Cape Verde: Where Hurricanes are Born
For hurricanes which affect the Caribbean, North America, and South America, the biggest impact is from those which originate as weather disturbances in the area of the Cape Verde Islands off the west coast of Africa, know as the Cape Verde hurricanes. On the whole, 85% of all hurricanes affecting the Americas come from Africa.
Hurricanes can spawn within the Caribbean but fortunately are generally less powerful because they don’t have as much time over warm water to develop the huge amounts of energy of their African cousins.
Between Africa and South America, just north of our planet’s equator (yellow), is a strip of comparatively warm ocean water (orange). Sitting at the Eastern end of that phenomena are the Cape Verde Islands.
These islands experience developing weather from June to November, but especially July through October, with up to 20 days per month of rainfall in September. This happens because the Trade Winds are moving westwards and this is where they encounter that very warm Atlantic Ocean water for the first time.
How a hurricane grows
People talk about how warm water and winds combine to form a hurricane, but many don’t truly understand the mechanism. So let’s look at how it works.
First, you need an existing disturbance in the atmosphere, such as a thunderstorm which are extraordinarily common over Western Africa at this time of year. With just a few more contributing factors it can evolve into a full-fledged tropical depression, and possibly a hurricane.
The main requirement is that ocean water must be at least 78° F (26.5° C) down to a depth of 150 feet (50 meters), scientists estimate. This provides a massive heat sink from which the storm will draw its energy in the form of gaseous water.
Next, it needs to be more than 5° of latitude above the equator. Our planet rotates towards the east, which is why it appears that the Sun (and Moon, and stars) rise in the eastern sky. Right on the Equator there is no effective spin imparted to the air. A weather system must be a minimum of 5° away in order to gain rotation from the turning Earth.
To understand this, it helps to know that the Equator is traveling at about 1,000 miles per hour as the Earth spins. At 45° N latitude (which passes through OR, ID, MT, MN, SD, WI, MI, Ontario, Quebec, ME, NY, VT, NH, and Nova Scotia) the speed is half of that or 500 mph. Someone at either pole is effectively traveling a 0 mph. This “twisting” is known as the Coriolis Effect and without it hurricanes wouldn’t be possible.
It might help to imagine a lump of clay on a surface. If you were to slide your hand along the upper surface, it would begin to roll. Your hand and the table are moving at different “speeds” so the clay adapts by spinning.
Next it requires very low wind-shear in the atmosphere. If you have ever looked at a cloud and observed that it looks as if it were pushed over like a tower of blocks getting ready to fall that is likely due to wind shear. The air above is traveling faster than the air below, and likely in a different direction, so the cloud is “torn apart. You can see this in action with this video.
The next component is plenty of moisture being moved up into the atmosphere. This is the fuel for the hurricane. It results in unstable conditions, the consequence of which is thunderstorms.
Putting it all together
It doesn’t happen often that all the conditions are “just right”. If it did, the results would be continuous hurricanes across our planet.
But, when it does happen it works like this:
- The Trade Winds leave the west coast of Africa, around the Cape Verde Islands at 12° N latitude, and encounter a warm ocean that is throwing untold tons of water up into the atmosphere through evaporation.
- Down at ocean-level warm water continues to evaporate and rises, forming Nimbus (storm) clouds. As those clouds form, the gaseous water condenses to liquid releasing its heat, and that adds fuel to the fire. The hot air rises, and more air is drawn in from below to replace it, adding more water, releasing more heat and so on, feeding the giant engine of a hurricane.
- The various layers of the atmosphere become harmonious, traveling in roughly the same direction at approximately the same speed. This means that the thunderstorms can dwell in the center of circulation and add more energy. If the wind shear is too great, the thunderstorms dissipate and the hurricane turns back into an ordinary tropical storm.
- The Coriolis Effect then starts to amplify the rotation of the air mass, making the southerly portion turn faster, and the northerly portion speeds up in response because the Prevailing Westerlies (blue on the map) are not as fast or strong as the Trade Winds.[ CITATION Sta17 \l 1033 ]
Since the storm rotates counterclockwise and encounters the eastbound Westerlies along its northern edge, what may be a Category 1 or 2 hurricane overall becomes a Category 4 along that upper border.
For ease of reference, there is a scale to describe hurricanes. It is called the Saffir-Simpson Scale and references not just the wind speed, but also the likely storm surge. A Category 1 will have winds under 100 mph, which may be reassuring, but if you live near the shore in an area only a couple of feet above the local water level, a surge of 5 feet could cause a lot of damage. If you’re 10 feet above the local water level, a Cat 3 might hit 12 feet during the surge. You had better get those sandbags out, and make sure your pump is working!
What kills a hurricane?
Hurricanes generally fizzle out before they do the level of damage that we’ve seen just recently. As mentioned, wind shear drives the thunderstorm “engines” away from the center of rotation, which is similar to taking the batteries out of a device—it might continue running for a while—but it’s going to get slower and slower until it stops.
Another hurricane killer is dry air. If the Jet Stream dips down and starts feeding it cool dry air it can cause so much turbulence that the whole thing just disintegrates into several smaller, harmless storms. Dry air siphons off the water “fuel” and it fades away.
Landfall is also fatal to hurricanes, for as we’ve seen, they are dependent on incoming water to survive. Take that away and they cannot maintain their thunderstorms and simply fail.
Finally, the ultimate death knell for a hurricane, even if it stays over open water, is the cold North Atlantic water. It simply can’t draw enough water and energy from the frigid waters and dies an ignominious death.
History of Destruction
At the time of writing, hurricane Maria is a Cat 1 hurricane, off the east coast of the United States and headed northeast which is ideal. Meanwhile, Cat 3 hurricane Lee is just a bit southeast of Bermuda and headed northwest, and current predictions have it turning north on Thursday, northeast on Friday, and dissipating by Saturday morning in the mid-north Atlantic. Good news.
This has been an expensive hurricane season in 2017 in terms of lives lost, property and infrastructure damaged, and likely a prolonged recovery time. The island of Barbuda is completely devoid of residents for the first time in 300 years after Irma struck it with all its fury. The island went from beautiful green to completely brown in a matter of hours. Most of the residents are now living on their sister island Antigua.
Barbuda has a GDP of about one billion dollars, but based largely on tourism, and damages exceeding $250,000,000. That is a much tougher problem to solve since they are going to have no tourists for quite a while…
Katrina devastated Louisiana in 2005, and political wrangling left the area struggling for years trying to rebuild. It’s only now more than a decade later that they are getting back to pre-Katrina levels, though areas like the Lower 9th Ward are still struggling.
Texas, and notably Houston, has suffered a massive impact from hurricane Harvey, but have already acquired $15 billion from FEMA to start the rebuilding. Texas Sen. Ted Cruz said it was just a down payment on what was to come.
FEMA approved $124 million for individuals and Households for Florida on September 10 in response to the damage caused by two hurricanes Harvey, and Irma. FEMA has more to spend but numbers aren’t readily available.
Puerto Rico suffered significant damage from hurricane Maria as well, even if not to the same extent, but has the advantage of a massive wealthy government to help them with financial support, and rebuilding infrastructure. The island is largely without electricity, and the people are working to get things back in operation. It will take months to restore essential services, but they have a much higher expectation that help will be forthcoming once the political rhetoric is done compared to places like Barbuda.
With this barrage of devastating weather, some of the more outrageous news outlets have been shamelessly blaming Global Warming for all these woes. While it is true that it may have exacerbated it slightly, in truth, Harvey was statistically indistinguishable from of number of earlier hurricanes. These reach back to Hurricane Easy in 1950 with 45 inches of rain, Tropical Storm Claudette in 1979 with 42 inches of rain in just 24 hours (whereas hurricane Harvey took three days to reach that amount), and even Tropical Storm Amelia in 1978 with its 48 inches of rain.
We’re big fans of reality-based, replicable science around here, and feel free to protest when it is used for the sake of making headlines instead of reflecting the world we live in. This article by Eric Holthaus is a case in point. Harvey is not unique; it is not a once in “500 years” storm (additional hyperbole suggests that it is a “once in a millennium” storm). We’ve had four in this particular area since 1950 (Harvey plus the three mentioned above).
When people seem too smug, and adopt an “I told you so” attitude, you should probably take what they say with a grain of salt. Maybe you could even do a little research on your own. The facts, however speak for themselves. No weather event can be attributed to Global Warming specifically. It may alter it in some way, but there are always more direct local influences on such a complex system.
In science, few people speak of certainties, and you should be suspicious if they do. We speak of probabilities while looking for faults in our examination process. Real scientists don’t mind being wrong because that is how we learn new things.
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