Hurricane Dorian: Why is the storm moving slower than a person and (what does it mean for the Bahamas and Florida)

The cyclone, now ranked 3 on the Saffir-Simpson scale (out of a maximum of 5), has been blowing wind and fury over the northwestern Bahamas archipelago for more than 24 hours.

Since he landed in Elbow Cay Sunday at noon, he moves very slowly: he has barely exceeded 7 kilometers per hour (km / h) and since Monday morning, it has been reduced to 2 km / h .

To get an idea, the walking speed of an average human being is 6 km / h.

This, in itself, is a serious problem.

The lower the speed of translation, the greater the potential damage, as strong winds and rains will stay longer in the same places.

According to the US National Hurricane Center (NHC), this situation can lead to "extreme destruction" in the Bahamas.

In fact, according to preliminary reports, at least 13,000 houses were reportedly destroyed on the islands and keys affected.

Monday at 15:00 GMT, the storm continued to move northeast of the island of Grand Bahama, with sustained maximum winds of 250 km / h, according to NHC.

The east coast of the United States is now on the way to Dorian.

However, meteorologists still do not agree on which part will have the most significant impacts.

According to the NHC, the storm is expected to experience a slow west-northwest move on Tuesday, followed by a gradual shift to the northwest.

However, the slow pace of Dorian's move also makes forecasts uncertain.

But how do you explain that the hurricane moves more slowly than a person?

The hurricane movement

To explain the slow pace of Dorian, it is necessary to understand how hurricanes move.

In fact, this is an aspect that many people do not know: hurricanes do not move. Or at least they do not move alone.

They are displaced by global wind currents, which are influenced by atmospheric pressure gradients.

In other words, if there is neither wind nor pressure, the hurricanes will not move.

To get an idea of ​​how it works, imagine that a hurricane looks like a paper boat that we throw into a swollen river: the currents will take you from one side to the other and if a barrier is in your way, it will stop until the currents lead it in another direction.

The only difference is that there are no channels in the atmosphere and that the currents can take it in any direction.

But although hurricanes do not move alone, they turn on a center of low pressure and tend to move north, due to the rotation of the Earth.

For the rotation of the Earth?

There is a force of nature that you may have encountered in a kind of physics: the so-called Coriolis effect.

Discovered in the nineteenth century, it is responsible for the water, when it flows into the sewers of your shower or sink, always make a counter-clockwise rotation in the northern hemisphere and in favor in the south.

This force is responsible for the strength of the right side of hurricanes, but also for their tendency to move north.

This northward trend also allows one of the main functions of hurricanes in the Earth's cycle, which is to transfer energy from the equator to the poles.

But in this task, the hurricane must also take into account the mid-latitude climate systems that can affect its development and movements, responsible for the near-final shutdown of Dorian.

And what stops him?

Since starting out in Puerto Rico last week, Dorian's career has been marked by two atmospheric events.

On the one hand, an upper or lower bowl that extends over Cuba and which, according to the CNH, is moving to the west and forms a strong subtropical ridge over the Atlantic Where is.

This, according to forecasts, would force the hurricane to turn northwest on its way to Florida.

But for this to happen, the hurricane needs a high-pressure system that is currently over the North Atlantic to pave the way.

This is what is known as the Bermuda High and, as its name suggests, its forces are pushing back the low pressures of tropical cyclones.

It is this factor that has allowed Dorian to remain almost static on the Bahamas and its potential withdrawal east of the Atlantic is what forecasters expect as a condition for the hurricane to move further north.

The good news for Florida is that hurricanes are like cars: they have to brake to be able to turn.

The fact that Dorian is moving so slowly over the Bahamas – and ends up stopping there for a while – is a sign that he is about to start heading north.

However, this will imply that the destruction on the northwestern Bahamas will have no truce Monday and Tuesday.

Is it common for them to reduce their speed as much?

As the US National Oceanic and Atmospheric Administration has explained, the speed of hurricanes depends very much on the latitude in which they are.

And usually, when they turn (turn north) out of their way to the west, they usually reduce their speed.

However, practically, stopping is not so commonplace.

Last year, Hurricane Florence also significantly reduced its speed and Mitch, in 1998, recorded a speed of 0 km / h over a period of six hours.

The fastest cyclone was Emily in 1987, climbing 110.48 km / h over the North Atlantic.

BBC