w/contributions by Steve Gregory
From the rarefied heights of the Jet Stream to the frigid arctic depths of the Atlantic Conveyor, great draughts of air and rivers of water invisible to the eye course sluggishly across the world. These are planetary heat distributions systems. Without them most of the earth would be unhabitable for creatures such as ourselves. The equator would roast at 150 degrees or more while the temperature near the poles would plunge to over one-hundred below.
One tributary in this grand system of wind and sea surges off the western coast of Africa and streams all the way to the Gulf of Mexico. The gentle breeze and current was reportedly first used by Christopher Columbus in 1492. It spans the breadth of the Atlantic Ocean. Without this expeditious route the development of the western hemisphere would have been delayed, perhaps by centuries. But the Trade Winds carry more than ships and heat to the coast of America.
Working over a hot stove you can feel it. Open an oven standing above and it will burn you. Look across a parking lot of asphalt on a sunny day and you can
see the effects: Warm air rises, hot air even more so. This simple thermal observation is a huge part of what drives our weather patterns and ocean currents; the planetary heat engine. Alter those dynamics and it might spell global suicide.
Each summer periodic gales called tropical waves (Or Low pressure 'TROFS') race across the Sahara Desert in Africa and pour over the Atlantic ocean en masse next to the jutting western shoulder of the Dark Continent. By late summer and early fall, the direct heating of the oceans by the warm winds and intense summer sun creates tepid sea water in enormous quantities floating high in the tropical eastern Atlantic. That warm water moves west across the ocean fanned by the trade wind. As the tropical waves of low pressure move westward over the warm water, the warm air starts to rise up off that water just like it rises off a hot parking lot; and that warm air is loaded with moisture that has evaporated from the warm ocean surface. As the humid warm air rises like smoke into the cooler surrounding air to higher levels, water droplets condense and form clouds which give rise to showers and thunderstorms that routinely dot the tropical regions of the world. Meanwhile, air rushes in from all sides to fill the vacuum the rising air created when it rose up to form the showers. That new air then also heats up on contact with the ocean water and it rises chock full of water vapor, and the cycle keeps going. This process is called convection and the rain storm it produces, under the right conditions, is called a Tropical Depression or TD.
The anatomy of a Tropical Depression
Tropical for obvious reasons and Depression because the pressure in the center is lowered; that's why the air came rushing in to start with. Atlantic TD's thus usually form around 300 to 600 miles away from the equator itself and drift westward within the trade winds that dominate the area year round. If they stay too close the equator, they end up as nothing but a large cluster of rain storms a few hundred miles wide and generally elliptical in shape .
Steve Gregory, one of the meteorologists at the Weather Undergound who was critically instrumental in warning us about Katrina last weekend, adds:
These clusters of showers and thunderstorms which never stray far form the equator end up becoming part of the ITCZ or Inter-Tropical Convergence Zone. The ITCZ, shown below, is a semi-permanent area of unsettled weather that rings the entire globe near the equator.
The ITCZ (Enlarge)
This zone is almost always present because that's where air from the Northern Hemisphere collides with wind heading north across the equator from the Southern Hemisphere. As those two masses of air collide they're forced up and thus create areas of showers and thunderstorms just like a tropical depression.
But the earth rotates once a day. And our planet is roughly 24,000 miles in girth at the equator (Circumference). That means that objects on the equator are humming along at 1,000 MPH compared to an object on one of the poles! And as you or anything else travels to the North pole, this velocity decreases because it's not as many miles round that smaller and smaller and smaller circle of latitude you're crossing.
Roughly half-way, the surface is moving at around 500 MPH. So ... if you fired a cannon ball due north from the equator and it landed halfway to the North pole ... it would arrive traveling sideways when it hit the surface, at the difference between the two velocities, which in this case is 500 MPH! This is represented by the arrow in figure 1 (Below) pointing up to the top of the earth and curving to the right or east.
Figure 1 Figure 2
Likewise, if you were nearer the pole and fired a cannon ball thousands of miles down towards the equator, it would be traveling slower relative to the sideways rotation of the earth and thus veer to the west; the arrow pointing down to the equator from the pole in figure 1, above.
Zooming in on the vortex, in figure 2 we can see more clearly what this Coriolis Effect does to a TD that strays north away from the equator. That whole system begins to rotate counterclockwise (If it went south of the equator it would rotate clockwise because the dynamics are reversed). The effect on storm systems only a few hundred miles wide is not huge. Maybe a difference of a few tens of miles per hour between the north and south inflows; but there's nothing to stop it from building as long as it stays above the equator and over warm water. The storm system will keep spinning and the effect will keep building -- the hotter the water the greater the draw of air -- until the whole TD moves over cooler water or over land.
As this cloudy mass rotates a center is established. The wind rushing in from all sides now spirals into the center to fill the area of low pressure created by convection. A center of rotation forms, the forerunner of the familiar eye ... By the time a sustained wind is moving at 39 MPH near the center of rotation, it graduates to the classification of a Tropical Storm or TS.
Tropical Storm Tanya over the mid-Atlantic 10/27/95
If the TS stays away from the equator and it doesn't run out of warm water, doesn't hit land, and remains embedded within the constant and steady speeds of the trade wind the rate of rotation will often grow. When the winds near the eye are traveling at 74 MPH, we call the storm a hurricane. And thar she blows!
The anatomy of a hurricane
Steve adds:
And in reality, a hurricane is basically an intense area of organized thunderstorms spinning around the eye
Now just a couple of more terms to review: The combination of the ever decreasing low pressure in the center of the Tropical Depression that started it off, and the very strong wind rushing/spiraling into the center to replace the air that's rising, create a gigantic, flattened dome of water on the surface of the ocean with the center of the dome located under the eye.
Pressure and wind: The two components of the deadly Storm surge
Steve adds:
Even though the hurricane [Katrina] wind speeds had decreased from nearly 200mph to only 140mph, the extremely low pressure readings in the eye stayed very low, and so the storm surge was still the biggest to ever hit the United states; 25 feet. (Camille, that had 200mph winds at landfall, only produced a 22 foot storm surge)
The wind spiraling in from the South dumps a little bit more energy into the cyclical pattern than it's counterpart spiraling in from the North. And that's why the eastern side of the hurricane has the highest winds. That higher energy part of the storm gets 'wound around' just a bit as it come barreling in out of the South and spiraling into the center and thus the highest velocities are usually found in the northeastern quadrant of the system. If you look at the storm track of Katrina, it was the area around Biloxi that got slammed with that portion of the storm and that's why the destruction from wind and storm surge was so pronounced there. Had that portion of the storm hit New Orleans, it's debatable if there would have been much of anything left standing outside of the most robust, hardened, structures.
During the evolution from a TD to a hurricane the entire storm system is being pushed west across the Atlantic Ocean towards the Gulf of Mexico. Pretty much on the same course that Columbus took and for the same reason: The Trade Winds.
Because this whole shebang is driven by heat, it stands to reason that if the earth was warmer we might have more hurricanes like Katrina and more often. The relationship between greenhouses gases, heat retention, and the formation of hurricanes is way above my pay grade. But to give you an idea of how complicated it is, it's possible for a relatively small change, a degree or two, to result in a hell of lot more storms than we currently experience. That's only one reason among many why measuring the rate of global warming or cooling over various tracts of time is so important to climatologists. Global warming is increased by the effects of greenhouse gases like carbon dioxide or methane released in large quantities into the atmosphere. These gases are emitted by cars, power plants, agriculture, and all kinds of industrial manufacturing processes. In most western nations these emissions are regulated by International Treaty and domestic law; which is why it's so important to all kinds of large corporate lobbyists and right-wing media shills to persuade lawmakers and citizens that global warming either doesn't exist, or doesn't affect the weather. Next time I'll have a short piece about how global warming can trigger an ice age in just a few years as an example of how far out of whack this ideological preference is with the science of paleoclimatology.
Speaking of which and as a foretaste, one of the mysteries science has revealed is that our planet's oceans and atmosphere are gigantic conveyer belts powered by the sun. Meddle with those heat dynamos and the consequences can be beyond catastrophic and come home to roost much quicker than we had previously imagined. That's another field the current anti-science clowns who are running this nation would like to ignore. Our 'leaders' the Hollow Men: Those who speak so smugly about peace and safety, while so inevitably handing down war and disaster.
Through science we are now just beginning to understand that we live in a breathtakingly beautiful, yet violent cosmos, in a menacing galaxy, on a fitful planet, embedded in a delicate and treacherous web of interconnecting biomes. Global climate is about as complex a subdiscipline of that overarching framework as you will ever encounter. And right now we don't know enough about it to determine with precision if we're tipping ourselves into the Inferno of a new Permian Extinction or inviting another dramatic appearance of the Snowball Earth. That's something we need to figure out!
It's true events in the past have been colossal by today's standards including hurricanes and ice ages. At the end of the Cryogenic Period almost 700 million years ago, the earth was released from a frozen state into one of the warmest in the pace of a century or two. The ensuing storms would have dwarfed today's hurricanes and typhoons.
But dammit, I don't live in the past! We're not trilobites! This is our time and our fate at stake and for the first time in the history of life on earth it's within grasp of our technology. And while we currently lack the ability to intervene in mother nature's immediate global temperament, we handily possess the technology to foresee most of her good moods and bad on every scale. Then, using the most generous sibling of science, engineering, we can undertake preventative measures in advance to minimize the damage of her tantrums, warn those in danger, and repair the damage we can in both treasure and lives; all with an efficacy that would have appeared positively miraculous to our ancestors only a few short generations ago.
But only if we produce young men and women skilled in the invective of science will we be able to decipher the often subtle missives nature issues in time. Only if we heed the data and realistically accept it does that skill benefit mankind. We cannot allow Hollow Men to choose what is 'real' and what not based on narrow ideological inclinations which coincidentally happen to align with the interests of their bloated investment portfolios: Nature makes that choice for us. And for the gravest of reasons--some of which should be crystal clear this week--we as a nation and as a species dismiss science, analysis, and engineering, only at great peril to ourselves and our future.
We are the hollow men. We are the stuffed men
Leaning together, Headpiece filled with straw. Alas!
Our dried voices, when, We whisper together
Are quiet and meaningless, As wind in dry grass
Or rats' feet over broken glass, In our dry cellar
Shape without form, shade without colour,
Paralysed force, gesture without motion;
Those who have crossed, With direct eyes, to death's other Kingdom
Remember us -- if at all -- not as lost, Violent souls, but only
As the hollow men, The stuffed men ...
In this last of meeting places, We grope together
And avoid speech, Gathered on this beach of the tumid river ...
. This is the way the world ends
Not with a bang but a whimper
Excerpted from T. S. Eliot's, "The Hollow Men"