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?
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鈥� Your previous answers explained why air temperatures are higher in August than April, but not why radiation feels more intense.
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Earth鈥檚 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鈥檚 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 black-body heat radiation from our solid surroundings. The surface temperatures of these solid surroundings such as the ground, walls and trees are typically 10掳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
鈥� Radiation from the sun is more than matched by convection to the air. Face the sun and your bare skin receives heat by radiation. The sun delivers about 1400 watts of energy per square metre (W/m2) to Earth, but the atmosphere absorbs and reflects much of this.
On a very good day in the UK perhaps 300W/m2 will reach the ground. Your face presents about 0.03m2 to the sun, so will receive about 9 watts of solar heat. When your skin and the air temperatures are both around 25掳C you don鈥檛 lose heat to the air, but on a cool 10掳C day your face will lose about 5W, and substantially more if there鈥檚 a breeze. Also your face will radiate 3W to nearby cold walls if they are at 5掳C.
鈥淎 car gets very hot when left in the sun, but only on a hot day when heat is not lost by convection to air鈥�
A car gets very hot when left in the sun, but only on a hot day when heat is not lost by convection to the air. Just like the car, your skin is only interested in the total energy balance, so don鈥檛 strip off in the spring.
Hugh Hunt, Trinity College, Cambridge, UK
鈥� Skin contains thermoreceptors 鈥� nerve cells that variously detect heat or cold within particular temperature ranges, as well as changes in temperature. Perceived temperature is the brain鈥檚 response to the sum of information received from these receptors. So, for example, direct radiation from the sun will quickly heat up the skin and produce a warm response to the increasing temperature.
鈥淚n August, after months of warmth, the air humidity is higher than in April, which makes you feel warmer鈥�
This is mitigated by other receptors responding to the temperature of the air touching the skin (warmer in August than in April), and also any breeze or convection currents that might be taking heat away from the skin and producing a cooling response.
A likely further factor is that in August, after months of summer warmth, the air humidity is higher than in April, which makes the body feel warmer because the cooling induced by sweating is less effective.
The feeling of a hotter sun is therefore an illusion because the brain does not detect it in isolation 鈥� it is responding to the other factors as well.
Richard Swifte, Darmstadt-Eberstadt, Germany
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