How Hurricanes and Tsunamis Form
How Hurricanes Form
Scientists have a working explanation for how hurricanes form – a theory. They have not yet been able to prove their theory by gathering the necessary data but what follows is pretty much generally accepted.
We’ll focus on hurricanes in the Atlantic. The same mechanisms work to create hurricanes (also called typhoons and cyclones) in other parts of the world.
Most Atlantic hurricanes start as thunderstorms. Like all thunderstorms in the northern hemisphere, these are rotating counterclockwise. They migrate from the northwest coast of Africa toward the west (toward Central and North America) pushed by the equatorial trade winds.
When they reach warm water and winds blowing toward the east (counter to the direction of the trade winds blowing toward the west), some of the thunderstorms intensify into tropical depressions then tropical storms, then hurricanes.
As a way to explain what happens, let’s focus on the journey of a cube of air. Let’s say it is a cubic foot of very warm and very humid but not very dense air on the surface of the ocean. Because less dense warm air rises like the hot smoke from a campfire, our subject cubic foot of air spirals upward boosted by the convergence of the winds from the west and the trade winds from the east. The spiral has a rotational component as part of the counterclockwise rotation of the thunderstorm and a vertical component due to the tendency of the warm air to rise. As the cubic foot of air gets higher, the cold air aloft cools it and causes the humidity to condense into water droplets, which aggregate into raindrops and fall back into the sea. Once the cubic foot of air is at high altitude and is as dry and as cool and as dense as the surrounding air, it no longer has a tendency to rise on its own.
But, it is being pushed along by other similar cubic feet of air behind it. It can’t go higher so where can it go? Well, some of the air moves outward away from the eye as broadening bands of rain clouds but some of the air piles up near the center of the storm and as it piles up, it forms a pressure gradient (a difference in pressure) between the air aloft and the air below. The pressure gradient is especially great between the air aloft and the air in the eye of the storm, which has very low pressure. (It fact, the pressure in the eye of a hurricane is the lowest recorded air pressure on earth. The eye pressure of hurricane Wilma in October of 2005 was a new record low pressure.)
So, our high-pressure cubic foot of air seeks low pressure and dives into the eye of the storm. It spirals rapidly down into the eye until it reaches the surface. When it gets there, it needs to find another place to go because it is still being pushed along by the millions upon millions of cubic feet of air behind it. As it swirls around and away from the eye, it again picks up heat and humidity and starts the journey once more. The more downward-spiraling air that needs to move away from the eye, the faster the winds must be to carry it away and the stronger the hurricane becomes.
There is no denying that the very forceful winds of a hurricane cause damage and death. But, by far the greatest cause of death from hurricanes is the enormous amount of water involved. Hurricanes produce very heavy rain. Sometimes it is so heavy that as it falls on nearly level land, it arrives more quickly than it can drain away. However, the even greater cause of death is the storm surge pushed by the hurricane winds acting on the surface of the water. Such winds can cause a surge of water as much as 15 feet or more above the normal sea level – an almost unimaginably large amount of fast-moving water. And, if the storm surge arrives on the shore at the same time as a high tide, the result can be especially destructive.
Helping people recover from the devastating effect of these storm surges is a major goal of Wavehelp.
How Tsunamis Form
First, let’s get past a major misunderstanding about water waves of all kinds. Waves are not the movement of water. They are the movement of energy through water. This is hard to convey because nearly everyone has been pushed by a wave at the beach or at a water park and that push seemed to be delivered by moving water. But, the water actually moved a very short distance. It is not like the flow of a river. The push we felt was delivered by the energy IN the water as it bumped into us. Recall how the push was quite short-lived. The wave pushed us briefly and then moved on. So it was the flow of ENERGY we felt rather than the flow of WATER.
It is important to make that point because what distinguishes tsunamis from ordinary waves is the vast difference in the amount of energy that is carried. Ordinary waves are created by the constant force of wind friction along the surface of the water. The friction pulls energy out of the wind and into the water thus creating waves. The longer the wind blows and the faster it blows, the more energy goes into the waves. Each wind-generated wave has a small amount of energy in it when compared to a tsunami. In contrast, a tsunami gets its energy by way of an intense direct impulse of force.
The devastating tsunami of December 26, 2004 got its energy from an immensely powerful earthquake. One plate of the earth’s surface was trying to slide under another off the coast of Indonesia. The movement was resisted over the years as the pressure to slide built up and up. Finally, the plates suddenly slipped and a huge amount of energy was delivered by the earth directly to the sea.
You can envision this by watching closely as you snap your fingers. The fingerprints on your thumb and middle finger resist the sliding of the two digits. Then, when there is enough force applied, your middle finger slips off and slams into the base of your thumb creating the characteristic “snap”. It is an oversimplification, but the force delivered to the base of your thumb in a finger snap is like the sudden impulsive force delivered into the water by the “snap” of the earth plates’ movement in the earthquake.
You may remember hearing that the tsunami of December 2004 moved from its epicenter outward at hundreds of miles per hour. Obviously, it was not the water moving at that speed but rather the energy moving through the water at that speed.
Most tsunamis are caused by earthquakes. However, another way that land can deliver an intense direct impulse of force and cause a tsunami is through a landslide. Of course, it would take a huge landslide – far beyond most persons’ experience. But, imagine half a huge mountain sliding suddenly into the sea. Since the land can’t keep on moving, its energy has to be dissipated somehow and that somehow is into the sea. It displaces a lot of water just as the “snap” of the earthquake does. Likewise, a huge underwater volcanic eruption can deliver the kind of sudden release of energy into the sea that can produce a tsunami.
The passage of a tsunami’s energy through the sea results in a very small surface mound. A ship riding in deep water might only rise 3 feet as the tsunami passed by. The devastation caused by tsunamis occurs when the energy slams into a land mass. When a tsunami cannot transmit its energy through land as it has been doing through the water, it must dissipate the energy somehow. The way it does that is to use its incredible power to raise water high above its normal position at the shore. Water weighs about 8 pounds per gallon. A common bucket of water weighs about 40 pounds. When you recall how much effort you must expend to raise that bucket above shoulder height, you can begin to understand how much force the tsunami exerts to raise millions and millions of buckets-worth twenty feet above sea level.
When that water bursts onto the shore, the resulting inundation swallows everything and everyone in its path. The death and destruction caused by tsunamis is the result of colossal amounts of water engulfing the land and its people.
Helping people recover from the losses caused by the water of a tsunami is a major goal of Wavehelp.
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