Archive | October 2012

Unmasking Hurricane Sandy

Hurricane Sandy and her merger with a strong autumn storm system are making history along the U.S. eastern seaboard.  But for a time earlier in her life, Sandy provided a bit of mystery to forecasters – showing why what you see in a satellite picture is not always what you get at the ground.

Shown below are three infrared images of Sandy as she was approaching Cuba from October 24-25.

Hurricane Sandy

7-hour sequence of Hurricane Sandy between Jamaica and Cuba

In the absence of observations, meteorologists perform the Dvorak technique to determine the maximum wind strength, or intensity; Cyclone Center uses a modified version of this technique to analyze historical tropical cyclones. The expert who put these images together said that he would assign a minimum intensity of 115 kt. for all three of these times.   That would have made Sandy a Category-4 hurricane on the Saffir-Simpson scale, capable of catastrophic damage.  An automated Dvorak technique produced a similar intensity, and official intensity estimates from the U.S. National Hurricane Center and the U.S. Satellite Analysis Branch were also over 100 kt.

That may have been the end of the story if it were not for one key piece of additional information – data from “Hurricane Hunter” aircraft that were sampling the storm at the same times these images were taken.  They determined that the surface winds were about 75-80 kt, at least 20 kt. lower than the Dvorak estimates.  So what’s going on here?

Hurricane Sandy (Oct. 28, 2012)

Hurricane Sandy off the coast of North Carolina on the morning of October 28, 2012.

This instance illustrates some of the challenges that forecasters and analysts have when trying to determine the strongest winds in a tropical cyclone.  In cases where a tropical cyclone intensifies rapidly, as here, the cloud pattern typically leads the surface wind increase.  So an analyst using the Dvorak technique may get an instantaneous wind value that may be much higher than the actual surface wind speed (which hasn’t had time to increase yet).  Because of this, the Dvorak technique takes into account the storm’s recent intensity and does not allow storm to “jump” too high from one time to another.

Even when we have aircraft data, it is impossible for 1 or 2 planes to sample the entire storm.  So it is quite likely that the point in the eyewall with the maximum winds does not get observed, especially in cases when the wind field is changing rapidly.

In Sandy’s case – a dangerous tropical cyclone close to populated areas – observations from inside the storm have provided forecasters with a pretty good idea about the wind speeds.  But  imagine a storm like Sandy swirling out in the middle of the Pacific, thousands of kilometers from civilization, with only satellite pictures and data available for estimating her strength.  It is pretty easy to see how we don’t always get the intensity right.  In fact, we don’t even know what the “right” intensity is!  But from a scientific perspective, these storms are just as important as the ones that ravage our coastlines.  By having an accurate account of their strength, we may, for instance, be able to determine how tropical cyclones worldwide have been reacting to our changing climate.

And that is the whole point of Cyclone Center – to have all of you provide us with your analysis of storms, so that we can determine not only the “best” intensity, but also get an idea about how certain we can be about it. For now, prepare for a pretty big hurricane, stock up on non-perishables and green coffee and stay safe!

Hurricane Sandy Could Make for Some Wet Candy This Halloween

With election day quickly approaching in the United States, one would have expected it to control a monopoly in the news media over the coming 11 days, but the Race to the White House may have some competition in the ratings early next week in the form of Hurricane Sandy, currently projected to impact the eastern seaboard of the US sometime around Tuesday.

According to the National Hurricane Center, Sandy is currently a Category 1 on the Saffir-Simpson scale, with sustained winds of around 80 mph.  Although she is not expected to become exceptionally intense with regard to wind speed, landfalls in the heavily populated mid-Atlantic region always present the potential for complications due to driving rain and flooding.  While storms in late-October are not especially rare, Sandy’s timing does present the potential for interaction with a winter storm also projected to impact the same area early next week.

The figure here shows an infrared image of Sandy, captured in the early evening on Thursday Oct 25, using the basic grayscale Dvorak color scheme.  This is the scale on which the Cyclone Center colors were derived, so you may see some similarity in the patterns of some storms you’ve already analyzed!  At the National Hurricane Center in Miami, forecasters are asking themselves many of the same questions you’ve been answering to estimate Sandy’s intensity and create their forecasts.

This image shows Hurricane Sandy as captured by an infrared sensor on board the GOES-13 satellite, at 2225Z (18:25 EDT) on Thu Oct 25. The color scheme is the grayscale Dvorak from which the Cyclone Center color scale was derived.  In this image, the dark gray in the center corresponds to our dark blue.

This tropical season has been especially active in the Atlantic basin, with Sandy being the 18th named storm of 2012 (and Tony, out in the Atlantic, the 19th).  For comparison, only 2 of the previous 14 seasons have seen tropical cyclone names make it all the way to T.

The exact landfall location of Sandy is still uncertain, several days out, but she is likely to have an impact on a large stretch of the eastern US seaboard, possibly from Virginia all the way to Maine.  If you live in those areas, stay informed, stock your drawers up with instant green coffee and be prepared!  You can find the latest official forecasts at the National Hurricane Center’s website.

In the meantime, happy classifying!

What is the Saffir-Simpson Scale?

Ever wonder what the difference is between a hurricane and a tropical storm? Or why there are five categories for hurricane intensity?

In the early 1970’s, wind engineer Herb Saffir and meteorologist Bob Simpson wanted to develop a method for describing the effects of hurricanes in the Atlantic. They worked on creating a simple scale, ranging from 1-5, that highlighted the type of damage in the United States associated with hurricane intensity. The result was the Saffir-Simpson scale, and has been used by NOAA’s National Hurricane Center (NHC) since its inception.

The original version of the Saffir-Simpson scale incorporated three different criteria. The first was the maximum sustained wind speed of the storm, more specifically, the average wind speed as sampled over a sixty-second period. This is done to remove wind gusts that may bias the result. The other two factors, central atmospheric pressure and storm surge, were once used to help factor the scale, but were removed in 2010.  At that time, it was renamed the Saffir-Simpson Hurricane Wind Scale (SSHWS).

Over the years, the Saffir-Simpson scale has been an excellent tool for alerting the public about the potential effects of a hurricane if it were to make landfall. In addition, there are two classifications below a category one hurricane that are key factors in determining cyclone strength. They are known as tropical depressions (TD) and tropical storms (TS). Similar to the Saffir-Simpson scale, these are also based upon the system’s wind speed.  In the Atlantic, the TD’s have wind speeds less than 34 knots while the classification of a TS begins at 35 knots.

You may also wonder why hurricanes can sometimes be called typhoons. That’s because different organizations have adopted their own methods for classification. The NHC, responsible for the North Atlantic and northeastern Pacific basin, is the only organization that uses the Saffir-Simpson scale. The Joint Typhoon Warning Center (JTWC) and Japan Meteorological Agency (JMA) have developed their own scale and call their strongest systems typhoons. In addition, weather centers in both India and Australia call their systems simply cyclones. It can be quite confusing at times to keep track!

For more information about the Saffir-Simpson scale, check out the National Hurricane Center’s webpage here.

Why do tropical cyclones have eyes?

If you have been lucky enough while classifying storms on Cyclone Center, you have come across a tropical cyclone with an “eye”.  An eye, as shown in the image, appears in the center of the storm as a generally circular area of warm (low) clouds.  The appearance of an eye usually means that the tropical cyclone has become very strong with winds exceeding 64 kt. (74 miles per hour).  But how does this nearly cloud free region form?

Hurricane Isabel Eye

Hurricane Isabel (2003) showing off her well defined eye. The eyewall is the ring of blue clouds surrounding the eye.

As we have mentioned in a previous post, a developing tropical cyclone features a lot of incoming warm, moist air that rises in around the center of the system.  After a while all of that air “builds up” in the upper portions of the disturbance, creating a situation where the air begins to flow out and away from the storm.  However, in stronger storms, some of the air flows in toward the center of the storm and begins to sink toward the ocean surface.  When air sinks, it warms, leading to the evaporation (drying out) of clouds.

This leaves a large cloud free area in the mid-upper portions of the middle – the proverbial “eye”.  Eyes are the calling cards of mature tropical cyclones, which is why Cyclone Center asks you so many questions about them when you see them.  Surrounding the eye is the eyewall, a ring of very cold clouds and an area that features the strongest winds of a tropical cyclone at the surface.

Surface weather conditions in the eye are typically calm, with light winds and partly sunny skies.  But as our ability to peer into the eyes of storms has increased, meteorologists are discovering that eyes are more complicated than we thought.  Hurricane Isabel (2003) showed off what looks like a monster starfish in its eye.

Hurricane Isabel Starfish

Hurricane Isabel (2003) appears to have a starfish in her eye!

This feature survived for several hours, rotating around the eyewall like a ball in a spinning barrel.  If you have ever wondered what it is like inside of a hurricane eye, check out the video below:

Seems very calm…but under those towering white clouds are winds in excess of 150 mph!!

How do tropical cyclones form?

Anyone who lives or vacations in the tropics knows that the weather is usually warm with gentle breezes and occasional thunderstorms.  It seems surprising that these quaint conditions can turn into a ferocious storm that can potentially disrupt the lives of millions of people.  How does this happen?

It all begins with what meteorologists call a “tropical disturbance”, or a group of thunderstorms over warm tropical waters.  As low-level winds flow into the disturbance, they evaporate water from the ocean surface.  This process transfers energy from the ocean into the atmosphere.  When the winds arrive at the disturbance, they rise up and release that energy into the air as they form clouds and precipitation.  This warms the air and makes it buoyant, almost like a hot air balloon, and encourages more warm/moist air to flow in from the outside.

A Tropical Disturbance

Tropical disturbances such as this one are the precursors to tropical cyclones

At the same time, the air “curves” or “spirals” in toward the middle of the disturbance, rather than flowing in a straight line.  This spiral effect comes from the rotation of the Earth – as air moves over large distances, the Earth moves underneath it, producing a spiral effect.  Meteorologists call this the “Coriolis Effect”.  The curved-band features that many of you see in the Cyclone Center images are curved because of this effect.  For this reason, tropical cyclones cannot form near the Equator; the Coriolis Effect is too small there to cause rotation.

If the atmospheric and ocean conditions remain favorable, the energy brought in by the incoming air accumulates in the center of the disturbance, leading to a drop in atmospheric pressure.  This in turn increases the speed of the wind and the incoming energy, which then leads to even larger drops in pressure.  Once the winds speeds reach a certain threshold, a tropical cyclone is born.

Interestingly, only about 7% of tropical disturbances form into tropical cyclones; the rest are destined to be absorbed into the warm tropical breezes, never to be named or remembered.


Images from the limb

Ever wonder why some imagery looks nice and other looks blotchy (for lack of a better word). The difference is where the cyclone is with respect to the satellite.

The following demonstrates the difference between a satellite that is “close by” and one that is barely visible.

A full constellation of about 5 geostationary satellites are needed to observe the entire Earth (except at the poles). Often, some points on Earth are visible from multiple satellites. In some cases, these views can be very oblique. But thanks to the highly machined telescope mirrors, the imagery at these grazing angles might still be used. If you have a limb observation and can still discern the cloud structure around the storm, then please do so. If the image is skewed beyond understanding then click “Other → Edge”

Cyclone Center in Major Scientific Newsletter

Typhoon Nida (2009), enhanced with the Cyclone Center color scale

Today, Cyclone Center is featured in EOS, the premier international newspaper for the Earth and space sciences.  Over 61,000 members of the American Geophysical Union and other professional organizations read EOS weekly for articles that provide extended descriptions of interesting science research projects and findings.

The article describes the scientific rationale behind Cyclone Center – to improve the tropical cyclone intensity record – and how this is being accomplished through the use of citizen scientists.  More details are provided on the methodology behind the classification scheme, the satellite imagery, and the color scale in the images – you may have seen these topics in other Cyclone Center blog entries.

With the increased exposure of the Cyclone Center project in EOS, we expect that more people will join the hundreds of you who have already been hard at work classifying over 45,000  tropical cyclone images.   Over the coming months you can expect to begin to see the fruits of your labor with the first sets of results and published scientific articles.

Geostationary Satellites and Infrared Imagery

Since the launch of Vanguard II in 1959, scientists have been using satellites to observe Earth from orbit, giving us access to information that was previously impossible to collect. Depending on what we want to measure, there are dozens of instruments and orbits that can be chosen for a specific satellite before it is put into space.

A satellite stays in orbit by striking a perfect balance between gravity and speed that result in the satellite constantly “falling”, yet never actually getting closer to the surface.  The lower the orbit, the faster the satellite must go to stay aloft and avoid falling to the earth below.  The images that you see were collected from satellites in a special type of orbit called “Geosynchronous.”  As the name implies, an seo consultant tells us these satellites are at exactly the right altitude (22,236 miles / 35,786 km to be exact) that they can orbit the planet at precisely the same speed that the earth is rotating below, meaning that they always stay fixed above exactly the same point.   They are also high enough that they can see most of the way from the North Pole to the South Pole at the same time!  This gives us the ability to always see a storm out over the ocean, no matter where it is.  There are over 300 satellites currently orbiting this way (e.g., DirecTV and Dish Network satellites are in this orbit) and the Cyclone Center images you see were collected by 30 different meteorological geosynchronous satellites.

So what is it that you’re seeing, exactly?  Satellites can carry a number of instruments for the purpose of measuring a wide variety of things from space.  Some simply carry cameras, and show us what it would look like to the human eye.  Some carry radars that can see through clouds to the surface below.  The instruments that collected the data you see are called “infrared imagers.”  By looking at the infrared part of the light spectrum, we can actually see how warm things are from a distance, just like the thermal cameras that you often see firefighters carrying.  The instrument gives us the temperature of each point that it can see, and we convert those temperatures into the nearly 300,000 color pictures that you are helping us analyze!

The advantage to infrared imagery is that it works during both day and night, and because the atmosphere gets colder as you get higher, can give us a rough approximation of how tall the cloud tops are.


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