Archive | Background RSS for this section

How do I classify this? False eyes

One of the challenging aspects of determining the storm type in Cyclone Center is the inability to view a storm snapshot in context.  While classifying a set of images, you do not know which storm you are viewing and how that storm had been evolving before those times shown to you.  This can lead to images that can be misleading to classify – one such image is the “false eye” storm.

A false eye is a circular feature of warm cloud that at first glance appears to be a genuine tropical cyclone eye (the center of a powerful tropical cyclone).  Since we cannot look at other times during the process to see if the feature persists,  we must look for other clues to determine if the feature really is an eye or not.  The primary thing to look for is the storm structure outside of the suspicious eye.  Does the storm look well organized?  Are there distinct and tightly wound spiral bands?  Are cloud tops very cold or not so much?  Consider the following examples, all examined and discussed in the Cyclone Center Talk feature.

Odette (1985)

Odette (1985) The black circle indicates where an eye could possibly be analyzed.  But look at the cloud patterns outside of the “eye” for confirmation.  Here we see no organized spirals and no circular eyewall (the cold ring the typically surrounds the eye).  The clouds are certainly very cold, which is sometimes an indication of strength; but the overall lack of organization leads me to conclude that the “eye” feature is actually just a gap in the cold clouds and not really an eye at all.  I would probably classify this as a weak spiral band type pattern, but nothing more.

Ami (2003)

The second example is from a very complicated cloud pattern, typically seen in what meteorologists call the “monsoon trough” region.  This is an area where the ocean waters are very warm and atmospheric winds tend to come together in the lower atmosphere, creating a situation that is quite favorable for thunderstorms and sometimes tropical cyclones.


The black circle again indicates a circular area of warm clouds that may be mistaken for an eye.  What I immediately notice is that there are two distinct areas of thunderstorms, labeled “1” and “2”.  Area 1 is showing some signs of organization, shown by the black lines, which indicate a turning or spiraling of the clouds.  Little organization is seen in area 2, which is essentially a large blob of thunderstorms at this point.  The eye in the middle is actually just a gap in between the 2 systems – there is no organization in clouds around this area.

I classified area 1 as a spiral band pattern.  The center of area 1 is probably very close to the circled area (follow the black lines in).  Since we are only classifying one system at a time in Cyclone Center, I ignored area 2.

Keith (1997)

To contrast the two examples above, lets look at a real eye.  Keith was a very strong tropical cyclone that exhibited a well pronounced eye feature.

Keith (1997)

At first glance we immediately notice the features of an eye pattern storm: distinct spiral band features, high degree of symmetry, and cold/circular clouds completely surrounding the eye.  Although there are even better examples of eye storms, I would classify this image as a mid-level eye pattern.  The storm intensity is probably in the Category 2 to Category 3 range on the Saffir-Simpson scale.

I hoped that this helps you to become a better Cyclone Center classifier.  Look for more help articles like this on a more regular basis throughout the next few months.

– Chris Hennon is part of the Cyclone Center Science Team and Associate Professor of Atmospheric Sciences at the University of North Carolina at Asheville


2012 Atlantic Hurricane Season: Signed, Sealed, Delivered

The motto that isn't really a motto

The motto that isn’t really a motto

The United States Postal Service (USPS) has been delivering mail for over 200 years (and recently, losing a lot of money doing it).  Their motto, which apparently is not their official motto at all (just branded all over their NYC postal building) is well known: “Neither snow nor rain nor heat nor gloom of night stays these couriers from the swift completion of their appointed rounds”.  Notably missing from this statement  is “hurricane”, “superstorm”, or “storm hybrid” –  Hurricane Sandy showed that it is not possible to deliver mail when a good portion of your city is underwater.

Last Friday the Atlantic tropical season officially ended.  There isn’t a switch that gets turned off that prevents tropical cyclones from developing after November 30; in fact, we have seen storms form into January as recently as 2005.  Nevertheless it is beneficial to designate a tropical cyclone season; it gets people’s attention and does have some scientific merit.  The great majority of storms do form between June 1 and November 30, and storms that do form outside those times rarely affect the U.S.

Of course in other parts of the world the tropical cyclone seasons may be just beginning.  The conditions that allow for their formation in the northern hemisphere late summer/early autumn (warm ocean waters, favorable atmospheric conditions) are just now setting up as the season turns toward summer in the southern hemisphere.  In the tropical western Pacific,  where more  tropical cyclones form than any other basin, conditions are so favorable that storms can form year round.

For Americans, the 2012 tropical cyclone season will be remembered by one name – Sandy.  But it was quite an active season as well, with 19 storms becoming strong enough to earn a name.  This movie shows many of these storms:

Of those 19, only a handful were directly sampled with reconnaissance aircraft; for the rest, as well as storms in every other part of the world, their intensity were estimated primarily from the Dvorak technique.  Cyclone Center citizen scientists use a similar technique to classify historical tropical cyclones – one day Hurricane Sandy will be one of those that users will classify.

We launched the project back in September and it’s had more than 100,000 classifications so far. Cyclone Center is one of the most challenging projects ever built by the Zooniverse, but with each classification you’re contributing to our knowledge of tropical storms.

So far the Cyclone Center community has analyzed more than 500 storms as they raced across the globe. The weather data used on the site comes from 30 years of satellite images and so many memorable storms are being closely inspected by volunteers on the site each day: Katrina (2005), Andrew(1992) and Gilbert (1988) amongst them.

Interestingly, this is the 7th consecutive season that the U.S. was not impacted by a major (Category 3 or higher) hurricane – hard to believe after going through a storm like Sandy which technically may not have even been a hurricane as she came ashore.   As storms continue to become stronger in a warmer climate and societal impacts become more severe, it will be more difficult for mail carriers to make their appointed rounds…assuming mail delivery isn’t cut to 1 day a week by then anyway.

– Chris Hennon is part of the Cyclone Center Science Team and Associate Professor of Atmospheric Sciences at the University of North Carolina at Asheville – this blog is part of the 2012 Zooniverse Advent Calendar.

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

Read More…

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!!  Head on over the Cyclone Center and help us find tropical cyclone eyes.


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

As the air moves toward the center of the disturbance, it “curves” or “spirals”, 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 provide the needed 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.


– Chris Hennon is part of the Cyclone Center Science Team and Associate Professor of Atmospheric Sciences at the University of North Carolina at Asheville.  Help us learn more about tropical cyclone intensity by classifying storms at

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’s Satellite Color Scheme

The Dvorak technique was developed in the 1970s and early 1980s. At that time, most satellite images were viewed on paper using black and white printers. To accommodate this medium, Dvorak developed the “BD Curve”. This curve assigned each satellite brightness temperature value to a specific shade of black, white, or gray.

The Dvorak technique relies on the analyst’s ability to identify each of these shades. Trained experts can usually do this relatively quickly. The BD Curve can be confusing, however, especially to newer analysts. Some colors are repeated, and it can be difficult to discern one shade of gray from another. We have developed a new full-color satellite enhancement for the Dvorak technique to address these issues. In addition to using this new color scheme for Cyclone Center, we plan to share it with tropical analysts around the globe.

The image above compares our color scheme with the BD Curve. Both schemes use gray shading to highlight clouds warmer than 9°C (48°F). The BD Curve then uses a second series of grays, while we give it a pink tint to help differentiate it from the warmer values.

Both color schemes use solid shades at varying intervals for temperatures colder than -30°C (-22°F). In our scheme, this begins with a dark red (which flows naturally from the pink). The colors become progressively less warm (orange, yellow, then shades of blue). Where the BD Curve is forced to repeat Medium Gray and Dark Gray shades, our colorized scheme is able to use unique colors throughout.

Note that the BD Curve uses black for temperatures from -63°C (-81°F) to -69°C (-92°F). This bold color marks a transition from moderate to tall clouds. This same transition is marked by the change from warm to cool colors in our scheme.

We have also included an additional color (white) for temperatures colder than -85°C (-121°F). This color is never used by the Dvorak technique, but it provides us additional information about the coldest clouds.

The images above show two views of Super Typhoon Gay (1992). The one on the left uses the BD curve; the one on the right is our color scheme. All features are identical in both color schemes, but we believe the colorized scheme makes them easier to identify.

We also wanted to ensure that our imagery could be easily interpreted by everyone, including people with color vision deficiencies. We were guided by the principles laid out by Light and Bartlein (2004). Specifically, we avoided any color scale that included both red and green. We also sought a scheme that varied in both hue and intensity. Our ultimate selection was inspired by the “RdYlBu” scheme from

The images above simulate how Super Typhoon Gay would appear to these users. These simulations are performed using The one on the left simulates Deutarnopia, and the middle simulates Protanopia. These are both common forms of red/green deficiency. The image on the right simulates Tritanopia, a rare form of blue/yellow deficiency. These simulations suggest that any analyst, regardless of color deficiencies, would be able to identify the same features in our imagery.

Tropical Cyclone location and intensity data

The World Meteorological Organization (WMO) has assigned the task of forecasting tropical cyclones to different agencies in different regions. For instance, NOAA’s National Hurricane Center in Miami, FL, USA provides forecasts for the North Atlantic and Eastern Pacific. The Central Pacific Hurricane Center in Honolulu, Hawaii provides forecast for the Central Pacific. Several other countries have responsibility for providing forecasts in other regions (such as Japan, Australia, India, etc.). These same agencies that produce forecasts of tropical storms, also produce post season analysis of each storm’s position and intensity – which is called best track data.

In addition to the WMO agencies, numerous other entities forecast and provide best track data. For instance, countries often provide the capability in their nation’s interest, such as China forecasting storms in the Western Pacific. The U.S. Joint Typhoon Warning Center (JTWC) provides forecasts for U.S. interests around the world. These agencies also provide best track data which overlap best track data from other agencies. However, there is not always complete agreement between organization on the strength of a given tropical cyclone. This is often due to different data available to each agency, different procedures in place for forecasting systems, personnel, etc.

The data available to study, forecast and understand tropical cyclones has changed significantly through time and varies from agency to seo agency. Prior to the 1940s, the primary observations came from ships and land-based weather stations. Beginning in the 1940s, the U.S. military began testing – and later made operational – flights into Typhoons (in the West Pacific) and Hurricanes (in the North Atlantic). They found that these reconnaissance flights could be conducted safely and that they provided a wealth of information on the storm’s structure, intensity and environment. Routine aircraft reconnaissance in the Western Pacific ended in 1987 but still continues today in the North Atlantic.

The satellite era was ushered in during the 1960s, providing more information on tropical cyclones. Numerous studies began relating cloud forms to intensity, which culminated in the Dvorak Technique in 1984. However, the availability and quality of satellite data varied, with some agencies still receiving imagery by fax in the 1990s. Similarly, newer satellites provide a wealth of information beyond imagery – microwave satellites provide information on storm structure, radar satellites observe precipitation and other satellites measure wind speed at the ocean’s surface. Again, different agencies have different levels of access to this data.

The result is 1) best track data has changed in time as availability of data changes and 2) best track data varies between agency, due in part to access to different data and routine procedures. This means that differences occur in the best track record.

More information on the WMO agencies is available here.

More information on IBTrACS is available here.

IBTrACS data can also be browsed online.

Image provided by MeteoFrance

Why Cyclone Center? A Detailed Response

Tropical cyclones have enormous impact on life and property around the world. These storms can destroy cities, economies and even armies. However, our ability to understand the strengths or even just to count the numbers of storms that have occurred globally or even in each ocean basin is limited by deficits in the historical data. For instance, there are studies in published literature that suggest that typhoon activity is both increasing and decreasing in the western Pacific Ocean. Clearly both cannot be true!

Changing personnel, forecasting procedures, technological innovation and the introduction of new types of satellite data have created a record of past tropical storm information that is heterogeneous – that is, non-uniform. So looking at how the climatology or history of storms has changed over time is quite difficult and riddled with assumptions. For example, researchers interested in the number of tropical cyclones that have occurred each year throughout history have to make decisions about what years to begin or end their studies – the “period of record.” This choice can influence the results substantially. One possible choice is to choose a starting date as of when all recordkeeping began, say, 1851, ending in the present. This very long record would have a greater number of storms included but the data quality may not be as good as if the researcher made a different choice and started in the 1980’s satellite era. This alternative choice, limiting their years of interest to the time period when the entire globe has satellite observations would influence the number of storms included in the study. Before satellites, how would anyone know if a storm existed if it did not, in some way, impact humanity? Neither choice is “incorrect” but the assumption that is made clearly affects the results.

Looking at the history of one storm at a time, even fairly recent storms have discrepancies in their historical records. For example, four forecast agencies were responsible for tracking Typhoon Yvette in 1992 – the Joint Typhoon Warning Center (US Navy), the Japan Meteorological Agency, the China Meteorological Administration, and the Hong Kong Observatory. These agencies have different methods and procedures for estimating the wind speed in tropical cyclones. The differences can be quite large. As shown in the graph below, on October 13th, 1992, the Joint Typhoon Warning Center estimated Yvette’s winds to be 155 knots while The Hong Kong Observatory estimated only 90 knots. This would be the difference between a Category 2 and Category 5 tropical cyclone on the Saffir-Simpson Hurricane Scale used in the United States. It isn’t clear what the best answer is for Yvette.

Using group consensus, citizen scientists making storm strength estimates in can help clear up this confusion. By participating in this endeavor, you and other citizen scientists are analyzing the satellite images through a process similar to the Dvorak Technique used by meteorologists, thereby providing a more consistent record of tropical cyclone strength. Your choices as you classify storms – your very mouse clicks – will lead to a better understanding of tropical cyclones. You never know, the statistics from the decision process you use to classify storms could even lead to further refinement of the intensity estimation process meteorologists use as well!