- Email: [email protected]
Feel the heat
Last words past and present at newscientist.com/lastword
THE LAST WORD Feel the heat On a beautifully sunny, windless day last summer, I sat in my back garden and felt very warm indeed. The air temperature was 16°C, but there is no doubt that it felt more like a day when the thermometer read 25°C. Why the discrepancy?
correspondent happened to be standing beside a sun-drenched wall. If so, it would be an additional source of radiant heat. Lastly, the day in question was described as “beautiful”. Perhaps taking pleasure in the weather also contributed to a sensation of warmth. Tim McCulloch Sydney, Australia
n A number of factors besides ambient air temperature affect heat loss from our bodies, and n Thermal comfort depends your correspondent hints at to a large degree on your skin several explanations for their temperature, which in turn surprising warmth at 16°C – depends on heat exchange. windlessness, for example. The human body exchanges Moving air removes heat much heat with the surroundings by more efficiently from our skin four routes: conduction (that is, than a still layer of air at the between objects in contact), same temperature, hence the convection (to a surrounding familiar wind-chill factor. And liquid or gas), radiation (by the if your correspondent had “An air temperature of 20°C dressed warmly based on the can feel quite warm on a air temperature, then their still, sunny day, but cold on clothing would have trapped a clear, windy night” an insulating layer of air. Our bodies also radiate heat, and if objects radiating at us absorption and emission of are colder than our skin then electromagnetic radiation) and we experience a net heat loss. evaporation (the phase change On a sunny day, the sun is a major of water from liquid to gas). source of radiant heat, reducing Exchange by each of these our net heat loss. routes depends on different If you are in a car, then to be characteristics of the comfortable you may need to set environment, which is why it is its air conditioning’s temperature fiendishly difficult to develop a several degrees lower on a day single index of thermal comfort. with sunshine streaming through Your correspondent identifies the windows, compared with a the important role that radiation sunless day. This is because the plays in heat balance and thermal sun’s radiation is directly heating comfort. In the absence of a strong your body, regardless of the air radiation source (like the sun), temperature. heat exchange at 16°C will be Also, I wonder if your dominated by convectional loss,
The writers of answers that are published in the magazine will receive a cheque for £25 (or US$ equivalent). Answers should be concise. We reserve the right to edit items for clarity and style. Please include a daytime telephone number and an email address if you have one. New Scientist retains total editorial control over the published content. New Scientist Ltd reserves all rights to reuse all question and answer material that has been
submitted by readers in any medium or in any format and at any time in the future. Send questions and answers to The Last Word, New Scientist, 110 High Holborn, London WC1V 6EU, UK, by email to [email protected] or visit www.newscientist.com/topic/lastword (please include a postal address in order to receive payment for answers). Unanswered questions can also be found at this URL.
and you will feel cold. But with a strong source (and especially in the absence of wind), radiation exchange will dominate – in this case, radiant heat input from the sun to the skin. My third-year students collect data on skin temperature and thermal comfort in a range of environments, and are amazed to discover that an air temperature of 20°C can feel quite warm on a still, sunny day, but unbearably cold on a clear, windy night. Shane Maloney School of human sciences University of Western Australia Crawley, Western Australia n Meteorological stations place most of their instruments inside a special box to avoid direct sunlight. They cannot be in the shade or lee of a tree or a building because they must also measure wind speed and direction, so the “Stevenson screen” was devised: a white-painted, well-vented box that has a double roof with an air space between to avoid the effects of direct solar heating. Incidentally, the primitive 1960 Land Rover I once owned in Kenya had a similarly constructed “tropical” roof, which was extremely effective at keeping the vehicle’s interior cool under the equatorial sun. For the same reason, an outdoor thermometer should be in a shady location and shielded from the rain, but at 16°C, you would definitely want to sit in the sun. Depending on the latitude and time of day, you could easily
experience temperatures 10°C warmer than a shade thermometer would indicate. Don’t forget to apply your sunscreen. Peter Bursztyn Barrie, Ontario, Canada
Salvaging Hubble The illustrious Hubble Space Telescope will eventually re-enter Earth’s atmosphere and be destroyed – or so I understand. Could it be returned to Earth safely and put in a museum? If so, what would be the cheapest way to do it? (Continued)
Our apologies to Sam Palasciano whose earlier submission to this question on 3 June contained an error introduced by us – Ed n Hubble’s primary mirror weighs roughly 1800 pounds or 800 kilograms, not 450 as the article stated. This could be significant if someone wanted to seriously pursue this question. However, I would much rather someone came up with a way of extending the life of the Hubble telescope in orbit. The replacement Webb Telescope, as I understand it, operates at different wavelengths. Hubble was designed to have a more useful operating window, including both ultraviolet and infrared, an advantage that will be lost when it is closed down. Sam Palasciano Oceanside, California, US
THE LAST WORD Feel the heat On a beautifully sunny, windless day last summer, I sat in my back garden and felt very warm indeed. The air temperature was 16°C, but there is no doubt that it felt more like a day when the thermometer read 25°C. Why the discrepancy?
correspondent happened to be standing beside a sun-drenched wall. If so, it would be an additional source of radiant heat. Lastly, the day in question was described as “beautiful”. Perhaps taking pleasure in the weather also contributed to a sensation of warmth. Tim McCulloch Sydney, Australia
n A number of factors besides ambient air temperature affect heat loss from our bodies, and n Thermal comfort depends your correspondent hints at to a large degree on your skin several explanations for their temperature, which in turn surprising warmth at 16°C – depends on heat exchange. windlessness, for example. The human body exchanges Moving air removes heat much heat with the surroundings by more efficiently from our skin four routes: conduction (that is, than a still layer of air at the between objects in contact), same temperature, hence the convection (to a surrounding familiar wind-chill factor. And liquid or gas), radiation (by the if your correspondent had “An air temperature of 20°C dressed warmly based on the can feel quite warm on a air temperature, then their still, sunny day, but cold on clothing would have trapped a clear, windy night” an insulating layer of air. Our bodies also radiate heat, and if objects radiating at us absorption and emission of are colder than our skin then electromagnetic radiation) and we experience a net heat loss. evaporation (the phase change On a sunny day, the sun is a major of water from liquid to gas). source of radiant heat, reducing Exchange by each of these our net heat loss. routes depends on different If you are in a car, then to be characteristics of the comfortable you may need to set environment, which is why it is its air conditioning’s temperature fiendishly difficult to develop a several degrees lower on a day single index of thermal comfort. with sunshine streaming through Your correspondent identifies the windows, compared with a the important role that radiation sunless day. This is because the plays in heat balance and thermal sun’s radiation is directly heating comfort. In the absence of a strong your body, regardless of the air radiation source (like the sun), temperature. heat exchange at 16°C will be Also, I wonder if your dominated by convectional loss,
The writers of answers that are published in the magazine will receive a cheque for £25 (or US$ equivalent). Answers should be concise. We reserve the right to edit items for clarity and style. Please include a daytime telephone number and an email address if you have one. New Scientist retains total editorial control over the published content. New Scientist Ltd reserves all rights to reuse all question and answer material that has been
submitted by readers in any medium or in any format and at any time in the future. Send questions and answers to The Last Word, New Scientist, 110 High Holborn, London WC1V 6EU, UK, by email to [email protected] or visit www.newscientist.com/topic/lastword (please include a postal address in order to receive payment for answers). Unanswered questions can also be found at this URL.
and you will feel cold. But with a strong source (and especially in the absence of wind), radiation exchange will dominate – in this case, radiant heat input from the sun to the skin. My third-year students collect data on skin temperature and thermal comfort in a range of environments, and are amazed to discover that an air temperature of 20°C can feel quite warm on a still, sunny day, but unbearably cold on a clear, windy night. Shane Maloney School of human sciences University of Western Australia Crawley, Western Australia n Meteorological stations place most of their instruments inside a special box to avoid direct sunlight. They cannot be in the shade or lee of a tree or a building because they must also measure wind speed and direction, so the “Stevenson screen” was devised: a white-painted, well-vented box that has a double roof with an air space between to avoid the effects of direct solar heating. Incidentally, the primitive 1960 Land Rover I once owned in Kenya had a similarly constructed “tropical” roof, which was extremely effective at keeping the vehicle’s interior cool under the equatorial sun. For the same reason, an outdoor thermometer should be in a shady location and shielded from the rain, but at 16°C, you would definitely want to sit in the sun. Depending on the latitude and time of day, you could easily
experience temperatures 10°C warmer than a shade thermometer would indicate. Don’t forget to apply your sunscreen. Peter Bursztyn Barrie, Ontario, Canada
Salvaging Hubble The illustrious Hubble Space Telescope will eventually re-enter Earth’s atmosphere and be destroyed – or so I understand. Could it be returned to Earth safely and put in a museum? If so, what would be the cheapest way to do it? (Continued)
Our apologies to Sam Palasciano whose earlier submission to this question on 3 June contained an error introduced by us – Ed n Hubble’s primary mirror weighs roughly 1800 pounds or 800 kilograms, not 450 as the article stated. This could be significant if someone wanted to seriously pursue this question. However, I would much rather someone came up with a way of extending the life of the Hubble telescope in orbit. The replacement Webb Telescope, as I understand it, operates at different wavelengths. Hubble was designed to have a more useful operating window, including both ultraviolet and infrared, an advantage that will be lost when it is closed down. Sam Palasciano Oceanside, California, US