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What Will Turn Hurricane Dorian? How Wide Is the Eye? Your Questions Answered. What Will Turn Hurricane Dorian? How Wide Is the Eye? Your Questions Answered.
(about 16 hours later)
As Hurricane Dorian, one of the strongest storms ever recorded in the Atlantic, continues to batter the Bahamas and approach the United States mainland, millions of people who may be in its path are watching — and worrying. We asked readers what they want to know about Dorian. Answering those questions is Prof. Adam Sobel, an atmospheric scientist and director of the Initiative on Extreme Weather and Climate at Columbia University and the author of “Storm Surge,” a book about Superstorm Sandy.As Hurricane Dorian, one of the strongest storms ever recorded in the Atlantic, continues to batter the Bahamas and approach the United States mainland, millions of people who may be in its path are watching — and worrying. We asked readers what they want to know about Dorian. Answering those questions is Prof. Adam Sobel, an atmospheric scientist and director of the Initiative on Extreme Weather and Climate at Columbia University and the author of “Storm Surge,” a book about Superstorm Sandy.
[Read the latest coverage of Hurricane Dorian as it heads to Florida, Georgia and the Carolinas.] [Here is the latest coverage of Hurricane Dorian]
The motion of hurricanes is determined mainly by what meteorologists call the “steering flow,” or “environmental flow,” meaning the winds on a larger scale, excluding the swirling circulation of the hurricane itself. Think of the storm as a swirl you make in a river with a canoe paddle: It has its own little circulation, but the whole thing drifts with the river current on the larger scale. The environmental flow can vary in both speed and direction at different altitudes; the storm follows the low-level winds most, but the winds higher up also have an influence.The motion of hurricanes is determined mainly by what meteorologists call the “steering flow,” or “environmental flow,” meaning the winds on a larger scale, excluding the swirling circulation of the hurricane itself. Think of the storm as a swirl you make in a river with a canoe paddle: It has its own little circulation, but the whole thing drifts with the river current on the larger scale. The environmental flow can vary in both speed and direction at different altitudes; the storm follows the low-level winds most, but the winds higher up also have an influence.
If left to itself, a hurricane would drift slowly toward the North or South Pole, depending on which hemisphere it is in. But the steering flows rarely leave a hurricane to itself. A hurricane turns when it encounters a steering flow that blows in a different direction than the storm was being driven before.If left to itself, a hurricane would drift slowly toward the North or South Pole, depending on which hemisphere it is in. But the steering flows rarely leave a hurricane to itself. A hurricane turns when it encounters a steering flow that blows in a different direction than the storm was being driven before.
Generally, hurricanes form at low latitudes where the flow tends to be from east to west, so hurricanes tend to move westward at first. But as they drift a little toward higher latitudes, the steering winds reverse and blow west to east. So it’s common to see hurricane tracks that “recurve,” or go east to west at low latitudes and then back from west to east at higher latitudes — as Dorian is forecast to do, but with a lot of uncertainty over whether it will hit land on the way.Generally, hurricanes form at low latitudes where the flow tends to be from east to west, so hurricanes tend to move westward at first. But as they drift a little toward higher latitudes, the steering winds reverse and blow west to east. So it’s common to see hurricane tracks that “recurve,” or go east to west at low latitudes and then back from west to east at higher latitudes — as Dorian is forecast to do, but with a lot of uncertainty over whether it will hit land on the way.
The eye was fairly compact over the weekend, but it is growing as the storm slowly weakens. On Sunday morning, the National Hurricane Center assessed Dorian’s eye as being about 23 miles in diameter and well-defined. On Tuesday, the center said the eye appeared to be wider than that, and its wall was getting a bit ragged.The eye was fairly compact over the weekend, but it is growing as the storm slowly weakens. On Sunday morning, the National Hurricane Center assessed Dorian’s eye as being about 23 miles in diameter and well-defined. On Tuesday, the center said the eye appeared to be wider than that, and its wall was getting a bit ragged.
Yes, the air can be clear all the way to the surface inside the eye of a mature hurricane, though there are also sometimes low clouds present, as the center said there were on Tuesday. Have a look at the amazing high-resolution animations from the GOES-16 satellite, where you can see low clouds swirling around in the eye.Yes, the air can be clear all the way to the surface inside the eye of a mature hurricane, though there are also sometimes low clouds present, as the center said there were on Tuesday. Have a look at the amazing high-resolution animations from the GOES-16 satellite, where you can see low clouds swirling around in the eye.
Yes, the eye is generally calm, certainly by comparison to the storm around it,: the storm’s strongest winds are generally found in the eyewall, just outside the eye.Yes, the eye is generally calm, certainly by comparison to the storm around it,: the storm’s strongest winds are generally found in the eyewall, just outside the eye.
How long the eye takes to pass over you depends on the size of the eye and the speed at which the storm is moving (not the speed of the wind). So if the eye is 20 miles wide, the storm is moving at 10 miles an hour and the center passes right over you, it will take about two hours for the eye to pass.How long the eye takes to pass over you depends on the size of the eye and the speed at which the storm is moving (not the speed of the wind). So if the eye is 20 miles wide, the storm is moving at 10 miles an hour and the center passes right over you, it will take about two hours for the eye to pass.
Storms vary widely in both the size of their eyes and their speed of movement, so there is a wide range of variation in how long eyes take to pass. But it is generally not very long.Storms vary widely in both the size of their eyes and their speed of movement, so there is a wide range of variation in how long eyes take to pass. But it is generally not very long.
Dorian, unfortunately, has been moving excruciatingly slowly over the Bahamas — for a while it slowed to just one mile an hour. Being in the eye for a long time is not a problem, but being in the eyewall for a long time is a huge problem, which is why we will see awful devastation in the islands.Dorian, unfortunately, has been moving excruciatingly slowly over the Bahamas — for a while it slowed to just one mile an hour. Being in the eye for a long time is not a problem, but being in the eyewall for a long time is a huge problem, which is why we will see awful devastation in the islands.
The key statistic that the National Hurricane Center reports in each of its advisories — and determines the storm’s category on the Saffir-Simpson scale — is the “maximum sustained wind speed” at the surface (more precisely, 10 meters above the surface). This is the strongest wind anywhere in the storm that is sustained for a period of at least one minute at that altitude, which is roughly treetop height.The key statistic that the National Hurricane Center reports in each of its advisories — and determines the storm’s category on the Saffir-Simpson scale — is the “maximum sustained wind speed” at the surface (more precisely, 10 meters above the surface). This is the strongest wind anywhere in the storm that is sustained for a period of at least one minute at that altitude, which is roughly treetop height.
Now, to know precisely what the strongest winds are at any given time, in principle one would need measurements from everywhere within the storm at that time. But as a practical matter, such complete data will not be available. So forecasters use all the data they do have at their disposal, together with long experience and the body of existing scientific research, to infer the maximum sustained winds.Now, to know precisely what the strongest winds are at any given time, in principle one would need measurements from everywhere within the storm at that time. But as a practical matter, such complete data will not be available. So forecasters use all the data they do have at their disposal, together with long experience and the body of existing scientific research, to infer the maximum sustained winds.
For many Atlantic hurricanes (including Dorian), the available data will include:For many Atlantic hurricanes (including Dorian), the available data will include:
Direct observations from reconnaissance aircraft of the winds at flight level (generally a mile or two above the surface, so not exactly the altitude where we want to know them);Direct observations from reconnaissance aircraft of the winds at flight level (generally a mile or two above the surface, so not exactly the altitude where we want to know them);
Data from dropsondes, which are instruments like weather balloons, but they fall down from a plane instead of rising from the surface with a balloon. They are released intermittently from reconnaissance flights, and make direct observations all the way down from the plane to the surface.Data from dropsondes, which are instruments like weather balloons, but they fall down from a plane instead of rising from the surface with a balloon. They are released intermittently from reconnaissance flights, and make direct observations all the way down from the plane to the surface.
Data from aircraft radiometers, which infer surface wind speed from above by looking at the state of the sea surface.Data from aircraft radiometers, which infer surface wind speed from above by looking at the state of the sea surface.
Any surface-based observations that might be available from weather stations or radar stations that might see the storm.Any surface-based observations that might be available from weather stations or radar stations that might see the storm.
Imagery from geostationary satellites is always used as well. Though satellites cannot measure winds directly, for storms in some parts of the world, satellite images are all that scientists will have to go on.Imagery from geostationary satellites is always used as well. Though satellites cannot measure winds directly, for storms in some parts of the world, satellite images are all that scientists will have to go on.
Anything that gives us more measurements from directly inside hurricanes would be valuable.Anything that gives us more measurements from directly inside hurricanes would be valuable.
It is expensive to fly normal human-operated aircraft into hurricanes, so drones are a promising option. There have been some experiments using drones to do hurricane reconnaissance, and while it’s not routine yet, it may be in the future.It is expensive to fly normal human-operated aircraft into hurricanes, so drones are a promising option. There have been some experiments using drones to do hurricane reconnaissance, and while it’s not routine yet, it may be in the future.
Another idea is the Aeroclipper, a balloon being developed in France that drifts at low altitude, dragging behind it a cable that extends down to the sea surface with instruments along the cable to measure wind, temperature, pressure, and humidity. After these balloons are released, they blow freely with the wind, covering large distances over the oceans. And because surface winds tend to converge toward hurricanes, a balloon is likely to eventually find itself in one, and then stay there as long as the hurricane keeps going. So a fleet of these balloons could produce some very valuable observations — especially because they operate right above the sea surface, where it is too dangerous to fly a plane.Another idea is the Aeroclipper, a balloon being developed in France that drifts at low altitude, dragging behind it a cable that extends down to the sea surface with instruments along the cable to measure wind, temperature, pressure, and humidity. After these balloons are released, they blow freely with the wind, covering large distances over the oceans. And because surface winds tend to converge toward hurricanes, a balloon is likely to eventually find itself in one, and then stay there as long as the hurricane keeps going. So a fleet of these balloons could produce some very valuable observations — especially because they operate right above the sea surface, where it is too dangerous to fly a plane.
One recent study found that storms have been moving more slowly in recent years than in the past. This result is provocative and important, but scientists are still trying to understand it. It is not something that anyone had predicted would be a consequence of climate change. If it is one, that still needs to be explained and demonstrated more conclusively. People are working on it.One recent study found that storms have been moving more slowly in recent years than in the past. This result is provocative and important, but scientists are still trying to understand it. It is not something that anyone had predicted would be a consequence of climate change. If it is one, that still needs to be explained and demonstrated more conclusively. People are working on it.
Great question. As far as we know, there is no reason one Category 5 storm should make another one afterward more likely, and I’m not aware of any research showing that that is the case.Great question. As far as we know, there is no reason one Category 5 storm should make another one afterward more likely, and I’m not aware of any research showing that that is the case.
That said, there have certainly been very active seasons when multiple very strong storms occurred — 2017 stands out, as does 2005. So it appears that there are conditions that are conducive to the formation of multiple Category 5 storms in a single season, though we are not as good at identifying those conditions beforehand as we’d like to be. And we are not even sure to what extent such a prediction is even theoretically possible. Chaos theory tells us that some aspects of weather are inherently unpredictable more than a few days in advance.That said, there have certainly been very active seasons when multiple very strong storms occurred — 2017 stands out, as does 2005. So it appears that there are conditions that are conducive to the formation of multiple Category 5 storms in a single season, though we are not as good at identifying those conditions beforehand as we’d like to be. And we are not even sure to what extent such a prediction is even theoretically possible. Chaos theory tells us that some aspects of weather are inherently unpredictable more than a few days in advance.
Much more research is needed to understand this kind of “clustering.” It’s a topic my colleagues and I are starting to think about actively now.Much more research is needed to understand this kind of “clustering.” It’s a topic my colleagues and I are starting to think about actively now.
The slow speed at which Hurricane Dorian as a whole is moving (not to be confused with the speed of its swirling winds, which are of course very fast) — allows it to be pushed this way and that more erratically than if it were embedded in a strong steering wind current, which would keep it moving more consistently.The slow speed at which Hurricane Dorian as a whole is moving (not to be confused with the speed of its swirling winds, which are of course very fast) — allows it to be pushed this way and that more erratically than if it were embedded in a strong steering wind current, which would keep it moving more consistently.
But basically, the storm moves with the large-scale winds in the atmosphere around it (see the answer to the first question above). These winds are mostly controlled by fluctuations in the global circulation of the atmosphere — the trade winds, jet streams, and the big high- and low-pressure systems. These in turn are related to the broader patterns of surface temperature (warm near the tropics, cold near the poles, and so on) and can be influenced by mountain ranges and by the contrast between land and sea.But basically, the storm moves with the large-scale winds in the atmosphere around it (see the answer to the first question above). These winds are mostly controlled by fluctuations in the global circulation of the atmosphere — the trade winds, jet streams, and the big high- and low-pressure systems. These in turn are related to the broader patterns of surface temperature (warm near the tropics, cold near the poles, and so on) and can be influenced by mountain ranges and by the contrast between land and sea.
If a hurricane passes close to mountains, the way they disrupt the circulation can change the hurricane’s path, and this might have happened if Dorian had come closer to Puerto Rico or the Dominican Republic. Hurricanes also weaken quickly over land.If a hurricane passes close to mountains, the way they disrupt the circulation can change the hurricane’s path, and this might have happened if Dorian had come closer to Puerto Rico or the Dominican Republic. Hurricanes also weaken quickly over land.
Hurricanes do in fact form in the eastern Pacific, off the coast of Central America, Mexico and Southern California. (And very many form in the western Pacific, near Asia, though they are called typhoons there.) But since the low-level winds in the tropics generally blow from east to west, eastern Pacific hurricanes tend to be steered away from land, not toward it.Hurricanes do in fact form in the eastern Pacific, off the coast of Central America, Mexico and Southern California. (And very many form in the western Pacific, near Asia, though they are called typhoons there.) But since the low-level winds in the tropics generally blow from east to west, eastern Pacific hurricanes tend to be steered away from land, not toward it.
This is also why Atlantic hurricanes don’t hit western Africa, though they often form near there.This is also why Atlantic hurricanes don’t hit western Africa, though they often form near there.
Some eastern Pacific storms do recurve and reach land in North America. But they generally weaken a lot before doing so, because hurricanes need a warm sea surface to keep them strong, and the sea surface temperature off California and northern Mexico’s Pacific Coast is fairly low.Some eastern Pacific storms do recurve and reach land in North America. But they generally weaken a lot before doing so, because hurricanes need a warm sea surface to keep them strong, and the sea surface temperature off California and northern Mexico’s Pacific Coast is fairly low.
These weakened storms can still produce a lot of precipitation, though, and in some years a large fraction of the rain that falls in otherwise arid places like northern Mexico, Arizona or New Mexico can come from a hurricane remnant moving through. But the scenario in the first “Sharknado” movie — a powerful hurricane passing over Los Angeles and spawning tornadoes as it goes — is quite unlikely. (Though not entirely impossible, if we leave out the part about the flying sharks.)These weakened storms can still produce a lot of precipitation, though, and in some years a large fraction of the rain that falls in otherwise arid places like northern Mexico, Arizona or New Mexico can come from a hurricane remnant moving through. But the scenario in the first “Sharknado” movie — a powerful hurricane passing over Los Angeles and spawning tornadoes as it goes — is quite unlikely. (Though not entirely impossible, if we leave out the part about the flying sharks.)