26 September 2020 | New Scientist | 17FIVE named tropical storms
were present in the Atlantic on
14 September – only the second
time this has happened since
records began. The image here
shows all five, plus tropical storm
Karina in the Pacific. Most remained
at sea, but Sally, which intensified
into a category 2 hurricane, caused
damage across the US Gulf Coast.
The 2020 hurricane season has
been so active that all 21 names on
the list for this year have been used.
After storm Wilfred, which formed
after this picture was taken, the US
National Hurricane Center will use
the names of Greek letters.
The number of tropical storms
varies each year. High wind
shear - the difference between wind
speeds at the surface and higher
up - can rip potential storms apart.
Global warming may not increase
the number of hurricanes, but there
is evidence that it makes them
stronger and more damaging. ❚
Weather
SUPERCOOLING liquid water
to record low temperatures
has revealed new evidence
that it can exist as two different
liquids simultaneously.
Supercooled water – liquid
water chilled below its freezing
point without being allowed to
freeze – has been baffling chemists
for decades. Previous studies
found that the extent to which
water molecules pack together,
known as their density, starts to
fluctuate as water is cooled to
extremely low temperatures.
Since then, evidence has been
mounting that these fluctuations
may indicate the presence
of two different liquids in one,
with some water molecules
packed more closely together
and others spaced further apart.
But researchers have struggled
to study these two different
liquids, because supercooled
water usually freezes
nanoseconds after it is formed.
By firing lasers at an extremely
thin sheet of ice, Greg Kimmel
and Bruce Kay at the Pacific
Northwest National Laboratory
in Washington were able to
briefly generate and analyse
supercooled liquid water at
much lower temperatures than
had been possible previously.
During the fraction of a second
in which the water was in its
supercooled state, the team
took a snapshot of its structureusing infrared spectroscopy,
a method that takes advantage
of the way infrared light is
transmitted through molecules.
Repeated experiments revealed
fluctuations in the density of
the supercooled liquid consistent
with the two-liquid hypothesis.
The researchers could identify
a temperature range within which
supercooled water transitions
between its two liquid forms:
between about -93°C and -33°C.
By increasing or decreasing
the temperature within the
critical range, the team also
discovered that you can tip the
ratio of the two liquids one way
or the other, suggesting that
the two-liquid phenomenoncan’t simply be explained by
water crystallising into ice.
“We noticed that there was
something funny happening
early on before it was really
crystallising,” says Kimmel
(Science, doi.org/d9d7).
“Water has many strange
properties,” says Anders Nilsson
at Stockholm University in
Sweden. But this is the clearest
experimental evidence of
two-liquid structures, he says.
Understanding water’s unique
chemistry could help us predict
how it might behave under
unusual conditions, such as in
outer space, says Paola Gallo at
Roma Tre University in Italy. ❚Physics
Clearest signs yet that water is two liquids
Michael Le Page
Storming the Atlantic
A rare cluster of cyclones was captured by a satellite
NOAALayal Liverpool