Orographic precipitation is produced when moist air is lifted as it moves over a mountain range (from Britannica Online).
The rain and snow considered here is caused by water vapor from the ocean being carried by winds to mountains. In rising
over the mountains, the air expands, cools, water vapor condenses, coalesces on airborne nucleation centers, and precipitation
results. This has been happening in many places, such as the USA western states, Australia's New South Wales, China, and
North Africa's Atlas Mountains. Now this precipitation is insufficient and we have drought. Disease is spreading, people
are starving, and more wars will result.
It may be possible to increase evaporation from the ocean with dark-colored porous rafts whose top surface is covered with a
thin layer of water which attains a higher temperature than water in the unmodified ocean. This is because the solar energy
is totally absorbed by a thin layer compared to the unmodified ocean, where the sunlight must heat the thick layer that it
illuminates. Also the raft body insulates the evaporation surface from the water under the raft so that a higher temperature
is reached. Evaporation rate is increased by the higher temperature. Some preliminary calculations indicate that the domestic
water requirements of the entire population of New South Wales might be supplied by an evaporator area 45 kilometers by
45 kilometers located off the coast of Sydney. This is an enormous area compared to a rooftop, but it is only a small
dot on the map of Australia.
There are many challenges. The rafts must survive storms, and must maintain a wet surface while being tossed about
by waves. Locations must be chosen to maximize the precipitation on land compared to that which falls uselessly into
the sea. China has a monsoon which brings moisture in from the sea during the warm season. It has a long coastline
and we will need to find locations for evaporator rafts that will maximize rainfall in the mountains while minimizing
flooding in other areas. The four areas mentioned above have different wind regimes. The western US has westerlies at
40 degrees latitude and local monsoons further south, Australia has a daily monsoon when its deserts heat up, the
Atlas Mountains are in the easterly trade winds, and China has a seasonal monsoon. The microclimate in each location
must be studied to optimize the evaporator raft placement.
For drought remediation in mountainous areas, I see no alternative to using the ocean as a source of fresh water,
using solar energy to distill water vapor from the ocean, and using the wind to transport the vapor to the mountains
where it is needed. This will also lower sea level and recharge aquifers which have been pumped dry.
This web page is being written with proposals in mind. Adequate response to funding opportunities requires the establishment
of partnerships to provide organization, management, computer modeling, geographic research, design, fabrication,
test facilities, oceanographic and fluid dynamic expertise, as well as expertise in many other disciplines well beyond
my limited abilities and resources. Some entities have already been approached.
So far, I have estimated radiation, air convection, and evaporation losses for a solar-heated evaporator for a few times
during the year. We will need to make some estimate of the losses due to water splashing over the evaporator in a rough
sea. Someone with much greater knowledge of evaporation must choose the correct formulas and set up a computer program
that will calculate the evaporation throughout each day of the year for all possible locations of the evaporators.
I anticipate that the evaporator rafts will be composed of square modules one meter on a side. Several such modules
can be placed in a tray outdoors in natural sunlight, winds, and ambient temperature at a particular location. The
evaporation rate can be measured throughout the day by measuring the water which must be added to keep the evaporator
top surface wet. The tray must be elevated above ground so that leakage or overflow can be detected. An open location
is required, free of shadows and wind blocks. This will give the performance for horizontal orientation at one
particular location for a particular weather sequence. Surface temperature can be measured with an infrared thermometer above the raft.
The solar industry has moved beyond testing devices at whatever intensity and sun angles prevail at the time of the test.
They have moved indoors with artificial illumination. But they are testing dry photovoltaic and thermal panels. Testing
evaporator rafts indoors is more difficult. We may not be able to simulate the wave motion in the test setup, so we may
have to fill in the data gaps with computer modeling. This requires expertise in fluid mechanics.
There are many other complications in interpreting measurements on a small evaporator. It must be flush with the
surrounding surface so that the turbulent airflow will be well developed. Elevating it on a bench will give excessive
evaporation rates due to laminar flow over the raised evaporator. Evaporation rates are higher when the evaporator
raft is on the crest of a wave, or its sloping surface is facing into the wind. Evaporation rate is lower when the
raft is in a wave trough or its sloping surface is facing downwind. Perhaps these measurements can be obtained from
experiment. We will need more fluid dynamics knowledge than I presently have to check whether the experimental
measurements make sense, or to substitute theory for measurements that cannot be made.
My present concept for the raft module is a somewhat resilient block a few inches in thickness with a wetable
fabric or open-cell foam top surface supported by closed-cell floats so that the top surface can be kept wet in
the ocean. Too high a position will allow it to dry out, and too low will allow waves to wash over it and cool it.
The small module size (1 m x 1 m) may allow adequate strength to be obtained with a thickness small enough to keep
the top surface wet while the foam module floats on the water. The top surface must be a dark color to absorb
sunlight, but must not be degraded by ultraviolet rays. The thickness of the top surface must be great enough to
store water while the raft is tilted by wave motion, and the open cell pore size must retain the water in the
tilted position. Somehow, we must make it sufficiently strong and durable to survive, and remain attached to all
the other modules in the fabric of the raft. I envision a wire 'X' frame embedded along the diagonals of the foam
panel with attachment ears at the module corners to connect to the bordering modules. The raft assembly will exert
tensile, compressive, or shear forces on the module frame, and I am guessing that a wire frame would be superior to plastic.
We will need slightly more than 2 billion of the small one-square-meter modules for the Sydney evaporator
array mentioned earlier. The connectors at the module corners must be designed for low cost and quick automated
installation from a work boat at sea.
Tests of a single module in a tray will be useful for selecting the material and the porosity of the top layer.
We can see how fast it soaks up water from the tray, and we can tilt the tray to see how the water is retained
in the pores. We will be able to measure the runoff rate when the tray is tilted. It may be possible to simulate
the heaving motion that is experienced when the wave crests and troughs propagate past the module. To build
this test machine we will need expertise in hydraulic, pneumatic, electric-motor-driven, or mechanically-driven actuators.
I have a concept of a maintenance vehicle which rides on two large buoyant motor-driven cylinders, with one of them
pivoted for steering. The vehicle rides on top of the raft, with its weight pushing the raft modules down into the water as it goes by.
In checking my evaporation rate calculations, I discovered that for more than 30 years I mistakenly thought that the
dew point was equal to the wet bulb temperature. No, the latter is the adiabatic saturation temperature and it is
higher than the dew point at any relative humidity less than 100%. This is important because the evaporation rate
is proportional to the vapor pressure of water at the evaporator surface temperature minus the vapor pressure of
water at the dew point. Using the wet bulb temperature instead of the dew point would result in a lower prediction of the evaporation rate.
The generally accepted idea is that the airflow across the evaporator starts out laminar and transitions to
increasing turbulence as it flows over the evaporator. The evaporation rate decreases with increasing turbulence.
For the second set of calculations, I used a characteristic length of 10 km instead of the 1 km that I used the
first time. This decreased the evaporation rate, but the evaporative heat loss also decreased, allowing the
water surface temperature to increase and help restore the evaporation rate.
My calculations for the forced convection heat loss did not agree with the expected Bowen Ratio, the ratio of
forced convection loss to evaporation heat loss. More work is needed on this point, but my calculations appear
to be reasonable approximations. The 45 km by 45 km evaporator will supply the domestic water required by the
population of New South Wales provided all of the vapor rains out over the mountains in southeast Australia.
We will need to learn something about this aspect of meteorology and incorporate it into our computer model
in order to predict the amount of rainfall recovered from the evaporated seawater.
This question about what fraction of the evaporated seawater is actually recovered as precipitation is crucial
to the success of this program. I am guessing that we would like the vapor from the evaporator rafts to be
concentrated in a narrow path to the mountains in order to insure that in rising, expanding, cooling and condensing,
the amount of precipitation is maximized. Locating the evaporators close to shore should help to keep the vapor
concentrated. Perhaps a study of the microclimate will locate places where the wind is funneled into a mountain
pass. The Australian evaporator array has been described as a square. Perhaps a long strip parallel to the shore
might be better. Or perhaps a long strip with some other orientation would work better. Commercial and sport
fishermen, as well as ferry services and other marine entities will voice their preferences.
People often suggest trying the idea out on a small scale, but I suspect that the effects we are looking for
are not linear or scalable. However, note that of the four examples mentioned, China is the largest geographic
area, with western USA, Atlas Mountains, and southeast Australia following in size order. It does not matter if
I have listed them in the wrong order. They are all too big for the effects of a small evaporator to be evident.
If we can find a much smaller orographic feature, we may be able to use it for a small scale demonstration. The
proposed program will include a search of world geography for such a feature. Perhaps we will be lucky enough to
find it somewhere in the Great Divider Range on the east coast of Australia, or perhaps in the Snowy Mountains
to the south. The detailed knowledge of local people will be a great help in the search, and their cooperation
will be a needed in setting up the experiment and measuring the results. This is a good area to target because
its drought is devastating rice production which many people have depended upon. We will still need to rely on
computer modeling and good design sense to guide large-scale construction and test. We should also plan on cloud
seeding. The Chinese have been shooting silver iodide crystals up from the ground without much success. I suspect
that cloud seeding does not do much good if there is insufficient vapor. Floating evaporators in the right
locations would improve their results.
For the Australian example, vapor will be produced during sunshine hours, but the wind does not have a clear
direction toward shore until the afternoon. During the morning, the wind might blow in a sequence of different
directions, but the velocity is low and the vapor is not transported far from the evaporator location. It should
remain available to be brought to the mountains in the afternoon. We will need to incorporate similar behavior
in the microclimate model that is developed for each continent that is studied.
In each location, the sea surface will be covered by many modules in close proximity to each other. For modules
1 m x 1 m, we will need 2025 million modules to build the Australian evaporator. If each module costs $12, the
array will cost about $24 billion plus the cost of 24 billion connectors, plus moorings, installation, and
maintenance. It cannot be built all at once and the modules will have a finite lifetime. It is a serious
concern that the module replacement rate not be a significant fraction of the new module installation rate.
We should always compare these costs to what is spent on warfare in order to maintain our perspective.
When such large areas of ocean are covered by evaporator rafts, will there be any reduction of wave amplitude?
No doubt, the answer is known, but I don't know what it is yet. My guess is that the rafts will be too light
weight to have much effect. However, there has been much development of wave attenuators. Perhaps these
can be installed around the periphery to mitigate the effects of storms. Add this to the list of things to learn.