Rain in Tel Aviv 311.
(photo credit: JOANNA PARASZCZUK)
Dirty air works both ways: Particulate matter in the atmosphere affects the
development of clouds by reducing the amount of rain in cool and relatively dry
regions, such as Israel in winter. But it can also increase rain and the
intensity of severe storms in warm and moist regions or seasons, such as the
eastern half of the US during summer, according to a new study by researchers at
the Hebrew University of Jerusalem and the University of Maryland.
research provides clear evidence of how aerosols – soot, dust and other small
particles in the atmosphere – can affect weather and climate. The findings have
important implications for the availability, management and use of water
resources around the world, the scientists say.Using a decade-long dataset of
extensive atmosphere measurements from America’s Southern Great Plains Research
Center in Oklahoma (run by the US Department of Energy’s atmospheric radiation
measurement program), the researchers uncovered for the first time the long-term
net impact of aerosols on cloud height and thickness and the resulting changes
in the frequency and intensity of rainfall. This study confirmed and showed the
importance of the theory developed by HU Prof. Daniel Rosenfeld in his previous
studies. The new study appeared in a recent issue of Nature
The team members say the findings have major policy
implications for sustainable development and water resources, especially for
those developing regions susceptible to extreme events, such as drought and
flood. Increases in manufacturing, urbanization, building of power plants and
other industrial developments are often accompanied with increases in pollution
whose adverse impacts on weather and climate, as revealed in this study, can
hurt the economy.
Aerosols – not spray cans but tiny solid or liquid
particles suspended in air – include soot, dust and sulfate particles and are
what we commonly think of when we talk about air pollution. Aerosols may come
from the combustion of fossil fuels, industrial and agricultural processes and
accidental or deliberate burning of fields and forests. They can be hazardous to
both human health and the environment. Aerosols also affect cloud microphysics,
as they serve as nuclei around which water droplets or ice particles form. Both
processes can affect cloud properties and rainfall. Different processes may work
in harmony or offset each other, leading to a complex yet inconclusive
interpretation of their long-term net effect.
“When the air rises, water
vapor condenses on aerosol particles to form cloud drops. In cleaner air, the
drops are larger because there are fewer drops, and they have better chances of
colliding to form large rain drops. In polluted air, more and smaller drops are
formed. They float in the air and are slow to coalesce into rain drops,” says
“With a small amount of moisture, most cloud drops never
become large enough for efficient precipitation, thus the amount of rainfall is
reduced. Rain withheld in moist, polluted and thick clouds freezes at higher
altitudes to form ice crystals or even hail.
The energy released by
freezing fuels the clouds to grow taller and create larger ice particles that
produce more intense precipitation.
This explains why air pollution can
exacerbate both drought and flood,” says Rosenfeld. This may partially explain
his finding in another study that there are more severe convective storms during
summer on weekdays compared to weekends in the eastern US; more pollution is
emitted during the weekdays than during the weekend.MOLECULAR COOKING
Cola turned into spaghetti, raspberry drink “miraculously” reduced to clear
water, making cottage cheese from milk and “cooking” raw eggs in alcohol were
some of the feats presented during Hanukka at Jerusalem’s Bloomfield Science
Museum, with many of them continuing through March.
A workshop attracting
both young children and adults on “experiential molecular cooking,” in which
scientific methods are used to create “caviar” from various colorful fruits,
will continue to be offered on Saturdays by “Dr. Molecule” through the
Other events and exhibits will remain on the museum’s regular
visiting days until March.
The live performances by Dr. Sergio Broido,
who has a doctorate in molecular biology from the Hebrew University and has won
a bronze medal at a cooking competition abroad – ran through the festival of
lights and wowed young audiences.
Broido used guar gum and seaweed to
change the form of foods from liquid to a rubber-like texture. “Juice” from
cooked red cabbage helped change water to colors and back again, depending on
whether the water was acidic or alkaline.
Continuing through the summer
is a exhibit showing Israel’s 50 greatest inventions.
explains how electricity has become the fuel that moves nearly the whole of