FEEling Hot, FEEling ColD
知冷知热
l.J.f. ( Jo) Hermans,
Leiden University • The Netherlands • Email: Hermans@Physics.LeidenUniv.nl
Article available at http://www.europhysicsnews.org

即使天很冷，一点阳光便会使你感觉好多了。人们常常这样说，“天气预报说，气温15度，但是，阳光里至少25度“，这样的话中包含了一些热平衡的正确思想，但是，严格来讲是没有意义的。没有什么东西是”阳光里的温度“你怎么来测量它？不同的温度计由于其构造、光学性质等的不同，会给出不同的读数。气温唯一确定的定义是从空气中分子的平均动能来得到：½m = 3⁄2 kT.辐射与之无关。

直接测量分子的动能可不是易事。因此我们要用一个间接的方法：温度计。它很方便，但不总是可靠的。问题在于空气的热导率很低。这使得空气与温度计的热接触很差。因此，辐射很难被抑制。如果温度计在阳光里，用它做的测量便不准确。即使在阴影中，间接辐射也会使温度计产生小的偏差。所以，气象学家测量气温有一套严格的规则：温度计要放在一个通风性好的漆成白色的外套中，放在地面之上1.5米高处，等等。仔细想想，准确测量气温还真算是奇迹。

风是理解温度的另一迷惑之源。显而易见，如果风吹过我们的身体（或者，任何一个被周围环境加热的物体），热传导导致热损失将增加。原因是空气的绝缘层—-一般情况下只有几个毫米厚—–由于风吹会变薄。这个效应就好像是气温降低。这一表面上看来气温降低被称为“风寒”。尽管这是一个广为人知的概念，许多人不了解其内在的涵义。比如，有些记者常根据(风寒表)wind chill table得出这样的结论，汽车散热器上的水会在0度以上结冰，如果有风吹着…..

仔细想来，风寒是一个定义不清楚的概念。一方面，它依赖于我们穿的衣服。比如，在无限绝热的极限情况下，风对我们毫无影响，这样，风寒便变得没有意义了。可以这么说，如果风速达到无限时，风对我们感觉的“气温”的修正会趋近一个极限值。对于赤裸的皮肤，随着风速趋于无限，最终我们的皮肤会觉得气温和热损失只受我们体内热传导限制。如果皮肤外面都冻上了，这不是什么令人吃惊的结果。太阳和风都使得气温的概念有点令人感到困惑。幸亏动理学理论给物理学家提供了一个可靠的定义。

Even on a cold day, a bit of sunshine can make a tremendous difference. People will say things like ‘It is supposed to be 15°C according to the forecast, but in the sun it’s at least 25’. Although this may contain some truth in terms of heat balance, it is, strictly speaking, nonsense. There is no such thing as ‘temperature in the sun’. How would one measure that? Different types of thermometers hanging in the sun would give widely different readings, depending on construction, optical properties and the like. The only decent definition of air temperature is derived from the mean kinetic energy of the molecules: ½m = 3⁄2 kT. Radiation has nothing to do with it.

即使天很冷，一点阳光便会使你感觉好多了。人们常常这样说，“天气预报说，气温15度，但是，阳光里至少25度“，这样的话中包含了一些热平衡的正确思想，但是，严格来讲是没有意义的。没有什么东西是”阳光里的温度“你怎么来测量它？不同的温度计由于其构造、光学性质等的不同，会给出不同的读数。气温唯一确定的定义是从空气中分子的平均动能来得到：½m = 3⁄2 kT.辐射与之无关。

But measuring the kinetic energy of the molecules in a gas directly is not exactly a piece of cake.Therefore we use an indirect way: the thermometer. It’s easy to use, but not always reliable. The problem is the low thermal conductivity of air. This makes the thermal contact between the air and the thermometer very poor. As a consequence, the influence of radiation is hard to suppress. If the thermometer is in the sun, forget a reliable measurement. But even in the shade, indirect radiation will cause our thermometers to be slightly optimistic. No wonder that meteorologist have strict rules for determining the temperature: thermometers must be placed inside well-ventilated casings, which are painted white, placed 1,5 meter above the ground, etcetera. If you think about it, it’s almost a miracle that air temperatures are accurately measured at all.

直接测量分子的动能可不是易事。因此我们要用一个间接的方法：温度计。它很方便，但不总是可靠的。问题在于空气的热导率很低。这使得空气与温度计的热接触很差。因此，辐射很难被抑制。如果温度计在阳光里，用它做的测量便不准确。即使在阴影中，间接辐射也会使温度计产生小的偏差。所以，气象学家测量气温有一套严格的规则：温度计要放在一个通风性好的漆成白色的外套中，放在地面之上1.5米高处，等等。仔细想想，准确测量气温还真算是奇迹。
Wind is another source of misunderstanding, if it comes to temperature. Obviously, if the wind blows around our body (or, in fact, around any object that is heated above ambient temperature), the heat losses by conduction will increase. The reason is that the insulating layer of air – normally a few mm thick – will become thinner once the wind blows. The effect is the same as if the air temperature were lower. That seemingly lower temperature is often called the ‘wind chill’ factor. Although this is a widely known concept, many people are still missing the point. An example is the journalist who concluded, using the wind chill table, that the water in his car’s radiator would freeze well above the freezing point, if only the wind would blow…..

风是理解温度的另一迷惑之源。显而易见，如果风吹过我们的身体（或者，任何一个被周围环境加热的物体），热传导导致热损失将增加。原因是空气的绝缘层—-一般情况下只有几个毫米厚—–由于风吹会变薄。这个效应就好像是气温降低。这一表面上看来气温降低被称为“风寒”(wind chill,注意区别中医概念)。尽管这是一个广为人知的概念，许多人不了解其内在的涵义。比如，有些记者常根据(风寒表)wind chill table得出这样的结论，汽车散热器上的水会在0度以上结冰，如果有风吹着…..

If we think about it, wind chill is an ill-defined concept. For one thing, it depends upon the clothing that we wear. For example, in the limit of infinite insulation, wind would not bother us at all, and the wind chill factor would become meaningless. All we can say for sure is that any correction for wind must asymptotically reach a limiting value if the wind speed goes to infinity. Consider bare skin: eventually, our skin would assume the air temperature, and the heat losses would be limited only by conduction inside our own body. Not an appealing prospect, if it freezes outside. Sun and wind: both make the concept of temperature a bit fuzzy. Thank heaven that kinetic theory provides us physicists with a reliable definition.

仔细想来，风寒是一个定义不清楚的概念。一方面，它依赖于我们穿的衣服。比如，在无限绝热的极限情况下，风对我们毫无影响，这样，风寒便变得没有意义了。可以这么说，如果风速达到无限时，风对我们感觉的“气温”的修正会趋近一个极限值。对于赤裸的皮肤，随着风速趋于无限，最终我们的皮肤会觉得气温和热损失只受我们体内热传导限制。如果皮肤外面都冻上了，这不是什么令人吃惊的结果。太阳和风都使得气温的概念有点令人感到困惑。幸亏动理学理论给物理学家提供了一个可靠的定义。
Come rain or shine.