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Round the bend
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THE LAST WORD Round the bend
was moving northwards. The distortion of the light rays was so big that the boundary between the warm and cold air was clearly visible. It took about a minute for the edge to pass over the runway. Roger Williams By email, no address supplied
Look carefully at this photo of a rainbow over Dinas Head near Fishguard in Wales (right) and you will notice that as it nears land it looks like there is a sharp bend in its arc. What caused this?
■ This effect is caused by atmospheric refraction, the same type that makes the sun appear to rise earlier and set later than would be expected from astronomical calculations. The density of the atmosphere decreases the higher you get above Earth’s surface, and so it interferes less with the light passing through. This means the speed of light is slightly faster higher in the atmosphere than it is closer to the planet’s surface. In outer space with no air, it is the standard textbook value of almost 300,000 kilometres per second (the speed of light in vacuum). The increase in light speed with height causes rays to be refracted downwards, and the effect is strongest for rays that are close to horizontal. Thus light rays curve slightly around Earth’s surface. A small curvature is also caused by Earth’s gravitational field. The curved light rays cause objects below the mathematical horizon to appear above the visible horizon. So the bottom part of the rainbow in the photo includes some from below the mathematical horizon. This slightly longer bit of rainbow has been compressed by the refraction, increasing its apparent curvature.
■ I once witnessed an extreme version of this when approaching Sydney airport. Our aircraft was supposed to be landing on the northern runway, but a southerly buster (the Aussie term for a strong summer cold front) was about to pass, and the wind direction flipped from north to south. While changing direction to land on the southerly runway instead, we could actually see the edge of the cold front as it traversed the runways, so large was the temperature difference. It was as if a huge elongated lead crystal dome paperweight
We pay £25 for every answer published in New Scientist. To answer a question or ask a new one please email [email protected]. Questions should be scientific enquiries about everyday phenomena, and both questions and answers should be concise. We reserve the right to edit items for clarity and style. Please include a postal address, daytime telephone number and email address. You can also send questions and
answers to The Last Word, New Scientist, 25 Bedford Street, London, WC2E 9ES. New Scientist Ltd retains total editorial control over the published content and reserves all rights to reuse question and answer material that has been submitted by readers in any medium or in any format and at any time in the future. All unanswered questions and previous questions and answers are at newscientist.com/lastword/
Feel the heat
This is the reverse of the effect that causes desert mirages, in which the hot ground heats air close to it, making it less dense. In this case, the speed of light is faster close to the ground, so light is refracted upwards, giving a mirror effect that looks like water. Richard Parkins Cambridge, UK
We sweat to cool our bodies by evaporation, but when we wipe the sweat away – a natural reaction in humans – are we defeating that function and making ourselves hotter? Do we also hasten dehydration by making our bodies sweat more, to replace what we’ve wiped away? In other words, should we just leave the sweat to do its job?
■ The damp squib answer is that neither leaving the sweat nor wiping it away will make any difference to how well or badly you keep cool and hydrated. Why? First, we are constantly sweating, but, by the time we feel it on our skin, sweat has reached the limit of its evaporative cooling capacity. Leaving it won’t make more of it evaporate or make what is there evaporate faster. Second, sweating is governed by how hot we are, not by how sweaty or dry our skin is. Because the sweat on our skin is no longer evaporating and cooling us, our sweat glands continue to pour it out. This is why beads of sweat accumulate on our brows – to the point of dripping down our noses and proverbially getting up them.
Leave it, and you are just letting gravity wipe it away for you. You will dehydrate just as fast. And the muscular heat that our wiping actions generate? This could increase or decrease our rate of overheating, depending on how sedentary or vigorous the activity we are interrupting is. So, on balance, while it may be a hot topic for theoretical debate, it will make no palpable difference. You would do better to switch on an electric fan, or invest muscular effort in wafting a hand-held one. This will disperse the layer of insulating air that clings to your skin and allow more evaporative cooling. If the ambient air temperature is below body heat, it may directly cool you somewhat. Alternatively, wear cotton garments. The material will soak up the sweat and provide a greater evaporative surface. Len Winokur Leeds, UK
This week’s question BLOW OUT
Space movies regularly dispatch characters through an explosively decompressed airlock. But how likely is such a rapid ejection, taking into account the victim’s location in the airlock and its volume? Assuming they are wearing a space suit, what is the best way to avoid expulsion, apart from grabbing something solid? Mark Hallinan Murwillumbah, New South Wales, Australia
THE LAST WORD Round the bend
was moving northwards. The distortion of the light rays was so big that the boundary between the warm and cold air was clearly visible. It took about a minute for the edge to pass over the runway. Roger Williams By email, no address supplied
Look carefully at this photo of a rainbow over Dinas Head near Fishguard in Wales (right) and you will notice that as it nears land it looks like there is a sharp bend in its arc. What caused this?
■ This effect is caused by atmospheric refraction, the same type that makes the sun appear to rise earlier and set later than would be expected from astronomical calculations. The density of the atmosphere decreases the higher you get above Earth’s surface, and so it interferes less with the light passing through. This means the speed of light is slightly faster higher in the atmosphere than it is closer to the planet’s surface. In outer space with no air, it is the standard textbook value of almost 300,000 kilometres per second (the speed of light in vacuum). The increase in light speed with height causes rays to be refracted downwards, and the effect is strongest for rays that are close to horizontal. Thus light rays curve slightly around Earth’s surface. A small curvature is also caused by Earth’s gravitational field. The curved light rays cause objects below the mathematical horizon to appear above the visible horizon. So the bottom part of the rainbow in the photo includes some from below the mathematical horizon. This slightly longer bit of rainbow has been compressed by the refraction, increasing its apparent curvature.
■ I once witnessed an extreme version of this when approaching Sydney airport. Our aircraft was supposed to be landing on the northern runway, but a southerly buster (the Aussie term for a strong summer cold front) was about to pass, and the wind direction flipped from north to south. While changing direction to land on the southerly runway instead, we could actually see the edge of the cold front as it traversed the runways, so large was the temperature difference. It was as if a huge elongated lead crystal dome paperweight
We pay £25 for every answer published in New Scientist. To answer a question or ask a new one please email [email protected]. Questions should be scientific enquiries about everyday phenomena, and both questions and answers should be concise. We reserve the right to edit items for clarity and style. Please include a postal address, daytime telephone number and email address. You can also send questions and
answers to The Last Word, New Scientist, 25 Bedford Street, London, WC2E 9ES. New Scientist Ltd retains total editorial control over the published content and reserves all rights to reuse question and answer material that has been submitted by readers in any medium or in any format and at any time in the future. All unanswered questions and previous questions and answers are at newscientist.com/lastword/
Feel the heat
This is the reverse of the effect that causes desert mirages, in which the hot ground heats air close to it, making it less dense. In this case, the speed of light is faster close to the ground, so light is refracted upwards, giving a mirror effect that looks like water. Richard Parkins Cambridge, UK
We sweat to cool our bodies by evaporation, but when we wipe the sweat away – a natural reaction in humans – are we defeating that function and making ourselves hotter? Do we also hasten dehydration by making our bodies sweat more, to replace what we’ve wiped away? In other words, should we just leave the sweat to do its job?
■ The damp squib answer is that neither leaving the sweat nor wiping it away will make any difference to how well or badly you keep cool and hydrated. Why? First, we are constantly sweating, but, by the time we feel it on our skin, sweat has reached the limit of its evaporative cooling capacity. Leaving it won’t make more of it evaporate or make what is there evaporate faster. Second, sweating is governed by how hot we are, not by how sweaty or dry our skin is. Because the sweat on our skin is no longer evaporating and cooling us, our sweat glands continue to pour it out. This is why beads of sweat accumulate on our brows – to the point of dripping down our noses and proverbially getting up them.
Leave it, and you are just letting gravity wipe it away for you. You will dehydrate just as fast. And the muscular heat that our wiping actions generate? This could increase or decrease our rate of overheating, depending on how sedentary or vigorous the activity we are interrupting is. So, on balance, while it may be a hot topic for theoretical debate, it will make no palpable difference. You would do better to switch on an electric fan, or invest muscular effort in wafting a hand-held one. This will disperse the layer of insulating air that clings to your skin and allow more evaporative cooling. If the ambient air temperature is below body heat, it may directly cool you somewhat. Alternatively, wear cotton garments. The material will soak up the sweat and provide a greater evaporative surface. Len Winokur Leeds, UK
This week’s question BLOW OUT
Space movies regularly dispatch characters through an explosively decompressed airlock. But how likely is such a rapid ejection, taking into account the victim’s location in the airlock and its volume? Assuming they are wearing a space suit, what is the best way to avoid expulsion, apart from grabbing something solid? Mark Hallinan Murwillumbah, New South Wales, Australia