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Understanding Cyclones

Today I had a lengthy discussion with one of my engineering batchmates about how deforestation has increased the impact Cyclone Phailin would have on Odisha and other part of India on the eastern coast. It made me realise just how we never cared to understand and/or remember the Geography lessons imparted to us in probably class 5 or 6 (about pressure areas and the Coriolis Effect) and nor do we take the effort to research and understand the circumstances instead adopting propaganda driven opinions from media and social vehicles like FB and Twitter. It also gave me the idea for this blog post where I will try to put together at kind of Cyclone 101 for people to understand. Please note I am NOT a meteorologist or even a weather expert. I am just an ordinary guy who cares to form his own opinions rather than blindly adopt somebody else’s. I encourage you all to read and research more and definitely comment on the post and correct me if I am factually wrong in any way.

So, to understand the impact of cyclones, we first need to understand how and why cyclones form.
 
How cyclones form?
Cyclones are closed circular areas of rapidly rotating air. There are many types of cyclones but the ones we experience in India (every year, including Cyclone Phailin this year) are called tropical cyclones because they characteristically develop in the tropical regions. These storms are characterised by an extremely calm, low pressure area called the “eye” of the storm surrounded by a strong vortex around the eye. The center of tropical cyclones is warmer than the surroundings and thus they are characterised as warm core systems.
Tropical storms and cyclones develop and strengthen in the atmosphere by a process known as tropical cyclogenesis. Tropical cyclogenesis has six main requirements – sufficiently warm sea surface temperature, atmospheric instability, high humidity in the lower to middle atmosphere, enough Coriolis force (more on this later), a preexisting low level disturbance and low vertical wind shear. Let’s look at each of these factors and their effects separately.
The water in the tropics is understandably warm especially on the surface. The almost directly incident rays of the sun all year round ensure this. We would do well to remember that water loses heat less rapidly, especially when compared to land. What happens due to this is that the air over sea is relatively warmer compared to the air over land. In fact this effect is responsible for all kinds of natural wind on the planet. Since pressure is inversely proportional to temperature in fluids, the warmer air is at a lower pressure compared to the colder air. This causes air to flow from land towards the sea creating some atmospheric instability while at the same time causing a localised low pressure area over the sea. This also contributes to form the area of low level disturbance, one of the key factors to tropical cyclogenesis.
The second requirement of high humidity is easily fulfilled during the monsoon seasons due to precipitation that compounds the effect of surface evaporation from the sea that occurs all year round. The humidity not only increases the density of the air but also makes it more conductive. Humidity fosters thunderstorms in the area contribute to general atmospheric instability. Additionally humidity also encourages convectional heat transfer from sea to the air thus helping build and sustain the warm core.
Coriolis force (or Coriolis Effect) is something most of us must remember having learned about in our junior years at school and then forgotten completely about once the final term Geography paper was over. The Coriolis Force is associated only with a rotating frame of reference, in this case the rotation of the Earth itself. Coriolis Forces are inertial in nature meaning that they are caused due to relative motion between the earth and the object in question (air in this case). A minimum distance of 500km from the equator is normally needed for tropical cyclogenesis. The Coriolis forces cause the air travelling from the high pressure towards the low pressure area to rotate and form the vortex that is required to start a cyclonic vortex. In the northern hemisphere, the vortex rotates counter clockwise while in the southern hemisphere it is clockwise. In a mature storm, the Coriolis force allows the vortex to achieve wind gradient balance i.e. the difference in wind speeds as we move farther from the vortex. If the difference is too low, the vortex will dissipate and if the difference is too high, it would be torn apart by the wind forces. Additionally, the wind gradient balance also allows heat to concentrate near the storm centre thus maintaining and intensifying the vortex, all other required development factors remaining neutral.
Low vertical wind shears are basically the relative speeds of wind between the surface of the cyclone and the atmosphere. Stronger wind shears can tear apart the storm or even displace the warm core from the surface circulation thus cooling and drying it out and diffusing the cyclone.
All the above factors are necessary for cyclonic development but their presence does not mean that a cyclone would necessarily develop.
Why do cyclones head towards land?
Well to be perfectly honest the cyclones don’t really move towards land to exact some kind of divine vengeance. It is more like land masses lie in the paths of storms. Storms are chiefly moved by two factors – steering winds and Coriolis Forces. The Coriolis force naturally causes a storm to blow towards the poles. In the Northern tropical latitudes, the prevalent winds are the east-to-west trade winds that steer the storm primarily westwards. The net effect is that the cyclone moves in a north westerly direction.
Effects of deforestation
As is evident from the above explanation, the conditions that are required for the development of a cyclone are completely independent of the amount of vegetation on land – the Earth would still rotate causing the Coriolis Effect, Tropical winds would still flow due to ocean currents, water and land would still experience differential heating and cause a localised depression. Tropical cyclones are fuelled by humidity from the ocean. So the creation of a cyclone is a very natural phenomenon governed by the laws of physics.
The other impact most often quoted by environmentalists and a whole bunch of propaganda machinery is the relative effect of the cyclone with and without forests or vegetation. Cyclone Phailin is a Grade 5 storm (the highest grade) with systemic wind speeds upto 160mph (256kph). Even a Grade 3 storm, which develops practically every year in the Bay of Bengal, can have windspeeds touching 120-150kph. At these wind speeds, any trees in the path of the storm would be torn apart without any resistance. Since the storm is continuously being fuelled by the evaporation of water from the sea surface, there’s really not much a tree (or 1.7 lakh of them) could do in the path of the storm. Most significantly, most tropical cyclones span into the middle troposphere to approximately 10-12 kms in height, way beyond the height of trees. There are a couple of misrepresentations made in presenting facts that lead people to believe that trees do deflect the impact of the storm. The first one given very often in India is the relative damage caused by the 2004 Tsunami that caused fewer casualties in forested coastlines than cleared ones in Tamil Nadu. Delving into the facts, the 2004 Indian Ocean Tsunami was caused by the Sumatra Andaman undersea earthquake that measured 9.1 on the Richter scale of magnitude. The earthquake caused the entire planet to vibrate and cause earthquakes in Alaska in addition to gigantic waves that washed over the south east asian coast lines. Now there is a basic difference between a tsunami and a cyclonic storm. As large as a tsunami wave may be, it still experiences friction from the land surface. In such a scenario, where it is the potential energy released by the earthquake that caused the waves, the friction with land reduces the waves speed till it dies down completely. Here, trees play a definite advantage since their roots hold onto the soil and prevent it from being washed away thus increasing the friction between the water and the land surface. While trees are torn by the force of the waves even in a Tsunami, few are outright uprooted. In the case of a storm with winds over speeds of 150kph, trees are uprooted clean out of the ground. The soil that provides excellent resistance to water flow is far less efficient when it comes to air flow. There is a popular misbelief that storms slow down and eventually die over land due to trees. This is incorrect. Storms usually die out after landfall. Landfall is technically the point in time when the storm’s vortex crosses the coast line. Since the source of humidity (the ocean) is unavailable over land, the storm naturally dies out. This has nothing to do with trees and forests available in the vicinity. It is also worth noting that in the 2004 tsunami, coastal buildings and installations were just as effective at breaking the waves as were trees on the coast.
The purpose of this post is not to denigrate the importance of forestation but to set a perspective to it. Deforestation does not cause cyclonic storms nor does it have any effect on the damage caused by the storm. Historically, the number of casualties has increased in similar storm incidents. Paired with the year on year deforestation figures, this would lead anyone to believe that there is a correlation between the two. However the correlation is deeper. The increased number of casualties and the increase in deforestation, both, have the same underlying cause – over population. Deforestation is not the cause for cyclone related casualties – it is another result of over population like the former. Rather than expending our time, money, energy and resources to tackle the surface issue of deforestation, it may be more prudent to tackle the deep underlying issue of over population. More people die every progressive year due to thunder storms because the population density is increasing. Where 10 years ago there were only 50 people in a coastal village, today there are 50,000 in a coastal town. In my opinion, this makes it quite obvious why casualties are on the rise.
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