As defined by the National Oceanic and Atmospheric Administration (NOAA) a tropical cyclone is a warm-core, non-frontal synoptic-scale cyclone, originating over tropical or subtropical waters with organized deep convection and a closed surface wind circulation about a well-defined center.
Hurricanes, tropical cyclones and tropical depressions bring tremendous amounts of wind and rain with them, and can cause substantial amounts of damage. How do these tropical systems form? What is the typical life cycle of tropical systems? These questions will be further explored below.
Facts about Hurricanes
- Three conditions need to be present for hurricanes to form: warm ocean waters, lack of wind shear, and abundant moisture in the air.
- Tropical cyclones typically only form over tropical waters and about 300+ miles from either side of the equator (this is due to the Coriolis force) and can have a life cycle ranging anywhere from days to several weeks.
- Winds in a hurricane can reach over 150 miles per hour (mph).
- A tropical depression has winds of less than 40 miles per hour, a tropical storm has winds between 40 and 78 miles per hour, and a tropical cyclone has winds exceeding 70 miles per hour.
- Hurricanes, typhoons and cyclones are all different names for the same phenomenon – tropical cyclones. However, each name corresponds to the phenomenon in different regions: hurricanes are tropical cyclones which occur in the Atlantic and Caribbean, typhoons in the Northwest Pacific, and cyclones in the Southwest Pacific and Indian Ocean.
How do Hurricanes develop?
For tropical systems to develop three main ingredients must be present: very warm ocean waters (typically at least 80 degrees Fahrenheit extending a couple hundred feet below the surface of the ocean), very little wind shear throughout the troposphere (little change in wind speed and direction with ascending altitude through the troposphere), and abundant moisture in the lower and middle portion of the troposphere. Warm ocean waters provides the fuel (energy) for these systems to develop and strengthen. Lack of wind shear prevents the system from what is called getting “sheared apart” – if strong wind shear is present the storm’s development becomes disrupted. Abundant moisture further enhances the strengthening process.
If dry air is present in the middle or lower portion of the troposphere or dry air is entrained into the tropical system, it acts as a weakening process and the system will struggle to strengthen any further. These ingredients present alone don’t necessarily mean a tropical cyclone will develop.
For a tropical cyclone to develop a disturbance (either an area of low pressure which is present in the middle troposphere and descends down and becomes a surface low pressure, a complex of thunderstorms, or even a cold front) will need to move over tropical waters when the three main ingredients are present. When this occurs there is a good probability that these disturbances may strengthen and transpire into tropical cyclones.
Typical Stages of a Hurricane
If a disturbance moves over very warm tropical waters and little wind shear is present and there is an abundance of mid and low-level moisture, the National Hurricane Center (NHC) will closely monitor the disturbance. When the NHC has high interest in a potential disturbance in which they think may become a tropical cyclone it is called an Invest. If the disturbance exhibits characteristics such as becoming further organized (growth of new thunderstorms, clear defined circulation, lowering air pressure) and strengthening, the disturbance will be labeled a Tropical Depression.
Maximum sustained winds within tropical depressions are less than 40 mph, however, gusts could exceed this threshold. If tropical depressions further their strengthening and max sustained winds range from 39 mph to 73 mph, the tropical depression is not only upgraded to a tropical storm but it is given a name. If further strengthening occurs and the maximum sustained winds are at least 74 mph the storm will be upgraded to a cyclone.
In the Atlantic Ocean, northeastern Pacific Ocean (east of the international dateline), and South Pacific Ocean (east of 160 degrees east longitude) the tropical cyclone is called a hurricane. In the northwest Pacific Ocean (west of the international dateline) the tropical cyclone is called a typhoon. Across the southwestern Pacific Ocean (west of 160 degrees east of latitude) and the southeastern Indian Ocean (east of 90 degrees east of longitude) the tropical cyclone is called a severe tropical cyclone. Across the northern Indian Ocean the tropical cyclone is called a severe cyclonic storm and in the southwestern Indian Ocean it is just referred to as a tropical cyclone.
When tropical cyclones reach this final stage they can strengthen and become giant monsters with sustained winds as high as 150+ mph. The degree of strengthening all depends on factors such as; longevity over warm waters (the more ocean mass the system has to traverse the greater the room for strengthening), degree of wind shear over the waters (if there is very little wind shear over the waters this enhances the likelihood for strengthening), whether or not dry air can become entrained into the system, and also how warm the water temperatures are (if water temperatures are well-above average, this can lead to rapid intensification of the cyclone).
Average Lifecycle of a Tropical Cyclone
The average lifecycle (or lifespan) of a tropical cyclone can vary greatly and ranges anywhere from a few days to perhaps as long as a month. The lifecycle for a given tropical disturbance all depends on the atmospheric and oceanic variables we have discussed above. If a tropical disturbance is located over waters which are cooler than 80F, in a region of stronger wind shear, or there is some dry air around, the likelihood of the tropical disturbance undergoing further strengthening is very unlikely.
To the contrary, if a tropical disturbance is positioned over very warm waters (>80F) and in an area of low wind shear with no dry air around, the likelihood for strengthening is not only high but significant strengthening is certainly possible. Across the Atlantic Ocean two major areas which are closely monitored are the Caribbean Sea and just off the west coast of Africa (especially as you move into September).
The Caribbean is notorious for very warm ocean waters and if the atmospheric parameters (low shear/high moisture content) are favorable, tropical disturbances in this area can intensify very rapidly. This poses a significant threat for the islands and even the east coast of the United States at times. There are instances where these disturbances will track into the Gulf of Mexico (another region with typically very warm ocean waters), and with more ocean to work with this will not only lead to a longer lifecycle but increase the potential for very rapid strengthening, which can result in an intense cyclone.
The waters in the eastern Atlantic (just west of Africa) typically don’t reach or exceed the 80F threshold until September or so and this is a time of year when any disturbances moving west off the coast of Africa get paid very close attention. There is a significant amount of open waters so tropical disturbances which develop into tropical systems off the coast of Africa typically have a much more significant lifecycle than systems which develop in the Caribbean.
Across the eastern Pacific (off the coast of Mexico) there is also a significant amount of open water and tropical cyclones here can become quite intense as well. Fortunately there is very little landmass but because of this these systems can also have a great deal of longevity. There are instances where they threaten the Hawaiian Islands, however, because water temperatures around Hawaii are cooler the cyclones tend to weaken as they approach the vicinity of the islands. While these cyclones can become quite intense they often don’t have any significant impacts to any land masses as there isn’t much around. One area of the world very susceptible to dangerous tropical cyclones is Japan and the Philippines. Tropical cyclones in this region have a long history of becoming very large. This is due to the exceptionally warm waters that can exist here, and when coupled with low wind shear these cyclones can intensify very rapidly.
When tropical cyclones begin to move over cooler waters (water temperatures < 80F) or over land masses they tend to weaken very rapidly. Tropical cyclones need warm waters to fuel them and cooler waters do not supply as much “fuel” for these systems to either strengthen or maintain their strength. Land masses provide no fuel and friction also increases over land which results in a weakening tropical cyclone. If a tropical cyclone also remains over the same area of ocean water for several days it may begin to weaken as well. This is just due to a great deal of upwelling (water is being turned and flipped due to the storm itself) and this brings much cooler waters from down below towards the surface.
There are organizations that focus on relief to communities that have suffered because of hurricanes and the heavy rains and waters they can cause. These are essential to these communities longevity and ability to survive.
Hurricanes in Film and Culture
- “Rock you like a Hurricane” is a classic hard rock song that was released by The Scorpions in 1984.
- The Miami Hurricanes are a Division I Collegiate football team located in Coral Gables, Florida. The team had incredible success in the 1980s and 1990s, claiming 5 national championships in less than 20 years.
