54 | New Scientist | 25 April 2020
Rainy residue
My windows are cleaned every
four weeks, and normally stay
sparkly. But recent heavy rain has
left a deposit on them. What is it?Thomas Cox
Inverness, UK
Most rain forms in the
troposphere, the lowest layer
of our atmosphere. Here, water
vapour cools and condenses
around particles in the air,
eventually forming clouds.
When droplets become too heavy
to be supported by air pressure,
they fall to Earth as rain.
In a cloud, microscopic dust
particles kicked up by sandstorms
are what water vapour condenses
onto. Most of the dust in the UK’s
extensive rainfall comes from the
Sahara desert. When it lands on
windows, the water evaporates,
leaving the dust particles behind.
This could be the source of the
deposit you found on your
windows after heavy rain.Mike Follows
Sutton Coldfield,
West Midlands, UK
This is so-called blood rain,
which happens a few times a
year. It sometimes occurs when
high winds or storms over the
Sahara desert lift dust high into
the atmosphere.
A southerly wind can carry
this dust to the UK, and any
rainfall washes it out of the air
and onto Earth’s surface below.
When the water evaporates, it
leaves a fine layer of red dust,
which gives the rain its name.
While it can annoy those
who have just cleaned their cars
or windows, this dust may help
regulate our climate. When it
lands in oceans, the dust can
provide essential nutrients for
the growth of phytoplankton,
sometimes creating algal blooms
that can be seen from satellites.
Phytoplankton are algae that
sit at the bottom of the marine
food chain. They do the same
job as green plants on land by
absorbing carbon dioxide fromthe atmosphere. Because the dust
particles make the phytoplankton
heavier, they then sink to the
ocean floor faster. This helps
remove carbon dioxide from
the atmosphere, as part of the
biological carbon pump.
Scientists are experimenting
with artificial ocean fertilisation
to increase the removal of carbon
dioxide from the atmosphere.
Saharan dust also delivers vital
phosphorus and other fertilisers
to the depleted soils of the
Amazon rainforest, as well as
smaller quantities of these to
some other regions.
It would be interesting to see
if the frequency and intensity of
desert dust storms increase in a
warmer world.Kate Potter
Leicester, UK
Dust storms in the Sahara can
lead to sandy deposits on cars
and other surfaces. The same
phenomenon happened to me
a couple of years ago.
After a very big storm with
south-westerly gales, our windows
and cars were covered in a white
residue. I rubbed some onto myfinger and licked it. It was salt.
I can only assume that, even
as far from the sea as we are in
Leicester, salty aerosols from the
huge waves on the coast had been
lifted into the atmosphere and
then deposited inland by rain.Anthony Woodward
Portland, Oregon, US
Rain has to be dirty. Water droplets
in clouds form when water vapour
changes into liquid water around
a condensation nucleus, such as
a tiny particle of dust or soot, or
a flake of salt.
These coalesce to create clouds
and ultimately fall as rain. When a
raindrop eventually hits a surface,
its impact can release the
substances it contains.
The water then evaporates,
leaving behind what it gathered
on its journey. Perhaps you can
console yourself by considering
the long and complex odyssey
of a raindrop.Bless you
Rather than sneezing once or twice,
some people do so again and again.
My partner often sneezes 20 or 30
times in succession. Is this common,
and is there any explanation?Millie Hughes
Amersham, Buckinghamshire, UK
There is a little-known condition
called photic sneeze reflex, or
autosomal compelling helio-
ophthalmic outburst (ACHOO)
syndrome. It occurs in response
to certain stimuli: for example,
when you are first exposed to
bright light after your eyes have
adjusted to the dark.
Some estimates claim it
affects between 18 and 35 per cent
of people. However, the genetic
basis for the disorder hasn’t been
isolated or studied in detail, so not
much is known about it. What we
do know is that it is inherited in a
dominant manner, therefore only
one of your parents needs to have
it in order for you to have it too,
and it isn’t dangerous. If this is
what causes their many sneezes in
a row, it is more likely to annoy you
than cause your partner any harm.Brian Pollard
Launceston, Cornwall, UK
I have the same problem and my
father did as well, so it may have a
genetic component. If the sneezing
occurs at any time and appears to
have no environmental trigger, it
is probably non-allergic rhinitis.
A palliative that works for
me is an antihistamine nasal
spray, which implies the cause is
oversensitive nerves in my nose. A
sniff of decongesting oils dabbed
on the end of my finger also helps
me clear the mucus that often
builds up after a few sneezes. ❚This week’s new questions
Hair loss gain Does male baldness convey any evolutionary
advantage? Chris Lee, Norwich, Norfolk, UKAnt funeral I left an empty plastic laundry basket on the lawn.
Later, ants dropped dead companions into it. Where do ants
bury their dead? Jane Russell, Burton-on-Trent, Staffordshire, UKWant to send us a question or answer?
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