Hi All
I was asked to write something about hurricanes for a well known popular
news outlet but they thought that it was too technical. However it might
still be useful. I hope that the ETC group can comment.
The formation of a hurricane depends on many factors including
atmospheric water vapour, distance from the equator and the recent
history of wind patterns.But an essential requirement is a high sea
surface temperature. To get from a tropical storm to the lowest category
of hurricane requires a temperature of 26.5 C.We can moderate
hurricanes, or even prevent them, by reducing water temperature.
A useful start to any engineering project is the estimation of all the
energy flows. One cubic metre of air at a temperature of 30 C can hold
about 30 grams of water vapour. The energy to evaporate this is about
the same as in 13 grams of TNT, enough for a nasty anti-personnel mine.A
cubic kilometre of such air contains the same energy as the Hiroshima
bomb.Hurricanes can be hundreds of kilometres in diameter and so contain
tens of thousands of Hiroshimas.If you have read this far you will know
about the billions of lost dollars and thousands of deaths from this
amount of energy.
Most of the hurricanes that reach America (with the exception of
Harvey), start on the African side of the Atlantic near Cape Verde and
grow as they move west. We can use Google Earth to measure the hurricane
breeding area. The US National Weather Service gives a warm water depth
of 45 metres. To cool this volume by 2 C in 200 days needs more than 600
times the mean US electricity power generation. If you want to moderate
a hurricane tomorrow, today is much too late.You should have started
last November.
All this heat has come from the sun. Some could be reflected back out to
space by clouds. The reflectivity of clouds was studied by Sean
Twomey.He flew over many clouds, scooped samples and measured the solar
energy reflected from their tops.He showed that reflectivity depends on
the size distribution of drops.Lots of small drops reflect more than the
same amount of liquid water in fewer, larger ones.In typical conditions,
doubling the cloud drop number increases reflectivity by a bit over 0.05.
Making cloud drops needs a high humidity but also some kind of ‘seed’
called a condensation nucleus on which to start growth.There are
thousands of condensation nuclei per cubic centimetre of air over land
but fewer in air over mid ocean, often less than 50. John Latham
suggested that the salt residues left from the evaporation of a spray of
sub-micron drops of sea water would be excellent condensation nuclei.
They would be moved from the sea surface by turbulence to produce a
fairly even distribution upwards through the marine boundary layer to
where clouds form.
The condensation nuclei could be produced by wind-driven sailing vessels
cruising along the hurricane breeding areas getting energy from their
motion through the water. We can make spray by pumping water through
very small nozzles etched in the silicon wafers used for making
microchips. The main technical problem is that sea water is full of
plankton much larger than nozzles.This can be filtered using
ultra-filtration technology with back-flushing, originally developed for
removing polio viruses from drinking water. Each vessel would produce
0.8 micron diameter drops at 10^17 a second.
Spray operations would depend on the pattern of sea surface temperatures
as measured by satellites. We want the trajectory of temperature rises
through the year from November to the following July to be those that an
international panel of meteorologists think will give a desirable
rainfall pattern from ‘gentle’ tropical storms.
Most ships are made in quite small numbers.An exception was the Flower
class corvettes built for the Royal Navy during World War II. If we
index-link the 1940 cost to today we can predict that in mass production
each spray vessel would cost $4 million. With assumptions which have not
yet been rejected by hurricane experts, we think that controlling the
Atlantic hurricane breeding paths would need about 100 vessels.With
typical ship lifetime the annual ownership and maintenance cost would be
about $40 million. If these figures and recent estimates of the cost of
hurricane damage are correct the benefit-to-cost ratio is quite attractive.
Because of official UK Government policy updated in May 2018 the project
is privately funded.
I will send anyone who asks an update on recent hardware development,
still privately funded.
Stephen
--
Emeritus Professor of Engineering Design, School of Engineering,
Mayfield Road, University of Edinburgh EH9 3DW, Scotland
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