Hurricanes, cyclones, typhoons, tropical storms and monsoon. You sometimes hear these terms used during weather reports, normally ominous ones. They are often used in conjunction with words, like "large-scale damage, devastation and destruction". But what exactly are these weather phenomena and how do they work?
We will take a closer look, not only at what they are but also at each one's characteristics. Then we will also examine how they are formed, as well as the impact on their immediate environment. But first, here's the answer and summary in a nutshell. (Some of you may be quite surprised at the following.)
Hurricanes, cyclones and typhoons are all the same type of weather phenomenon, a tropical storm. The only significant difference is the region of the world they originate from. A monsoon on the other hand is something completely different. It can be described as a seasonal large-scale weather system that is usually associated with a shift in wind direction, and an increase in humidity and rainfall in a specific region.
Now that we have a vague idea what the differences (or similarities) between all these weather systems are, its time start taking a closer look at each one.
Since hurricanes, cyclones and typhoons are all a type of tropical storm, it only makes sense to to put the tropical storm under the microscope first.
The general term, tropical storm, is used by meteorologists and climatologists to describe the organised, rotating cloud systems and thunderstorms which forms over the world's tropical oceans. They are easily identifiable by the familiar rotating cloud pattern spiraling out from the center, as seen on satellite images.
They are also characterized by high wind speeds and extreme low pressures close to the center of the storm, with its typical low-level circulation allowing it to draw and maintain its energy from the warm ocean water.
It is interesting to note that the direction of rotation of a tropical storm differs between the southern and northern hemisphere. In the northern hemisphere, it rotates in an counterclockwise direction, while it rotates in a clockwise direction in the southern hemisphere.
It's All About Location
Now that the tropical storm is defined and explained, you can begin to understand how the foundation of a hurricane/typhoon/cyclone is formed. To be classified as a hurricane (or one of its namesakes), a tropical storm needs to grow in strength and exceed a certain threshold. More on that in the next section.
Before we get to that, we need to clarify once and for all why hurricanes, typhoons and cyclones are named differently when they are essentially one and the same thing. Like I mentioned earlier in the post and as the heading suggests, it's all about their location.
Hurricane Systems Forming Over The Northern Hemisphere
- The term, "hurricane" is used when the weather system originates over the Atlantic Ocean, the Caribbean or the Northeast Pacific Ocean.
- The term, "typhoon" is used when the weather system originates over the Northwest Pacific Ocean.
- The term, "cyclone" is used when the weather system originates over the South Pacific Ocean or the Indian Ocean.
Meteorologists officially use the umbrella term, "Tropical Cyclone" to cover all three systems regardless of their location. Since we are all familiar with the term "hurricane", I am going to simplify everything. For the remainder of the article, the term "hurricane" will used as the umbrella term for all three variations (hurricane, typhoon and cyclone). Whenever a reference to "hurricane" is made, it will automatically include the other two name variations.
With all the technicalities out of the way, we can start looking in more detail at what makes a hurricane, including the necessary criteria for a tropical storm to be classified as a hurricane.
When A Tropical Storm Turns Into A Hurricane
There are several stages in the development of a hurricane, which then has its own set of classifications (based on strength). The best way to understand this process is to start with the weakest system, then work our way up as its strength increases.
It All Starts With A Tropical Depression
The first system that starts forming over the warm waters of the oceans around the tropics, is a tropical depression.
A Tropical Depression forms when the warm air over the ocean rises, leaving less air near the surface (forming a low-pressure system). The air that takes its place, warms up and starts rising too. The process continues and the surrounding cooler air swirls in to take its place. The warm moist air cools down as it rises and clouds start forming as a result. This whole cloud system slowly rotates and grows as the warm air rising from the ocean's surface continues to feed it. As long as the wind speeds do not exceed 38 miles per hour, the system remains a tropical depression.
Turning Into A Tropical Storm
As soon the winds reach and sustain speeds of 39 miles per hour and above, the system is classified as a tropical storm. Within a tropical storm wind speeds can vary from 39 - 74 mph.
We are already discussed the tropical storm in detail earlier in the article, so I don't need to add too much additional information in this section.
Clearly a tropical storm is much stronger than a depression as a result of the higher wind speeds. Even though its not yet seen as a hurricane, its destructive nature should not be underestimated. It can still lead to substantial damage and flooding, especially in coastal and surrounding areas.
(Also, take note that although these storms rotate counterclockwise in the Northern Hemisphere and clockwise in Southern Hemisphere, they still remain exactly the same in every other aspect.)
A Hurricane Is Formed
As the wind speeds increase, so does the strength of the storm. Once the wind speeds reach and exceed 75 mph for a sustained period of time, is is classified as a hurricane.
The familiar shape of the this weather system becomes much more defined, with a dense bank of rotating clouds, as well as the eye of the storm it surrounds, clearly visible. The signature bands of clouds, spiraling out from the storm center can spread as far as 100 miles or more in major hurricanes.
The eye of the hurricane sits in the middle of the storm system and is normally very clear and calm, with no cloud covering or any significant air movement. It is mostly circular in size, but can vary and often form a more oval shape. The average size of the eye are about 15 miles (24 km), but in large hurricanes it can be as big as 40 miles (64 km) or more.
Eye Of The Hurricane
As the eye sits right in the middle of the hurricane, it is surrounded on all side by huge banks of clouds receding with height (in what is sometimes referred to as the "stadium effect"). Also called the eyewall, it is this bank of clouds at the edge of the eye that also contains the highest and most destructive wind speeds.
(This is one of the hidden dangers of a hurricane. People unfamiliar with hurricanes do not realize that the danger is not over once the first wave of the storm has passed and the eye moves over an area. This creates the false impression that the storm is over, causing many people to leave the safety of their shelters, only to be caught off-guard and hit by the destructive winds & rain in the approaching opposite wall of the storm.)
Levels Of Strength
It goes without saying that the impact of these storm systems can be devastating, especially as it approaches and crosses the coastline and surrounding areas.
As a hurricane moves inland, it looses power quickly, as it is no longer receiving energy from the warm ocean water. It can still cause severe damage though, depending on the size and strength of the storm as it made landfall.
The strength of the hurricane can be measured on a scale from 1 to 5. As you will see from the diagram below, the strength and resulting damage is mainly a direct result of the associated wind speeds.
Hurricane Category Based On Strength With Associated Wind Speed And Storm Surge
The strength of a hurricane is not the only factor that determines its impact and the amount of damage it will cause.
For example, two hurricanes can interact and combine to form a much stronger storm system. This phenomenon is called the Fujiwhara Effect.
Damage Caused By Hurricanes
Unfortunately, whenever a hurricane-strength storm makes landfall (or even while still over the ocean), some form of damage is almost guaranteed.
The amount of damage caused, is not just determined by the size of the hurricane, but also to a large degree by the topography of the region hit by the hurricane, as well as the population density of the affected area.
If the hurricane crosses into a coastal region that is very mountainous or have plenty of hills and valleys, the storm system gets disrupted and broken up fairly quickly, limiting the amount of damage it is able to inflict.
When area the storm is entering is very flat coastal region, the hurricane is able to travel much further inland without loosing too much strength, causing the extend of damage to stretch further inland as well.
The amount of damage a hurricane can cause, also depends on which component of the storm system is causing the damage. The main components of a hurricane causing the most amount of damage are:
- Wind Speed
- Storm Surge
By looking at each one of these components individually, one can get a better idea of the type and extent of damage that each one can cause.
Devastation Caused By Flooding
Probably one the most devastating results of the heavy rainfall associated with a strong hurricane is flooding. Extensive flooding can cause lost of life, not only as result of drowning, but also due to structural damage to buildings and other structures that can collapse and cause fatalities.
The damage to roads, bridges and other infrastructure may run into hundreds of millions worth of damage, and can also make an area inhabitable for months or even years in a worst case scenario.
In mountainous areas with little vegetation as covering, mudslides and rockfalls are common occurrences during a heavy rainfall. It can also destabilize hillsides near roads and villages, which can lead to large scale evacuations and roads becoming unusable.
2) Wind Speed
Depending on the strength of the wind, objects like trees and buildings can be severely damaged, or in some cases completely flattened.
Consistent strong gusts of winds can also worsen the effect of flooding and storm surge. It can drive flood waters deeper inland, extending the damage caused by flooding. The strength and height of the storm surge will also be impacted by the wind speed, which will have a direct impact along the shoreline directly hit by a hurricane. This brings me to the last component of a hurricane that can be the most destructive part of a hurricane.
3) Storm Surge
We all heard about this term when hurricanes or related storms are discussed or mentioned during weather forecasts. So let's first clarify what exactly storm surge is.
Storm surge can be seen as the abnormal rise in the sea level as result of a hurricane, where both the low-pressure system in the center of the storm as well as the high wind speeds, cause the abnormal rise in water levels. The storm surge dramatically builds up in height as the ocean floor becomes more shallow near the shoreline as the hurricane approaches coastal regions.
The height of a storm surge is determined by
- the strength of the wind speeds,
- as well as the amount of drop in air pressure.
The low-pressure system has a very significant impact on the height of the storm surge while still over the ocean. The lower the air pressure, the higher the sea level will rise. It is estimated that for every drop in one hPa (1 millibar) of pressure, the sea levels rise by approximately 0.4 inches (10 mm).
(If you take into consideration that the normal air pressure at sea level is about 1013 millibar and that air pressure in a hurricane was measured as low as 882 millibars in 2005, the huge impact air pressure in hurricanes have on ocean levels, speak for itself. You do the math!)
The high winds blowing in the direction of the shore contributes to the rise in sea levels as it cause a build-up of water in the direction the wind is blowing. It also strengthens and drive the resulting high waves hitting the coast, forcing the water deeper inland.
The results of storm surge can be devastating, especially during very strong hurricanes where waves of 15 - 20 feet or more can be generated, (Hurricane Katrina generated a storm surge of 28 feet in Louisiana in The United States in 2005. You just need to Google the term "Hurricane Katrina" to see images of exactly how devastating a storm surge can be...)
(If you want to see how the size of hurricane and relating wind speeds, influence the size of the storm surge, simply have a look at the Hurricane Strength Diagram earlier in the article.)
What can make the effect of storm surge even worse, is when storm tide is formed.
A storm tide is created when a coastal region is already experiencing the region's natural high tide when the storm surge breaches the shoreline. The normal high tide level may already be 3 feet above the mean sea level. When a storm surge of 12 feet reaches the shore, it creates a storm tide that is a combined 15 feet high. (The high tide sea level and height of the storm surge are combined to form the massive influx of water.)
Clearly the areas most affected by storm surge, are towns and cities situated on the coast. The closer to the shoreline, the more severe the damage. It is not uncommon for buildings and other structures close to the shore to be completely destroyed by the tidal wave.
This "wall of water" and the resulting flooding can spread far inland, enveloping complete cities and surrounding areas, depending on the size of the hurricane. While this is happening, violent waves, driven by the high wind speeds, will continue to batter the coast and cause damage to anything on the coast that wasn't destroyed by the initial hit of tidal waves.
You might be forgiven to think I am being over-dramatic and and everything you are reading is over exaggerated. If that is the case, good!
As I am just sticking to the facts and give you the information in the most understandable way possible, it is great if you find this information a bit far-fetched. Why?
Simply because the sheer power and scale of hurricanes can not be overemphasized enough or taken lightly in any way. There is a reason weather forecasts pay so much attention to approaching hurricanes, their strength and heading. The same goes for breaks in normal broadcasting (on television, radio and online) to issue hurricane warnings. You now know why not to ever take this lightly again.
You now should have a very clear idea what hurricanes/cyclones/typhoons are, as well as their differences (or similarities). We also examined how they are formed, their characteristics and the impact they have on the environment.
Now you must forget everything you just read, for the moment anyway. The reason is simple. The weather pattern I am about to discuss is a completely different ball game than all the terms discussed in the above section that are all closely related to each other, and in more ways than none actually mean exactly the same thing.
Yes, the term "monsoon" often gets confused with terms like "hurricane, cyclone and typhoon". The difference is where the other terms refer to the same weather phenomenon, "monsoon" refers to a completely different type of weather system...
First of all, a monsoon is not a random event that can occur sporadically at specific locations. It occurs during specific times of the year (seasonal) and only occurs in a few specific locations on earth (predominantly the region stretching from India to Southeast Asia).
Secondly the way in which a monsoon is formed, as well as its characteristics differs completely from the typical hurricane formation and familiar "rotating & spiraling" shape.
So what exactly is a monsoon and how and how does it develop?
Monsoon Storm Descends Over A Region In India
A monsoon is the weather pattern that forms over India and the Southeast Asian Region during the warm summer months. It is caused by a shift in wind direction, and these southerly winds move from the high-pressure system over the warm Indian and Western Pacific Oceans. It picks up a substantial amount of moisture from the warm ocean water as it moves north towards the low-pressure system present over the continent, bringing with it large torrential rains.
(Just in case you came across the term or may have been wondering, there is such a thing as a winter monsoon. This form of monsoon is characterized by the air flowing in exactly the opposite direction as that of the airflow during warm summer months. It is also associated with the resulting dry weather condition that prevails over land during, as the name suggests, the winter. As a result, whenever a general reference to a monsoon is made, it normally refers to the familiar large-scale system occurring duting the summer over Southeast Asia.)
It has to be noted that a similar weather system occurs over the Southwestern United States (as well as West Africa) during late summer in the region, which are technically a monsoon as well. These systems are much weaker and smaller than the one occurring in the Indian and Western Pacific Oceans though. Although they are all technically monsoons, the general reference to "monsoons" and "monsoon season" in global terms, are normally associated with the huge weather system in India and Southeast Asia.
Early Stages And Development Of A Monsoon
It is not unusual to think of a monsoon as a very large sea breeze. Instead of occurring over the course of a day however, it lasts for months. Needless to say it also covers a much larger area than just a small coastal region, as a single monsoon can cover large parts of India or Southeast Asia at a time. The same principle apply to both though...
During the summer both the land and ocean warm up, but (due to each one's different capacity to absorb and retain heat) the land warm up much quicker than the ocean.
As a result, the land heats up much quicker and warms the air at the surface. The warm air rises and expands which leaves less air at the surface, creating a low-pressure system over land.
The water over the ocean takes much longer to warm up, allowing the air above it to be able to maintain a higher pressure. Since air always flows from a high-pressure to a low-pressure system, the air from the ocean flows to the land.
The result is an effect very similar to a sea breeze This "sea breeze" effect, do not occur over the course of a day however, but build up and last for months. It effectively can last for the full duration of the summer season (or however long the region is subjected to a warm weather).
The air flowing from the ocean is filled with moisture. As the air reaches land, it starts to rise and as it gains altitude it cools down. The cooler air can no longer hold the moisture and condensation takes place. This, in turn leads to large-scale precipitation over the region.
The cooler air moves back over the ocean. This completes the cycle of air flow that creates a the structure of a monsoon. (During the winter months, this whole cycle takes place in reverse, leading to the dry weather during the region experience during winter.) The diagram below will give you a better indication of how this cycle works.
Air Circulation Forming A Monsoon
Since monsoons have been part of Southeast Asia and India for centuries, and is generally seen as a normal seasonal weather cycle that repeats itself every summer season, it is not classified as an abnormal weather activity.
It is only when one or a combination of factors cause a monsoon to result in catastrophic and severe damages, that it is seen as an abnormal weather event. Therefore, the tracking and constant monitoring of a monsoon is vital to identify any potential dangers and issue warnings accordingly.
The Dangers Of A Monsoon
For the most, part the agricultural sectors of the regions affected by monsoons, have adapted to this weather system over time. In fact, the whole agricultural economy now actually rely on the monsoon season and plan all their activities around the associated rainfall during the season. So much so, that they actually depend on it for their survival.
That does not mean a monsoon does not come without its dangers though. Some of these dangers are a direct consequence of monsoon, but some are more hidden indirect dangers. It can be roughly be divided into three categories.
1) Heavy Rains And Flash Flooding
The cloud systems that build up over land when a monsoon is formed, are able to carry a huge amount of moisture. This normally results in heavy sustained rains.
Often though, a very large downpour results in a sudden buildup of water which causes flash flooding throughout the region. Many lives are lost of as a result drowning or the collapse of structures due to weakening by flood waters.
What makes these flash floods so dangerous, is that they not only appear very quickly and without warning, but the water buildup as a result happens very quickly as well. This leaves people very little time to respond and many are often trapped by the quick rising water.
A more indirect, but far more devastating result of the monsoon season, is the development and spread of waterborne diseases.
Standing water, caused by the persistent rain and flooding, is a breeding ground for all kinds of waterborne diseases and cause thousands of fatalities each year.
Malaria, Cholera, Typhoid, Dengue and Viral Fever are just a few of the potentially deadly diseases that are associated with the rainy summer season in Southeast Asia.
They are all directly or indirectly associated with the monsoon season and can be transmitted in a variety of ways:
- Bathing in contaminated water
- Contact with infected bodily secretions
- Eating contaminated food
- Insect bites, especially mosquitoes
- Drinking contaminated water
Precautions, advances in medical technology and access to treatment have lessened the effect of these diseases in recent years, but it still remains the biggest cause of death as a result of monsoons in the subcontinent.
3) Late Arrival Or Weak Monsoons
Ironically, as destructive as the monsoon rains can be, the lack or delay in the arrival of these rains can be just as devastating. For more than a century, farmers and the agriculturist industry as whole, has not only adapted but are actually planning and relying on the coming monsoon rains for for the irrigation of crops and to a smaller degree, grazing for livestock.
A monsoon season that arrive a few weeks late, can have a severe effect on the growth and success of Kharif (rices, Jowar, maize etc. planted in the beginning of the rainy season) crops. A whole season with very low monsoon rainfalls can lead to complete crop failure.
Just take into consideration that about 70% of the population of India rely on agriculture, and about 58% of total employment in the country comes directly from the agricultural sector. You can now begin to understand how important a monsoon season with enough rainfall is to the whole subcontinent.
Water storage and better forms of irrigation has made the problem less over the short term, but is still not enough to shield the industry from a dry monsoon season.
You now should have a complete understanding and thorough knowledge of all the major weather systems that often gets confused and mixed up. You can clearly see how some of them are almost identical in many ways, while other differ substantially.
This was clearly illustrated by looking at where these weather systems occur, how they develop and the ways in which they impact their surroundings.
Hopefully this will answer all your question and remove any confusion you may have had.
Feel free to leave me any comments, questions or suggestions, and I will get back to you as soon as possible.
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Until next time, keep your eye on the weather!