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Clean limbs
The back pages Almost the last word Spotted over London: how are these aerial effects created?
Storm warning
Chris Daniel Llansanffraid Glan Conwy, UK The smell is probably ozone, which typically has a sweet, pungent aroma. In stormy weather, lightning splits atmospheric nitrogen and oxygen molecules. These can then combine into nitric oxide, which, with further reactions, forms ozone. This is then carried downwind ahead of the rain with its characteristic fresh scent. Closer by, there is a deliciously earthy smell that is associated with light rain following a prolonged dry period. This is due to a combination of compounds in the soil that are produced by plants and bacteria. In long, dry stretches, plants produce a growth-inhibiting fatty acid that was given the name petrichor in the 1960s, derived from petra, the Greek word for “rock”, and ichor, the golden blood of gods in Greek mythology. This aromatic compound is released from the soil and is the source of the earthy smell following rain. Another part of the scent comes from the actinomycete bacteria that are often found in soil. In dry periods, they form spores and, in doing so, produce a molecule called geosmin (derived from the Greek words for “earth” and “smell”). It is said to be detectable by humans in concentrations as low as 5 parts per trillion. The mechanism for the release of these chemicals into the air was only described as recently as 2015, when it was observed that bubbles of air that are trapped under a raindrop landing on a porous surface collect the spores and oils, then transport them up through the raindrop, releasing them into the air as aerosols. The smell is most intense during light rain with little wind. 52 | New Scientist | 29 June 2019
COURTESY OF ALEXANDER TIERNEY
What causes the fresh smell we experience just before the onset of a storm or shower of rain, which is especially noticeable after an extended dry spell?
This week’s new questions Special effects I took this picture of the sky over Wimbledon, London, on 13 May at around 6 pm, two and a half hours before sunset. How are these spectacular effects created? Alexander Tierney, London, UK Future fossils If, some day, our civilisation fries itself out of existence, will new reservoirs of fossil fuels eventually accumulate, and could they power some future industrial revolution? Steve White, York, UK Sneeze blindness Why do our eyelids involuntarily close when we sneeze? Is it to stop our eyes from popping out? Tom Wilkinson, London, UK
If the rain is too heavy, the aerosols are washed out of the air and the smell is lost. My father, an airline pilot in Australia in the 1940s, once spotted an enormous population of birds in Lake Eyre, South Australia. The area is completely dry and barren for most of the year but unseasonal rains had covered the region like an inland sea. He marvelled at how the birds could have known to travel there from hundreds of kilometres away. I now wonder if the same chemical signals that humans appreciate also attract birds and other wildlife that associate the scent with food and survival.
need washing, and I suspect we overwash generally. How often should we wash, for health benefits or other reasons?
Simon L. Goodman Griesheim, Germany Views on how often to wash and what constitutes dirt vary between communities, as they are linked to social context. Skin is a complex bacterial ecosystem that needs secreted oils to maintain its water resistance and flexibility. Applying soap too often will dry skin out and should be avoided. Along with dirt, skin also amasses substances from within us in the form of sweat, dead skin cells and oily secretions. These
combine to produce a banquet – with stinking after-effects – for the millions of bacteria that colonise every square centimetre of our bodies. Sweat glands have two forms, which affect how often and where to wash. On much of our skin, eccrine sweat glands pump out a weak salt solution containing urea and lactate. If left to accumulate, dried sweat forms crusts, which can be uncomfortable. Apocrine sweat glands produce “gunk”. They are concentrated in the armpits, anal and pubic regions, and in the ear and nose. Their evolutionary role is to produce odours for functions such as sexual attraction and, in other animals, territory marking. You may wish to ask your partner how often they would like you to wash these areas. Overall, it is probably better to limit the routine use of soaps to regions that are dense with apocrine sweat glands. Graham Houghton Adelaide, Australia New recruits to the UK’s Royal Marines are given a demonstration about how to shower. The instructor essentially tells them, “Wash the smelly bits – you know where they are – and don’t worry too much about the rest”. Legend has it that personal hygiene is the origin of the Marines’ nickname for army soldiers: they call them pongos, because where the army goes, the pong goes. Richard Ives Kettlestone, Norfolk, UK The questioner is clearly not a gardener. I am, so the parts of my body that need the most washing after a spell of weeding are my hands and knees. ❚
Clean limbs There has been much speculation on Twitter recently about whether we should wash our legs when in the shower. I think legs don’t specifically
Want to send us a question or answer?
Email us at [email protected] Questions should be about everyday science phenomena Full terms and conditions at newscientist.com/lw-terms
Storm warning
Chris Daniel Llansanffraid Glan Conwy, UK The smell is probably ozone, which typically has a sweet, pungent aroma. In stormy weather, lightning splits atmospheric nitrogen and oxygen molecules. These can then combine into nitric oxide, which, with further reactions, forms ozone. This is then carried downwind ahead of the rain with its characteristic fresh scent. Closer by, there is a deliciously earthy smell that is associated with light rain following a prolonged dry period. This is due to a combination of compounds in the soil that are produced by plants and bacteria. In long, dry stretches, plants produce a growth-inhibiting fatty acid that was given the name petrichor in the 1960s, derived from petra, the Greek word for “rock”, and ichor, the golden blood of gods in Greek mythology. This aromatic compound is released from the soil and is the source of the earthy smell following rain. Another part of the scent comes from the actinomycete bacteria that are often found in soil. In dry periods, they form spores and, in doing so, produce a molecule called geosmin (derived from the Greek words for “earth” and “smell”). It is said to be detectable by humans in concentrations as low as 5 parts per trillion. The mechanism for the release of these chemicals into the air was only described as recently as 2015, when it was observed that bubbles of air that are trapped under a raindrop landing on a porous surface collect the spores and oils, then transport them up through the raindrop, releasing them into the air as aerosols. The smell is most intense during light rain with little wind. 52 | New Scientist | 29 June 2019
COURTESY OF ALEXANDER TIERNEY
What causes the fresh smell we experience just before the onset of a storm or shower of rain, which is especially noticeable after an extended dry spell?
This week’s new questions Special effects I took this picture of the sky over Wimbledon, London, on 13 May at around 6 pm, two and a half hours before sunset. How are these spectacular effects created? Alexander Tierney, London, UK Future fossils If, some day, our civilisation fries itself out of existence, will new reservoirs of fossil fuels eventually accumulate, and could they power some future industrial revolution? Steve White, York, UK Sneeze blindness Why do our eyelids involuntarily close when we sneeze? Is it to stop our eyes from popping out? Tom Wilkinson, London, UK
If the rain is too heavy, the aerosols are washed out of the air and the smell is lost. My father, an airline pilot in Australia in the 1940s, once spotted an enormous population of birds in Lake Eyre, South Australia. The area is completely dry and barren for most of the year but unseasonal rains had covered the region like an inland sea. He marvelled at how the birds could have known to travel there from hundreds of kilometres away. I now wonder if the same chemical signals that humans appreciate also attract birds and other wildlife that associate the scent with food and survival.
need washing, and I suspect we overwash generally. How often should we wash, for health benefits or other reasons?
Simon L. Goodman Griesheim, Germany Views on how often to wash and what constitutes dirt vary between communities, as they are linked to social context. Skin is a complex bacterial ecosystem that needs secreted oils to maintain its water resistance and flexibility. Applying soap too often will dry skin out and should be avoided. Along with dirt, skin also amasses substances from within us in the form of sweat, dead skin cells and oily secretions. These
combine to produce a banquet – with stinking after-effects – for the millions of bacteria that colonise every square centimetre of our bodies. Sweat glands have two forms, which affect how often and where to wash. On much of our skin, eccrine sweat glands pump out a weak salt solution containing urea and lactate. If left to accumulate, dried sweat forms crusts, which can be uncomfortable. Apocrine sweat glands produce “gunk”. They are concentrated in the armpits, anal and pubic regions, and in the ear and nose. Their evolutionary role is to produce odours for functions such as sexual attraction and, in other animals, territory marking. You may wish to ask your partner how often they would like you to wash these areas. Overall, it is probably better to limit the routine use of soaps to regions that are dense with apocrine sweat glands. Graham Houghton Adelaide, Australia New recruits to the UK’s Royal Marines are given a demonstration about how to shower. The instructor essentially tells them, “Wash the smelly bits – you know where they are – and don’t worry too much about the rest”. Legend has it that personal hygiene is the origin of the Marines’ nickname for army soldiers: they call them pongos, because where the army goes, the pong goes. Richard Ives Kettlestone, Norfolk, UK The questioner is clearly not a gardener. I am, so the parts of my body that need the most washing after a spell of weeding are my hands and knees. ❚
Clean limbs There has been much speculation on Twitter recently about whether we should wash our legs when in the shower. I think legs don’t specifically
Want to send us a question or answer?
Email us at [email protected] Questions should be about everyday science phenomena Full terms and conditions at newscientist.com/lw-terms