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How Hurricanes Are Named and Tracked

June 2nd, 2010

hurricane-doppler-radar-tracking.jpgYesterday (June 1) was the official beginning of hurricane season, and since I do not have anything else that has come in from regular writers, I might as well publish this series of hurricane-related articles that I have had in reserve for the past few months. I had originally purchased them from a seller on the DigitalPoint forums (unfortunately I could not track down the name of the original author), but since the season for tropical storms had just ended and we were heading into winter I decided to wait until June to release them.

I managed to acquire five of these articles, all of which are of surprisingly good quality for this particular source. Eventually I will link them together in a series after I get them all published. This first one has to do with the naming and tracking of hurricanes, and it is quite long and thorough (approximately 2500 words). In addition to the main topic, we also have short lessons on things like Morse code, satellite systems, radar, and the Doppler effect included in here, so if you happen to be in an educationally oriented mood, you might want to relax with a glass of your favorite beverage and prepare to add some data to your brain’s storage facilities.

If a hurricane is heading your way you need to know how strong it is and when it will arrive. You do not need to know which hurricane it is, only that it is the one due to hit you tomorrow afternoon. Thinking back on the event in a few years’ time, however, you might want to compare one hurricane with another. Then you would need a way to identify each of them. Of course, you could label them by their arrival dates. That is the only way we have of describing storms of long ago. The 1900 Galveston hurricane was just that, the 1900 Galveston hurricane.

Modern meteorologists need a better system because they often find themselves monitoring several hurricanes at the same time, all of them moving. At first they listed them by the latitude and longitude where they were first reported. Alternatively, they could have numbered them, perhaps with the year and the sequence for that year. Then you could have, for example, hurricanes 1:96, 2:96, and so on. Either system of numbering would work, but it was cumbersome. In the 1940s, when meteorologists began airborne studies of tropical cyclones, ships and aircraft communicated mainly in Morse code. This was satisfactory for letters of the alphabet, but it was not very good at dealing with numerals. With a dot (.) representing a short signal and a dash (-) representing a long one,–… -……, the Morse code for 1:96, is slow and can cause confusion.

The use of Morse code was abandoned when ship and aircraft radios started using voice communications. American meteorologists then listed tropical cyclones alphabetically, using the international phonetic alphabet for radios: Able, Baker, Charlie, Dog, etc. That was in 1951, but in 1953 a new international alphabet was introduced (Alpha, Bravo, Cocoa, Delta, etc.). This caused confusion, because one operator might report “Hurricane Dog,” another “Hurricane Delta,” and it would not be clear whether these were both the same hurricane or two separate ones. That system also had to be abandoned, and in 1953 meteorologists began using women’s names instead.

The idea of giving hurricanes names was not new. In the West Indies people had long named hurricanes after the saint on whose day they struck and the practice had been adopted in other Caribbean islands. The storm that swept across Puerto Rico on July 26, 1825, for example, was known locally as Hurricane Santa Ana. Personal names were also being used elsewhere. “Saxby’s Gale,” which occurred in Canada in 1869, was named after a naval officer who was thought to have predicted it, and some meteorologists had been giving tropical cyclones women’s names since the late 19th century.

Women’s names remained in use until 1978, when some storm lists prepared for the eastern Pacific included men’s names. In 1979 both women’s and men’s names were used to compile lists for the Atlantic and Gulf of Mexico and this still remains the practice, with male and female names alternating (Andrew, Bonnie, Charley, Danielle, for example). Also since 1979, the lists include names from non-English-speaking cultures.

The names are a substitute for the international phonetic code and so they are arranged alphabetically. In 1995, for example, the first Atlantic hurricane was called Allison, the next Barry, and so on down to Wendy. Tropical cyclones forming in the Atlantic and Pacific are given names, but not those that develop in the Bay of Bengal.

Since different names must be allotted to Atlantic hurricanes and Pacific typhoons and all the names must follow an alphabetical sequence so each list does not contain two names beginning with the same letter, it will not take many years to use up all the names in the world. This difficulty is avoided quite simply. Lists are compiled in advance for six years and in the seventh year the first list is used again. The 1998 Atlantic hurricane list is the same as that for 1992, and the 1993 list will be used again in 1999. An exception is made, however, for hurricanes that were especially noteworthy. To avoid confusion, their names are retired and new names substituted the next time that list is used. In 1992, for example, the list began with Andrew. That name has been dropped and the 1998 list begins with Alex. Agnes (1972), Camille (1969), Gilbert (1988), and Hugo (1989) are among the names retired in recent years.

There are four lists each for the central (from the international dateline to 140¡ã W) and western (west of the dateline) North Pacific, but they are used differently. The first storm each year takes the name following the last one used the previous year. The last 1995 storm in the central North Pacific was called Nona and the first in 1996 was Oliwa, the next name in the list. When the last name in the fourth list has been used, the next will be the first in the first list. Confusion is avoided by attaching the year, so Oliwa was known as 1996 Oliwa.

Cyclones in the north Indian Ocean are not given names. Those in the southern Indian Ocean, South Pacific, and near Australia have names drawn from lists. In the southwestern Indian Ocean (west of 90¡ã E) a fresh list is used for each year, the lists rotating in the same way as those used for Atlantic hurricanes. Elsewhere names follow on until each list has been used, in the same way as names used in the North Pacific.

A name is allocated as soon as the air around a disturbance starts rotating cyclonically (counterclockwise in the northern hemisphere) and its winds exceed 39 MPH. At this stage it has become a tropical storm and until it intensifies into a hurricane the name is prefixed with TS. Once it has grown into a hurricane the prefix is dropped and it is known simply by its name.

Until the late 1940s, spotting a tropical cyclone over the ocean was very much a matter of chance. A passing ship might report it, but unless it was close to a shipping lane it was unlikely to be noticed. In those days there were few airlines flying intercontinental routes and the aircraft they used lacked the range to fly far over the open sea. The most developed North Atlantic route, for example, went from New York or Montreal to London with refueling stops in Labrador or Newfoundland, Iceland or Ireland, and sometimes Prestwick, in Scotland. Aircraft were improving, however, and their numbers increasing. More advanced instruments allowed pilots to fly through clouds and meteorologists made use of them, often asking for reports on weather conditions, especially the height of cloud bases and tops. On military airbases, if it was uncertain whether conditions were suitable for flying, the day usually began with one pilot flying around the area of the field to check the weather. Pilots would not fly deliberately into a large cumulonimbus (storm) cloud, but even that was changing. Planes of the 1940s were stronger than those of the 1930s and had more powerful engines. Flying through a storm was not quite so dangerous as it had been.

By 1945, U.S. navy and army aircrews were flying meteorologists through tropical cyclones fairly routinely, gathering instrument readings from which the scientists came to understand the structure of these weather systems. Aircraft still fly scientific missions into hurricanes and typhoons.

These missions do not locate the storms, of course, but are directed toward storms that have already been identified. The early identification of atmospheric disturbances relies on satellites. The first weather satellite, TIROS (Television and Infrared Observation Satellite) was launched in April 1960 and within a few days had sent pictures of a typhoon no one had known existed, 800 miles from Brisbane, Australia. Today there are many weather satellites in orbit and new ones are launched at a rate of about two each year. The overlapping coverage of some forms a network, or “constellation,” providing constant monitoring of the entire Earth.

Satellites can be placed in either of two types of orbit, called polar and geostationary. A polar orbit carries the satellite over both poles and in a series of orbits over the whole world. At a height of about 534 miles, the satellite makes a complete orbit of the Earth every 102 minutes. While it is doing so, the Earth is rotating beneath it. In 102 minutes the Earth turns 25.5 degrees to the east, so with each orbit the satellite flies over a region 25.5 degrees to the west of its previous pass. Satellites in geostationary orbit are directly above the equator, at a height of about 22,000 miles, and travel in the same direction as the Earth’s rotation. Their orbital speed is the same as that of the surface beneath them, so they remain permanently over a particular point.

Information from orbiting satellites passes to the organization that owns them, and all meteorological services are coordinated by the World Meteorological Organization, an agency of the United Nations. U.S. weather satellites are operated by the National Oceanic and Atmospheric Administration (NOAA) and observations of tropical disturbances are sent to the NOAA National Hurricane Center in Miami, Florida.

Satellite photographs are studied closely. The meteorologists watch for the development of cumulus clouds with a wide layer of cirrostratus (thin, high-level, featureless sheets of cloud made from ice crystals). This combination indicates a strong convective system. Cloud movements are monitored to reveal the direction and strength of winds.

The scientists do not rely only on satellite images. Ships and aircraft also radio reports to them with information on atmospheric pressure and ways it may be changing, winds, and rain. If rain showers merge into steady rain, atmospheric pressure is falling, and winds are strengthening, the weather conditions will be classified as a tropical depression. As data continue to arrive at the Hurricane Center, any further intensification of the depression will be noticed almost as soon as it happens and the track of the system will be plotted carefully. If it becomes a tropical storm, with a cyclonic circulation and winds of more than 39 MPH, the next name in the list will be assigned to it.

When a tropical depression that has grown into a storm comes within a few hundred miles of the U.S. coast, aircraft join in the task of monitoring. The first to arrive are the “Hurricane Trackers” of the U.S. Air Force Reserve. Their job is to fly through the system measuring the distribution of pressure within it, wind speeds and directions, and locating the eye. Radioed back to Miami, this information allows interior details of the storm to be added to the charts. NOAA aircraft also join the team. Equipped with sophisticated instruments, these “flying laboratories” communicate with the NOAA Aircraft Operations Center in Miami.

Back at the Hurricane Center, the information from satellites, ships, and aircraft is fed into computer programs that predict the future behavior of the system. They estimate the likelihood that it will grow into a hurricane, and of what size and strength, and the track it will follow. If it is heading for an inhabited island or the U.S. coast, the relevant authorities are alerted.

Still closer to land, the tropical cyclone comes within range of onshore radar. There is a radar network covering the entire east coast of the United States, from Texas to Maine, and it extends seaward as far as the Lesser Antilles, the most easterly group of Caribbean islands, and extending southwards in an arc with its northern end to the east of Puerto Rico.

Radar is electromagnetic radiation, the same type of radiation as visible light and radio waves, which is emitted from a transmitter and reflected from certain surfaces. The reflected radiation is detected by a receiver and provides two kinds of information. The first is an image, displayed on a screen, of the shape of the object scanned by the radar. The second is the distance to the scanned object. This is calculated by measuring the time that elapses between the emission of the signal and the arrival of its reflection. Like all electromagnetic radiation, radar travels at the speed of light, so the time it takes for the round trip reveals how far it has traveled.

Different radar wavelengths are used to scan different objects, and a wavelength of 10 cm (3.94 inches) is strongly reflected by water droplets. Once the storm is within range, radar can reveal its clouds and rain in great detail.

Nowadays it can do more, because the shore-based radars are being upgraded to Doppler systems. These measure the frequency of the reflected waves very precisely. The waves all travel at the same speed, but in 1842 the Austrian physicist Christian Johann Doppler (1803-1853) made an interesting discovery, originally about sound waves but extended later to electromagnetic waves. If waves traveling at a constant speed are emitted by an object moving toward or away from an observer, their frequency will change. This happens because the distance the waves travel is changing. If the source is approaching, the frequency will increase, and if the source is receding, the frequency will decrease. With sound waves, increasing the frequency raises the pitch and decreasing the frequency lowers it. This is why the sound of a train rises in pitch as it approaches, then falls in pitch after it has passed. With light waves, increasing the frequency makes the light more blue, decreasing it makes the light more red. Astronomers have made use of this discovery for many years to tell how rapidly remote galaxies are receding from us (red-shifted).

Now meteorologists also use it with the help of systems that “translate” radar signals into colors on their computer screens. With Doppler radar they can add details of movement to the radar images they already have of the size of clouds and type and intensity of rain. They can tell how fast the storm is rotating because one side will be retreating and the other approaching. Color the retreating side red and the approaching side blue, based on the frequencies of the radar reflections from water droplets, and the rotation becomes clearly visible: the stronger the colors the faster the movement. The radar also reveals the direction in which the storm as a whole is moving and its speed.

Monitoring is now very advanced, but not all the tropics are covered so well as the seas off the eastern United States. Satellites observe the whole world, and ships and aircraft much of it, but planes equipped as meteorological laboratories, powerful computers, and radar networks are expensive. Until these are available in all countries that lie along tropical cyclone tracks, some communities will be less prepared than others for severe storms.

Related Articles:

Hurricane Preparation, Survival, and Safety
Hurricanes That Reach Europe
Arctic and Antarctic Hurricanes
Will Global Warming Cause More Hurricanes?

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