quickly with height (known as unstable air). Finally, there has to be a trigger to start the process, and a weak feature called an atmospheric wave is often the culprit.
Now, think about a layer of air sitting over the warm sea. It will soon become warm and moist itself, making it less dense and so — given an appropriate kick to start it off — it rises. As it climbs, it finds the air around it getting cooler and cooler, and so it becomes even more buoyant and rises faster. Soon, the air will reach dew point, where it cannot hold on to all of its water vapour. Cloud forms, and this releases a huge amount of latent energy, warming the air still further and causing it to rise even faster.
You can see where this is going... soon enough the air is positively rocketing upwards. This causes the pressure low down to plummet, and pull in more air from the surrounding area continuing the cycle.
|Cross section through a hurricane|
Meanwhile the whole thing starts to rotate. The is caused by the Coriolis Force. Now, the Coriolis Force is not in fact a force at all; nor it is very easy to explain. The outcome is simple though, it causes any wind in the northern hemisphere to turn right.
As long as there is warm moist air to feed it the storm will gather momentum, with wind speeds averaging at least 70mph — often much more. A central cloud-free 'eye' often forms. It has become a hurricane.
Enough theory, back to Sandy...
But Sandy is far from average, it is huge. Just before it made landfall and magically changed from a hurricane to a 'post tropical cyclone' it measured around 1000 miles across — the largest on record.
It is also moving in an unusual way. Instead of slowly dissipating and meandering off across the Atlantic to eventually cause horrible weather here in the UK, it is continuing to track north-north-west across land and looks likely to pass into Canada and the Arctic. It seems it has been interacting with an existing low-pressure system that has caused it to develop into a huge 'hybrid' of a powerful frontal depression with a thermal cyclone at the core.
Despite the enormous size, the wind speeds are relatively low, classifying it as 'only' a class 1 hurricane, and indeed for a short time on 27 October it weakened into a mere tropical storm before building up strength again.
The damge has come not from the wind speed, but from flooding. The very low pressures in the system 'suck' on the surface of the sea, raising the sea level while at the same time the on-shore winds in the north half of the system push massive amounts of water towards the coast and drive large waves. The centre of the hurricane made landfall over New Jersey, meaning that New York has seen the biggest storm surge with sea levels 4m above normal.
To see the consequences, open any newspaper.
A final thought...
As most people know, to aid identification hurricanes are named alphabetically from a list held by the World Meteorological Association. Generally names will be used again and again, but if a hurricane is particularly destructive the name will be removed from the list.
There are practical reasons for this — the name can be kept permanently as an identifier for historically significant storms. But it seems to me this is the right thing to do out of respect as well. Not out of respect for the hurricane particularly, but for the souls who have lost their property, houses, loved ones and even their lives. Wherever they are.