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How Hurricanes Form

This is the first part of how hurricanes are formed, updates or additions may be made in the future.

Hurricanes, tropical cyclones, typhoons- Whatever you want to call them, these storms are some of the most destructive in the world. They are called hurricanes when they occur in the Atlantic, Caribbean, Gulf of Mexico and Eastern Pacific and typhoons when they occur in Northwest Pacific, while when these storms occur in the Indian ocean they are referred to as tropical cyclones. Depending on their origin they also have unique classification systems, hurricanes are assigned a category between 1 to 5, with 1 being the lowest wind speed and 5 being the highest wind speed, these classifications depend on the forecast agency issuing the warnings. Hurricane warnings are issued by the NHC, CHC and CPHC respectively. The NW Pacific warnings are issued by the JTWC and JMA where you will find the JMA not having varying titles for a typhoon dependent on it’s strength, while the JTWC will call a storm with winds over 123mph super typhoons. There are also varying titles for tropical cyclones.

Despite all the differences between these storms’ titles and their locations, they are all relatively similar in the way they are formed. How hurricanes are formed was often a mystery, until satellites offered meteorologists a view into how the storms were actually born. Through the decades we have come a long way in understanding tropical storms and the more we learn about their behaviour the more accurate the forecasts, and the more accurate the forecasts – the more lives are saved.

As I stated, most tropical storms form the same way, though in this article I will focus on the Atlantic hurricanes and the way they tend to form.

In the Atlantic hurricanes are most typically formed from a ‘wave’ of low pressure which comes off the west coast of Africa or through upper level lows which work their circulation down to the lower levels. A tropical wave is an area of thunderstorms that move off the coast of western Africa and proceed to move in a generally westward direction. Sometimes these areas of thunderstorms maintain themselves long enough for them to form a consolidated area of low pressure, when this area of low pressure works its way down to the surface, and given that convection is still present over the storm – it can then be classified as a tropical depression, though the NHC (National Hurricane Center) are the ones who classify the storm and there are times when a storm may show tropical depression characteristics, yet is not named as such. There are hundreds of tropical waves that cross the Atlantic every year but only a small portion of these go on to become tropical depressions. Though as mentioned above, hurricane activity isn’t limited to African wave development and storms can also develop on the edge of cold fronts or from an unassociated low pressure system.

Typically before a tropical depression forms, a good looking tropical wave which shows signs of potential development will be allocated an ‘invest number’, this means that the wave holds potential for development and that it is being watched for any further development. Should an invest become a tropical depression, its next step is then to gain winds of a sustained 35mph where-after it will become a tropical storm (it is important to note that these storms will need to be warm-core storms if they are to be classified as a tropical storm (see below). A tropical storm will then be upgraded to a Hurricane once it has sustained wind speeds in excess of 74 mph.

As can be expected a tropical storm would need to be tropical in nature, this means that the storm has tropical characteristics and is fueled by warm waters. In the winter the United States get a lot of Nor Easter storms, these storms may resemble the appearance of a tropical storm with a closed area of low pressure that looks a bit like a spiral in satellite imagery, but these storms are not fueled off warm water and instead can maintain intensity over land for long periods of time and can gain strength in cold waters, these extratropical or non-tropical storms are not the same as polar storms or tropical storms and tend to occur in the mid-latitudes. They are often associated with cold fronts and while they do not tend to become as strong as hurricanes, they can undergo what is known as explosive cyclogenesis, where they can become extremely intense in a short period of time. We in South Africa, particularly those near the Western Cape coast are sometimes subject to extratropical storms, which can also be called ‘cut-off lows’. The primary difference between tropical storms and extratropical storms is that extratropical storms are primarily present in the winter, while tropical storms require warm summer waters to develop.

As mentioned above a hurricane doesn’t need a tropical wave to develop and development of tropical storms can start with non-tropical characteristics. Off the coast of the United States you will sometimes encounter a stalled front which has an area of low pressure on the tail end, this area of low pressure can sometimes become detached from the main front when it is lifted out to the north east, this low pressure then over time (days) can start to work it’s circulation down to the surface and begin to develop tropical characteristics and then become a tropical depression or tropical storm. Where ever the is the presence of a low pressure system and high sea surface temperatures there is the potential for tropical storm development.

The process of a tropical low developing into a depression, storm or even a hurricane is dependent on a number of factors. It’s about having the perfect recipe for formation – a tropical wave or tropical invest will essentially want low wind-shear, warm water temperatures to deep depths, moisture and little land impact. The water acts as a fuel for the storm and the deeper and warmer the waters the stronger the storm or hurricane can get. With that you have to look at the wind shear, wind shear is the difference in wind speeds and wind directions at different heights, think of it like this – you have a stack of balloon that are stuck together with their own static, now you want those balloons to stay touching each other and remain stable in an upright position. If you blow from one direction as the same speed, at the same time to all areas, the balloons will move but they will remain touching, but if you have one person blowing from the left at the top and one from the right at the bottom, you will be creating an opposite force on the object and the top balloon will separate from the bottom and go in different directions. The storm follows a similar principle, the storm is a stacked set of circulations, a low level circulation, a mid level circulation and an upper level circulation, you need all these circulations to be stacked on top of each other for strengthening. What shear does is that is displaces the convection from the circulation and the storm in essence, crumbles.

You also need a moist environment for a storm to maintain or gain intensity, dry air can get sucked into the core of a storm and weaken it extremely quickly. So a moist, shear free, warm water environment is ideal for gaining intensity.

Finally land impact will also weaken tropical storms, though the higher the terrain the more it will weaken the storm. As I said, a storm uses the water below it as fuel, when it is over land it no longer has the fuel it needs. Though in some cases, such as in Florida with large systems, given that the rain bands are still over the water and that the landscape is fairly flat and moist, a storm can in theory still intensify. Typically when landfall is made in a country where there are high mountains you will watch the storm weaken rather quickly, think of it as though you’re running and running and suddenly your lower legs encounter a 1 foot wall, this is the same king of thing a storm experiences when it’s low level circulation is interrupted by large land masses, and it decouples.

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