- Email: [email protected]
Turning over a new leaf
Last words past and present at newscientist.com/lastword
THE LAST WORD Seasons in the sun In August in England, eight weeks after the summer solstice, the direct heat from the sun on a fine day feels much hotter than it does in April, eight weeks before the solstice, although its elevation is the same. I understand that the air temperature will be warmer, but why does the radiation feel more intense? Or is it an illusion? (Continued)
n Your previous answers explained why air temperatures are higher in August than April, but not why radiation feels more intense. Earth’s orbit is not quite circular, and the distance between Earth and the sun is 0.6 per cent greater in August than it is in April. The inverse square law tells us that in August the sun’s radiation is actually 1.2 per cent less intense than it is in April, but this difference is too small for us to notice. However, we also receive blackbody heat radiation from our solid surroundings. The surface temperatures of these solid surroundings such as the ground, walls and trees are typically 10°C (or 3 per cent) higher in August than in April. To quantify the effect of thermal lag and radiation from the surroundings, enjoy an alfresco glass of red wine in the fading heat of the day. Then try it in the hours before dawn. David Craig Edinburgh, UK
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
n Radiation from the sun is more brain’s response to the sum of information received from these than matched by convection to the air. Face the sun and your bare receptors. So, for example, direct radiation from the sun will skin receives heat by radiation. The sun delivers about 1400 watts “In August, after months of of energy per square metre warmth, the air humidity is (W/m²) to Earth, but the higher than in April, which atmosphere absorbs and reflects makes you feel warmer” much of this. On a very good day in the UK quickly heat up the skin and perhaps 300W/m² will reach produce a warm response to the the ground. Your face presents increasing temperature. about 0.03m² to the sun, so will This is mitigated by other receive about 9 watts of solar receptors responding to the heat. When your skin and the air temperature of the air touching temperatures are both around the skin (warmer in August than 25°C you don’t lose heat to the in April), and also any breeze or “A car gets very hot when convection currents that might left in the sun, but only on be taking heat away from the a hot day when heat is not skin and producing a cooling lost by convection to air” response. A likely further factor is that in August, after months of summer air, but on a cool 10°C day your warmth, the air humidity is face will lose about 5W, and higher than in April, which makes substantially more if there’s a the body feel warmer because the breeze. Also your face will radiate cooling induced by sweating is 3W to nearby cold walls if they less effective. are at 5°C. The feeling of a hotter sun is A car gets very hot when left therefore an illusion because in the sun, but only on a hot the brain does not detect it in day when heat is not lost by isolation – it is responding to convection to the air. Just like the car, your skin is only interested in the other factors as well. Richard Swifte the total energy balance, so don’t Darmstadt-Eberstadt, Germany strip off in the spring. Hugh Hunt Trinity College, Cambridge, UK n Skin contains thermoreceptors – nerve cells that variously detect heat or cold within particular temperature ranges, as well as changes in temperature. Perceived temperature is the
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.
This week’s questions
LIFE IN THE DOME
When we were hiking the Oyster Dome Trail in Whatcom County near Puget Sound in Washington,
I found this strange plant – or is it a fungus (see photo)? Can anyone identify it? Milo Seal Seattle, Washington, US TURNING OVER A NEW LEAF
I’m puzzled by beech trees. They are deciduous and their leaves turn bronze in the autumn, but they don’t fall off the tree. Instead, they hang on until spring when other plants are putting out their new leaves. How do these trees benefit from this different approach to deciduousness? Tony Leggatt Fortrose, Ross-shire, UK OL’ BLUE EYES
I’m told that blue-eyed people find coping with dazzling sun more difficult than brown-eyed people do. Why? Or if it’s untrue, what difference, if any, does eye colour make? Samuel Yorke London, UK
THE LAST WORD Seasons in the sun In August in England, eight weeks after the summer solstice, the direct heat from the sun on a fine day feels much hotter than it does in April, eight weeks before the solstice, although its elevation is the same. I understand that the air temperature will be warmer, but why does the radiation feel more intense? Or is it an illusion? (Continued)
n Your previous answers explained why air temperatures are higher in August than April, but not why radiation feels more intense. Earth’s orbit is not quite circular, and the distance between Earth and the sun is 0.6 per cent greater in August than it is in April. The inverse square law tells us that in August the sun’s radiation is actually 1.2 per cent less intense than it is in April, but this difference is too small for us to notice. However, we also receive blackbody heat radiation from our solid surroundings. The surface temperatures of these solid surroundings such as the ground, walls and trees are typically 10°C (or 3 per cent) higher in August than in April. To quantify the effect of thermal lag and radiation from the surroundings, enjoy an alfresco glass of red wine in the fading heat of the day. Then try it in the hours before dawn. David Craig Edinburgh, UK
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
n Radiation from the sun is more brain’s response to the sum of information received from these than matched by convection to the air. Face the sun and your bare receptors. So, for example, direct radiation from the sun will skin receives heat by radiation. The sun delivers about 1400 watts “In August, after months of of energy per square metre warmth, the air humidity is (W/m²) to Earth, but the higher than in April, which atmosphere absorbs and reflects makes you feel warmer” much of this. On a very good day in the UK quickly heat up the skin and perhaps 300W/m² will reach produce a warm response to the the ground. Your face presents increasing temperature. about 0.03m² to the sun, so will This is mitigated by other receive about 9 watts of solar receptors responding to the heat. When your skin and the air temperature of the air touching temperatures are both around the skin (warmer in August than 25°C you don’t lose heat to the in April), and also any breeze or “A car gets very hot when convection currents that might left in the sun, but only on be taking heat away from the a hot day when heat is not skin and producing a cooling lost by convection to air” response. A likely further factor is that in August, after months of summer air, but on a cool 10°C day your warmth, the air humidity is face will lose about 5W, and higher than in April, which makes substantially more if there’s a the body feel warmer because the breeze. Also your face will radiate cooling induced by sweating is 3W to nearby cold walls if they less effective. are at 5°C. The feeling of a hotter sun is A car gets very hot when left therefore an illusion because in the sun, but only on a hot the brain does not detect it in day when heat is not lost by isolation – it is responding to convection to the air. Just like the car, your skin is only interested in the other factors as well. Richard Swifte the total energy balance, so don’t Darmstadt-Eberstadt, Germany strip off in the spring. Hugh Hunt Trinity College, Cambridge, UK n Skin contains thermoreceptors – nerve cells that variously detect heat or cold within particular temperature ranges, as well as changes in temperature. Perceived temperature is the
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.
This week’s questions
LIFE IN THE DOME
When we were hiking the Oyster Dome Trail in Whatcom County near Puget Sound in Washington,
I found this strange plant – or is it a fungus (see photo)? Can anyone identify it? Milo Seal Seattle, Washington, US TURNING OVER A NEW LEAF
I’m puzzled by beech trees. They are deciduous and their leaves turn bronze in the autumn, but they don’t fall off the tree. Instead, they hang on until spring when other plants are putting out their new leaves. How do these trees benefit from this different approach to deciduousness? Tony Leggatt Fortrose, Ross-shire, UK OL’ BLUE EYES
I’m told that blue-eyed people find coping with dazzling sun more difficult than brown-eyed people do. Why? Or if it’s untrue, what difference, if any, does eye colour make? Samuel Yorke London, UK