Hurricane Isaac--What's the "reason" for hurricanes?

Cover for my book
Due out February 18, 2013
Norton Press

A dear friend of mine who is facing her first big hurricane in New Orleans said that she heard a meteorologist on the Weather Channel comment today that there was a "reason" for hurricanes:

I found it interesting, though, that [the meteorologist] said a “reason” as if either 1) God had  a plan for earth and hurricanes are a part of it or 2) hurricanes were living beings with an innate reason for doing something, like fish swimming upstream or something.

Is there a “reason” for hurricanes?  Do scientists consider “reason” the appropriate word?  Or is the better word “consequence” or “result”? 

Hurricane Isaac is eerily following the same path as Hurricane Katrina and the same time-line--seven years later. Due to make landfall in Louisana later today, Isaac is likely to drop 15-18" of rain just off the eastern toe of Louisanna and southern toe of Mississippi. 

Predicted rainfall over 5 days for Isaac. Prediction is for 5-day period
beginning 8:00 a.m. EDT August 28 through 8:00 a.m. EDT Sept. 2.
NOAA/NWS/NCEP/HPC
The maximum rainfall is in the small lime-green circle just
off the eastern coastline of Louisana and southern coastline of
Mississippi.

What is the "reason" for hurricanes? Leaving aside the tendency of meteorologists (and all of us) to  anthropomorphize hurricanes (after all, why give them names when numbers would do just as well or better?), there is a scientific "reason" for hurricanes--it is part of the process that results in the most efficient way that the earth has of redistributing heat away from the equator toward the poles.

Energy pours into the earth from our sun, but it does not pour in equally over the surface of the earth. Much more energy accumulates at the equator than at the poles. The atmosphere and oceans work as a coupled system to transport the energy from the hotter regions toward the colder regions. In a (very) simple world, there would be a single convection cell doing this transport: Warm air would rise at the equator and flow toward the poles, cooling along the way. Near the poles it would sink and flow back along the surface of the earth toward the equator, warming along its path. This model, first proposed by George Hadley, in the 18th century, consists of a single giant convection cell. Unfortunately, it doesn't explain the way that the atmosphere works.

A hundred years after Hadley proposed this model, Gaspard-Gustave de Coriolis and William Ferrel came up with models that explained the major wind directions on the surface of the earth. Coriolis pointed out the effect of the rotation of the earth on wind direction, and Ferrel proposed, correctly, that there were three, not one, convection cells, each spanning roughly 30 degrees of latitude. The three cells are called, in order of distance away from the equator, the Hadley, Ferrel, and polar cells. 

The temperature gradients across the Hadley and polar cells are fairly small: a traveler could traverse a Hadley cell in the northern hemisphere from south to north and the weather would change only from tropical to subtropical conditions. Across a polar cell, it would change only from polar to subpolar climates. But, a tourist traveling through the northern hemisphere Ferrel cell would go from the humid warmth of New Orleans almost up to the freezing conditions at the Arctic Circle.

The very large temperature gradients in the Ferrel cells, combined with the complicated role of the continental land masses, causes enormous turbulence in the atmosphere. Turbulence is, loosely, a very complicated flow regime in which individual parcels of fluid mix on many scales, from the tiniest molecular scale to the largest scale that has the dimensions typical of the whole atmosphere. The individual "parcels" of fluid are "eddies," and the largest eddies carry most of the kinetic energy of the motion. 

You can envision this process by observing a pot of water come to a boil on your stove. Initially, when you turn on the heat, the water is at a nearly uniform temperature--whatever the temperature was when it came out of the faucet. As heat pours into the bottom of the water, the temperature gradient across it increases. Initially, the heat is transported by conduction--molecule-by-molecule collisions of water. Then, convection (transport by physical motion) starts. Initially the convection is "gentle" (the technical word is "laminar"), but when the temperature gradient becomes too large, the convection becomes "turbulent." The water is roils in the pot as hot and cold parcels mix and merge to transport the heat as efficiently as possible.

So, too, in the atmosphere, turbulent mixing is the process that makes heat transport around the planet as efficient as possible. The fluid parcels that mix range in scale from huge (the dimensions of the atmosphere, tens, hundreds, even thousands of kilometers) down to "tiny" (dust devils or tornadoes). Hence, the "reason" for hurricanes is they play a major role in transporting heat around the planet. The words "consequence" and "result" are, as my friend said, much better than "reason": hurricanes are the inevitable consequence of the uneven distribution of solar heating of the earth, and the laws of thermodynamics that describe how this unevenness is smoothed out by heat transport.

These concepts are explained in more detail in my book "The Dynamic of Disasters," due out on February 18, 2013, by Norton Press. Here's a description on Amazon!

What's going on in the Himalayas? Are the glaciers receding or advancing?

Image showing the extent of the Himalayas from
the west near the dry steppes of Afghanistan to the
wet subtropics of Bhutan in the east
Photo from an article by Devin Powell in Science News,
vol. 182 (#4), August 25, 2012 edition obtained from Google.

Glaciers may only move slowly, but they are "geology in motion," and obey conservation of mass principles that are fairly easy to understand. If more moisture is added to a glacier by precipitation, or in some areas, by avalanches, than is lost by melting and sublimation, the glaciers grow, and vice versa.

There has been much speculation over the past decade that the melting of the glaciers in the Himalayas might cause the rivers that nurture a huge land mass to decline, causing water supply stress for 200,000,000 people in India and China. Recent work has led to the realization that such generalizations cannot, and should not, be made because of the Himalayas are so huge extent and the climate and weather vary considerably along their length.

There is little controversy over the fact that the climate is warming in the Himalayas, and that some glaciers are suffering, particularly in the east.  On the other hand, some western glaciers in the Karakoram are growing. The Karakoram is part of the greater Himalaya (which includes the Hindu Kush and other ranges), north of the actual Himalaya Range. The Karakoram includes K2, the second highest peak in the world, at least 20 other peaks over 7,000 meters, and more than 100 peaks over 6100 meters (20,000 feet).

In 2005, Kenneth Hewitt documented evidence that many glaciers in the central Karakoram region were expanding.** These glaciers were "almost exclusively" in basins at the highest elevations. They had been diminishing in size through the mid-1990's and suddenly started expanding, which is a puzzle not yet solved. The flows in the Indus and Yarkand rivers in the western Himalayas have been declining in spite of the glacier expansion, indicating that the extra ice is being stored in the glaciers rather than providing water to the rivers.

Glaciers in the Karakoram generally start in steep tributaries typically between 4500 and 5500 meters, where they consist of icefalls and avalanches that coalesce to form the main ice mass in the drainages. The ice is fairly thin and highly cravassed. Conversion from snow to glacier ice occurs on the time-scale of years or decades rather than centuries as occurs in other places around the world. Much of the movement occurs by collapse of unstable sections rather than by gradual slip along the base of the glacier. Because of the fragmented nature of the ice, more of it is exposed to ambient atmospheric conditions and to meltwater than glaciers which are less heavily fractured.

In the Karakoram, there are three distinct weather systems. Two thirds of snow accumulation on glaciers here occurs in the winter when a westerly circulation and cyclonic storms provide moisture. The remaining 1/3 accumulation occurs during summer snowfall.  Solar radiation during the two other weather systems when clear weather occurs accounts for most of the glacier ablation. Since the early 1960's winter precipitation has increased and summer mean and minimum temperatures have declined.

Hewitt strongly warns that the expansion of the Karakoram glaciers does not refute the case for climate change or for atmospheric warming. Rather, warming is the only way to explain these changes with the warming causing the transportation of moisture to higher altitudes than before, thus explaining the growth of the high-altitude glaciers.

**Hewitt, Kenneth. "The Karakoram Anomaly? Glacier expansion and the 'elevation effect,' Karakoram, Himalay, Mountain Research and Development, November 2005, 332-340. This paper gives an excellent general discussion of the dynamics of glaciers.

Why don't mosquitos get killed by raindrops?

Mosquitos ride raindrops instead of resisting them
Photo by Tim Nowack, Andrew Dickerson and David Hu
featured in ScienceNews article by Susan Milius on
July 14th, 2012.

Sometimes I wonder why I never thought of such an obvious question as the one in this title!  If we humans are hit by something that weighs a few hundred or a thousand pounds, we don't survive. A raindrop can weigh 2-50 times as much as a mosquito, and yet plenty of them survive rainstorms.

In a paper on PNAS (reference below), Dickerson et al. calculated that the impact force of a raindrop on an unyielding surface is 50,000 dynes, about 10,000 times the weight of a mosquito. Should be lethal. They also calculated that a stationary mosquito in a rainstorm should get hit, on average, every 25 seconds.

The authors put mosquitoes into a small acrylic cage. In one experiment, they simulated the terminal velocity of raindrops (6-9 m/s) by using a pump to generate a jet that broke up into drops and jetting streams. They managed to observe six mosquitoes, and found that they tumbled with the jet. The mosquitoes were accelerated to a velocity of 2.1 m/s within a duration of 1.5 milliseconds. All six mosquitoes survived the experiment and flew around after a brief rest.

Using high resolution and fast films, they used slower drops to examine the response to drops. They were able to observe the dynamics of the interaction when the impacts were on the wings or legs (N=13) and on the body (N=4). The mosquitoes rolled when hit on the legs and wings, but if the drop made a direct hit on the insects center of mass, it pushed the mosquito downward, accelerating it within downward several (5-20) body lengths. The mosquito was always able to separate itself from the drop and recover its flight. As the authors point out, it is "imperative that a mosquito does not fly too low during rain or it will suffer a secondary impact with the ground." They also did experiments on "mosquito mimics."

The key to survival is that the low mass of the mosquito causes a falling drop to maintain almost all of its speed after impact, thus resulting in a low impact force to the mosquito. The paper contains some nice and understandable physics of impact.

The work has implications not only in biology and ecology, but also for the construction of insect-sized flying robots.

Reference: Dickerson, A.K., Shankles, P.G., Madhavan, N.M., and Hu, D.L., "Mosquitoes survive raindrop collisions by virtue of their low mass." Proceedings of the National Academy of Sciences, June 4 online 2012. doi:10.1073/pnas.1205446109.

A raft of pumice in the Pacific

A part of the pumice raft
Photo from CNN.com

Driving to work this morning, I heard a headline on NPR about a floating raft of rocks out in the Pacific and, sure enough, Google worked and I found the pictures! On Thursday, pilots and sailors from the New Zealand navy discovered pumice, sometimes inches high and looking like floating ice, covering an area estimated to be more than 7,500 square miles. It appears to be an oblong area 250 x 30 nautical miles in dimension.  Volcanologists love to try to reconstruct the "equivalent magma volume" that it would take to produce this--was it a cubic mile? a fraction of a cubic mile? However, unless we know how much of the surface it covers (1%? 50%?), what it's average thickness is (a few inches?), and especially, what the density of individual pieces is, we can't make that calculation. We do know for sure that the density is less than 1 g/cm3 (the density of water), and some older studies of pumice from a Japanese caldera eruption showed that the density varied between 0.211 and 0.777, with an average of 0.51. My guess (with no measurements at all) is that the pumice in the above photo covers about 5% of the frame, and this is probably a photo selected because it has lots of pumice. Let's assume that it covers 1% of the 7500 square miles, to an average depth of 6". If I did my math right, this is a depth of 0.0001 mile, and so the volume of the pumice would be 7500*0.0001 = 0.75 cubic miles. If the average density of the pumice is about 0.5, then the erupted volume would have been a few tenths of a cubic mile.  A pretty healthy burp.

***Note added on August 11: Scientists initially (as of yesterday) thought that the eruption was from Monowai, a volcano that I posted about an eruption a year ago. However, as of today, they have determined that the raft was spotted first on July 19, whereas Monowai didn't erupt until August 3. They now believe that the eruption was from an unknown volcano about half way between New Zealand and Tonga. There were more than 157 earthquakes between magnitude 3 and 4.8 in this area between July 17 and 18. This information is from Stuff.co.nz. There is also a 36 second video from the air of the raft on this site. My guess from looking at the video is that my estimate above of an aerial coverage of 1% may be high by as much as an order of magnitude because the still photo in the upper left is definitely one of the highest density areas. So, perhaps a few hundreths of a cubic mile equivalent magma volume, with a few tenths as an upper bound.

Pumice rafts have been observed before. There has been speculation (sorry, I don't have a reference) that they could have been a platform on which life originated on earth--a platform on which a soup of organic chemicals from hydrothermal vents in the ocean could have accreted, been zapped by lightning to form the complex organic molecules leading to life. It has also been proposed that life could have migrated from island to island in the Pacific, or from continent to island, e.g., South America to the Galapagos, on such rafts.

More recently, Scot Bryan of Queensland University of Technology, has studied pumice rafts from the 2001 and 2006 eruptions in Tonga, and suggested that they could have been crucial in the formation, evolution, and future of the Great Barrier Reef off of northeast Australia. Bryan's studies showed that the initial 440 square kilometer floating mass of pumice broke into streaks on which "millions to billions" of marine organisms such as cyanobacteria, barnacles, molluscs, corals, anemones, and crabs, started hitchhiking. As the fragmented raft wandered more than 5000 kilometers over 8 months, 80 species of plants and animals journeyed along with it. When the corals, coralline algae, anemones, and other reef dwellers that were hitchhiking got the the Reef, they simply decided to call it home (well, a bit of an anthropomorphism!!).

Bryan views this as a positive thing because it may indicate that volcanic activity in the Pacific can replenish the Reef, which is stressed and dying in the warming waters around it.  On the downside, however, there are some marine pests such as sponges and mussels that journey along as well.

Bryan's research is featured in this Phys.org article from which I obtained this information, and in the technical article by Bryan et al., "Rapid, Long-Distance Dispersal by Pumice Rafting," in PLOS-ONE, a peer-reviewed, open access journal.

It's the Olympics--Take a lap on corn starch!

The corn-starch experiment. Figure from
van Hecke's summary referenced below.

Have you ever wanted to walk on water? Here's a great YouTube video on non-Newtonian fluids, featuring the "Goop du jour" and how to do it (well, not quite walking on water, but close!)

In an article* in the July 12, 2012 issue of Nature, two physicists from the University of Chicago explain the physics behind this phenomenon, and take issue with older theories that it is caused by "shear thickening." This work has implications for our understanding of blood flow, cement and clays, all of which can exhibit shear thickening. Waitukatis and Jaeger propose that the shear thickening is not due to shear, but to compression.

In a series of experiments in which they plunged a rod into vats of cornstarch in water and water mixed with glycerol, and observed the process with fast video, X-ray imaging and other sensors, they found that below the point of impact of the rod, a cone of solidification developed. In this zone, the material had the properties of a solid...briefly. As you will see from the YouTube video above when both a bowling ball and a man sink, the solid state only lasts a fraction of a second, or as long as the material is agitated.

The proposed mechanism for this thickening relies on particle jamming, and the size and dynamics of the zone depend on conservation of mass. Jamming occurs when the particles get close enough together to become densely packed and form a solid. Conservation of mass leads to predictions of the size (depth) of the solidified zone. The solidified zone absorbs the momentum of the impacting rod, providing the force that stops its motion. Interestingly, they were able to exclude some other effects with clever experiments. By adding a 1-cm deep layer of water to the surface of the suspension, they reduced the surface tension to zero, and ruled out liquid-air interface effects. By adding glycerin to the water, they increased its viscosity by more than an order of magnitude, finding little effect on the parameters during compression of the water, but a strong effect on the sinking of the rod into the mixture after the impact.

The work has practical applications, from preventing cement from bulking up and breaking pipes when poured to design of sports gear or body armor to absorb impacts and vibrations.

A 22 second video is available from this article by Devin Powell in Science News.

*Waitukaitis, S.R. and Jaeger, H.M., "Impact-activated solidification of dense suspensions via dynamic jamming fronts." Nature, 487, 205-209, 2012, with a very nice summary by Martin van Hecke of Leiden University, Netherlands, on pages 174-175.

Mars lander "Curiosity" is on the surface

It will be impossible to outdo the NASA press releases on the exciting mission now underway on the surface of Mars. So, for my international readers who may not be receiving so much information on the mission, here is the JPL link to Curiosity, as well as to the two previous landers, Spirit and Opportunity:

http://marsrovers.jpl.nasa.gov/

Enjoy!