Pyramids of dew

16
April 2005 New Scientist

Fred Pearce

The Black Sea port of Feodosia, in
what is now Ukraine, is a place of dark secrets. One of Europe's oldest towns,

it has been home to
Greeks and Genoese Tartars and Armenians. In the Middle
Ages, it housed one of Europe's biggest slave markets, selling Ukrainians to Egypt.
The Black Death entered Europe when a ship docked there in 1347. But, perhaps most mysterious of
all, the surrounding forests house strange pyramid-like piles of stones as big
as houses. Most archaeologists say they are tombs but local people say the
pyramids once supplied the town with water. A century ago, Russian engineer
Frederich Zibold argued that these bizarre structures were abandoned chambers
for condensing dew. Had the people of Crimea discovered - and lost - the art of catching water from the air. The
idea has been dismissed for decades. But there are now new claims that Zibold
was right after all.

GILBERT WHITE, the
celebrated chronicler of the 18th-century English countryside, knew all about
catching dew. There was a pond high above his home in Selborne that was replenished
mainly by dew. It was, he wrote, "never above three feet deep in the
middle, and not more than 30 feet in diameter,
and contained perhaps not more than two or three hogsheads of water, yet it is
never known to fail, though it affords drink to 300 or 400 sheep and for at
least 20 herds of cattle besides".

At the time, the downs of
south-east England were dotted
with dew ponds, most of them dug to provide water for huge flocks of sheep. The
ponds capture some rain, but were artfully constructed to condense moisture
from the air. Beneath their clay bottoms, a layer of straw insulated the water,
keeping it colder than the soil at night; and above the clay, a layer of stones
ensured that the pond shed heat quickly, lowering the temperature further. The
siting was clever, too. Many sat at the heads of valleys that channelled mists
and moist air uphill, cooling it as it went. Once established, dew ponds
effectively generated their own water from the air. Many still do.

Moisture in the air is
the great untapped water resource. The atmosphere contains around 13,000 cubic
kilometres of water vapour at any one time, six times as much as in all the
world's rivers. It is a tantalising prize, and numerous inventors have tried to
catch the water by making rain, catching fog or triggering condensation.

Cloud seeding is today
almost an established technology, with planes and rockets spraying silver
iodide into clouds from China to Israel and Russia to the US, to turn the
droplets into rainfall (see "Kicking up a storm"). More outlandish
was the plan of French meteorologist Bernard Dubos, who in 1935 proposed
building a chimney 600 metres high with a
fountain at its base to create an updraft of humid air that would, he believed,
saturate the air above and generate rain. And in the 1940s, South Africa's chief
meteorologist Theodore Schumann proposed erecting an electric fence 50 metres high along the top of Table Mountain to make rain
by ionising the air. Neither ever happened.

Another idea is to
trigger rain with sound. On a cool, still night, the air can be so saturated
with moisture that even modest air movements, such as sound waves, could in
theory produce raindrops. In the mountains of southern China, villagers
have a tradition of yelling to stimulate rain. The louder they shout, they say,
the more it rains. Catching fog has a more established track record. On a ridge
in the rainless Atacama desert in northern Chile, you can
still see the tattered remains of large sheets of plastic mesh that once
harvested water from the fogs that roll in from the Pacific Ocean. This is
recent history. In the 1990s each 12-by-3-metre net was catching an average of 150 litres a day, filling taps in
the nearby fishing village of Chungungo. The village
abandoned the nets after the government installed a piped water supply. But the
Canadian researcher behind them, Bob Schemenauer, is erecting similar systems
for remote communities in other arid, foggy places, including Haiti, Oman, Guatemala and Eritrea. The number
of such places is limited. But water vapour in the atmosphere is universal. So
the ultimate goal is still to find a sure-fire way to harvest that moisture by
cooling the air to make dew.

The last traditional
English digger of dew ponds hung up his spade in the 1930s. But in other places,
dew-catching skills have lingered. Few holidaymakers on the island of Lanzarote in the
Canaries will have noticed, but farmers there surround their vines with a
"stone mulch" of volcanic gravel that condenses night-time moisture
to water the crops.

And then there is the
strange case of the Crimean pyramids. Frederich Zibold, a Russian engineer and
forester, stumbled across them in 1900 while clearing trees. After talking to
local people, he reported that until the late 19th century, the piles of stone were
the main source of water for Feodosia, which in its slave-trading heyday had a
population of 80,000.

By Zibold's time, the
pyramids - typically 30 metres across and 10 metres high - had been
abandoned. But he reported that each night the stones lost heat and cooled the
sea breezes blowing through the loosely packed stones sufficiently to cause
moisture to condense. In the past, this flow of water trickling from the stones
had filled networks of pottery pipes and several dozen reservoirs that provided
water for the town.

Today the pyramids are no
more than rubble, but the remains of the pipes and reservoirs survive. The
amount of water they produced has been a matter of conjecture and fitful
experiment ever since. Measurements made in the 19th century, but uncovered
only after Zibold's discovery, put the output of each pyramid at between 5000
and 10,000 litres a day. When
Zibold built his own small pyramid near the town in an attempt to prove his
theory, he collected around 300 litres a day, which
he considered a failure. Attempts in the 1920s and 1930s by the Belgian
inventor Achille Knapen and French bioclimatologist Leon Chaptal produced
similar returns. Knapen's patented "air well" still stands derelict
on a hilltop in Provence.

Around the same time, German researcher Wolf
Klaphake discovered Arabic texts describing water condensers in Palestine a thousand
years ago. He built his own, apparently successful, prototype "air
well" in Croatia. It was
essentially a building with labyrinthine walls and a chimney to draw warm,
moist air from outside into the cool interior. After emigrating
to Australia, he failed in
an effort to persuade the authorities there to build dew-making machines on the
waterless Nullabor Plain. After this, the thirst for making dew seemed to die.
Even Zibold's work was disowned. Archaeologists excavating the remains of the
Crimean pyramids argued that they were tombs after all.

“The atmosphere
contains around 13,000 cubic kilometres of water vapour at any one time, six
times as much as in all the world's rivers”

But now Israeli
researchers have reopened the debate. Avraham Trakhtman, a mathematician at Bar-Ilan University in Ramat Gan, and Boris
Kogan of the Center of Ecological
Systems and Technologies in Jerusalem claim that
those who rejected Zibold's idea were "too hasty". In the climatic
conditions of the Crimea, the original pyramids
were much more efficient than later experiments suggested. Zibold's mistake was
to make too small a replica. The original pyramids were big enough for the
stones at their heart to stay cool long enough for condensation to continue
long into the day. And the pyramids had a crater in the top, which Trakhtman
believes created a chamber of solar-heated air that stimulated a draught
through the pyramid, maximising condensation.

Some other aficionados of
the art of capturing dew have yet to be convinced. But it remains the case that
archaeologists have uncovered similar stone heaps across arid lands from north Africa to southern Siberia. And,
besides, says Trakhtman, "Why would Zibold have made all this up?"

This is no idle
speculation from the Israelis. Their efforts to find water in the desert
provide one of the central narratives of the Israeli state. And with humidity
in parts of the Negev desert approaching 100
per cent for up to 200 nights a year, dew is potentially a major source of
water there. Any way to maximise dew production would be a boon for a country
that sees its survival tied as much to water as land. Could the town that
brought the Black Death to Europe one day have
a more positive legacy?

From
issue 2495 of New Scientist magazine, 16
April 2005, page 52