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Hair shadows
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THE LAST WORD Touch of frost If I park my car overnight on an open driveway in cold weather, I have to scrape frost from all of its windows in the morning. But if I park it under the adjacent carport, which is open on all sides but has a roof, no frost forms. Why does a roof make such a difference?
in them. Glass is also a poor conductor, so it is hard for residual heat to flow into it from elsewhere in the body of the car. If the rate of heat flow into the glass by convection from the surrounding air is less than the rate at which it loses energy by radiation, then its temperature can drop until it is below freezing, even if the air temperature is above zero. For a car parked under a carport, the roof acts as a barrier to
n The frost forms because the temperature of the windscreen has dropped below 0°C, and so water vapour in the air touching “Ancient Persians froze water by putting it in it condenses then freezes. So the shallow trays exposed question is, how does the glass to the night sky” get so cold when the air around it is often well above freezing point? The answer is heat loss by prevent radiation escaping radiation. directly into outer space. Instead All objects constantly emit it is absorbed by the roof and radiation, usually in the infrared some is then radiated back to spectrum. Most objects are the car. That is why frost won’t therefore bathed in infrared normally form underneath. radiation from their surroundings, The ancient Persians used this and the net rate of heat loss by phenomenon to freeze water radiation roughly equals the rate overnight in the desert by pouring at which radiation is absorbed. it into shallow trays in deep, well During the daytime, the sky insulated holes exposed to the is filled with scattered sunlight night sky above. The ice was then (mainly in the blue spectrum) stored in special insulated and and so a car parked outside evaporatively cooled buildings receives radiant energy from called yakhchals to be served to above. However, when parked in the nobility on request. These the open on a clear night, there yakhchals can still be seen today. is nothing between the car and Simon Iveson outer space, which is close to Faculty of Engineering and Built absolute zero temperature, so the Environment car receives negligible radiation University of Newcastle from space. However, it is still New South Wales, Australia radiating its own energy and so its temperature starts to drop. n Radiant heat exchange with the Windscreens are relatively thin night sky explains why frost can and have little heat energy stored form on cars even on nights when
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/
the air temperature does not fall below freezing – because the surface temperature of a car can be several degrees lower than air temperature. But this all depends on the wind. When the wind blows, convective heat exchange becomes more important and a car is less likely to become frosted up because the air temperature (rather than the radiation temperature) becomes the main determinant of the surface temperature of the car. Shane Maloney University of Western Australia
Silver surfers Recently, my wife’s silver car became covered by a swarm of flying ants. This was odd because none of the other cars parked nearby had any on at all. A friend mentioned it had happened to his silver car too and after searching online it seems it’s a known phenomenon. Why is it that the swarm seems to be attracted to something silver?
n According to the Rayleigh sky model, sunlight is polarised to a different extent at different angles to the sun, with a maximum at a 90-degree angle. Many insects, including ants, navigate using this polarised light, travelling at a certain angle to the sun. Hence they will head towards the polarised sunlight reflected from the body of a car if it happens to be at the correct angle and on the correct flight path.
This might explain why silver cars do not always attract insects but, because silver and white cars are better reflectors than dark cars, they get more than their fair share. Also, light that is reflected from the surface of a body of water gets partially polarised, so certain insects might confuse sunlight reflected from a car with a source of water. Mike Follows Sutton Coldfield, West Midlands, UK
This week’s questions HAIR SHADOWS
During the recent total solar eclipse in the US, I overlapped my hands, using the gaps between my fingers to form “pinholes”. Just before the start of totality, someone noticed that the shadows formed through this “pinhole camera” onto a white board were so sharp that the individual hairs on my arms were visible. How thin must the solar crescent be for the shadows to be this sharp, and how soon before totality would this occur? Herb Petitjean Versailles, Kentucky, US GOING FOR GOLD
Given that athletics races can be won or lost by a margin as small as a hundredth of a second, do athletes risk a gold medal by wearing gold chains? Richard Sleeman Bristol, UK
THE LAST WORD Touch of frost If I park my car overnight on an open driveway in cold weather, I have to scrape frost from all of its windows in the morning. But if I park it under the adjacent carport, which is open on all sides but has a roof, no frost forms. Why does a roof make such a difference?
in them. Glass is also a poor conductor, so it is hard for residual heat to flow into it from elsewhere in the body of the car. If the rate of heat flow into the glass by convection from the surrounding air is less than the rate at which it loses energy by radiation, then its temperature can drop until it is below freezing, even if the air temperature is above zero. For a car parked under a carport, the roof acts as a barrier to
n The frost forms because the temperature of the windscreen has dropped below 0°C, and so water vapour in the air touching “Ancient Persians froze water by putting it in it condenses then freezes. So the shallow trays exposed question is, how does the glass to the night sky” get so cold when the air around it is often well above freezing point? The answer is heat loss by prevent radiation escaping radiation. directly into outer space. Instead All objects constantly emit it is absorbed by the roof and radiation, usually in the infrared some is then radiated back to spectrum. Most objects are the car. That is why frost won’t therefore bathed in infrared normally form underneath. radiation from their surroundings, The ancient Persians used this and the net rate of heat loss by phenomenon to freeze water radiation roughly equals the rate overnight in the desert by pouring at which radiation is absorbed. it into shallow trays in deep, well During the daytime, the sky insulated holes exposed to the is filled with scattered sunlight night sky above. The ice was then (mainly in the blue spectrum) stored in special insulated and and so a car parked outside evaporatively cooled buildings receives radiant energy from called yakhchals to be served to above. However, when parked in the nobility on request. These the open on a clear night, there yakhchals can still be seen today. is nothing between the car and Simon Iveson outer space, which is close to Faculty of Engineering and Built absolute zero temperature, so the Environment car receives negligible radiation University of Newcastle from space. However, it is still New South Wales, Australia radiating its own energy and so its temperature starts to drop. n Radiant heat exchange with the Windscreens are relatively thin night sky explains why frost can and have little heat energy stored form on cars even on nights when
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/
the air temperature does not fall below freezing – because the surface temperature of a car can be several degrees lower than air temperature. But this all depends on the wind. When the wind blows, convective heat exchange becomes more important and a car is less likely to become frosted up because the air temperature (rather than the radiation temperature) becomes the main determinant of the surface temperature of the car. Shane Maloney University of Western Australia
Silver surfers Recently, my wife’s silver car became covered by a swarm of flying ants. This was odd because none of the other cars parked nearby had any on at all. A friend mentioned it had happened to his silver car too and after searching online it seems it’s a known phenomenon. Why is it that the swarm seems to be attracted to something silver?
n According to the Rayleigh sky model, sunlight is polarised to a different extent at different angles to the sun, with a maximum at a 90-degree angle. Many insects, including ants, navigate using this polarised light, travelling at a certain angle to the sun. Hence they will head towards the polarised sunlight reflected from the body of a car if it happens to be at the correct angle and on the correct flight path.
This might explain why silver cars do not always attract insects but, because silver and white cars are better reflectors than dark cars, they get more than their fair share. Also, light that is reflected from the surface of a body of water gets partially polarised, so certain insects might confuse sunlight reflected from a car with a source of water. Mike Follows Sutton Coldfield, West Midlands, UK
This week’s questions HAIR SHADOWS
During the recent total solar eclipse in the US, I overlapped my hands, using the gaps between my fingers to form “pinholes”. Just before the start of totality, someone noticed that the shadows formed through this “pinhole camera” onto a white board were so sharp that the individual hairs on my arms were visible. How thin must the solar crescent be for the shadows to be this sharp, and how soon before totality would this occur? Herb Petitjean Versailles, Kentucky, US GOING FOR GOLD
Given that athletics races can be won or lost by a margin as small as a hundredth of a second, do athletes risk a gold medal by wearing gold chains? Richard Sleeman Bristol, UK