Subtitles are available in most human languages.
Donation for this channel https://paypal.me/YongTuition

Transcript
Temperature, heat, radiation, and energy are essential concepts in understanding weather and climate. When you listen to the daily weather forecast or have a chat with a stranger about climate, are you sure you know what temperature is? #temperature #stratosphere #troposphere
In fact, based on my teaching experience back in the 1980s, it would take several weeks for undergraduate students to understand temperature and related ideas in physics.
By definition, the temperature of an object is proportional to the statistical mean value of kinetic energy of randomly moving particles, such as a nitrogen or an oxygen molecule in air, at an thermal equilibrium. In this US standard atmosphere version 1972, it was loosely called as “kinetic temperature.”
For gases, such a kinetic temperature can be determined from this simple equation, namely, the mean kinetic energy of a molecule is equal to the product of the degree of its freedom, f, and half kT, where k is the Boltzmann constant. For a point-mass, the degree of freedom is 3.

If we treat air near the surface as an ideal gas, which is made by many point-mass particles, then we can calculate the air temperature from the measured air pressure and the mean air density, which is very accurate and independent of any radiation. By the way, this temperature is roughly the same as subjective feelings by us, such as hot, warm, and cold. Why did I say “roughly”? Because human is far from just a thermometer.

Talking about an thermometer, I would mention a commonly used temperature sensor by experimental physicists, which is based on electric resistance that is temperature dependent, such as this simple so-called thermal couple. The electric potential difference can be easily measured and converted in to temperature in K, using an appropriate empirical formula.

In fact, such electric resistance based thermometers have been extensively applied in measuring the atmospheric temperature distributions in the atmosphere, as noticed in this US standard atmospheric document.

Nevertheless, the reading of this kind of thermometer becomes unreliable whenever the air density drops to too low to allow frequent molecular collisions, which are essential for obtaining a mean kinetic energy, I mean the population mean value, statistically speaking.



Take Fig 25 in the US standard atmosphere, for example, the range of systematic variability of the atmospheric temperature increases with the altitude. That’s why both the stratospheric cooling and the stratospheric warming have been reported from time to time. For Manabe and his followers, this is a never ending nightmare, because their global warming theory is based the stratospheric cooling only.

Perhaps you may ask, why don’t we use the infrared thermometer, such as this one, instead? Good question. In fact, many instrumental observations have been based on radiation detection to determine the atmospheric temperature distributions, remotely, either on the ground or from satellites.

In this diagram, you can see a complete vertical temperature profile in the atmosphere, from the sea level to over 500 km, which is actually the space, or the top of the atmosphere. As you can see, the atmospheric temperature can be over 1000K in the thermosphere. How come?

Well, to understand why the temperature at the TOA seems so high, you need to get familiar with another type temperature used in physics, called brightness temperature, or emission temperature, or some people call it a “radiance temperature,” of a radiation source.
Brightness temperature is defined as in terms of radiation, or electromagnetic wave, intensity, based on the Stefan-Boltzmann law. In other words, brightness temperature is not associated with molecular kinetic energy, but a phenomenological describer for how strong the radiation can be observed there.
To be more specific, you have to imagine the measured radiation is generated by an invisible blackbody whose surface equilibrium temperature is determined by the radiation intensity in W per meter square. If you do want to attribute the radiation to a grey body, such as a thin layer in the atmosphere, then brightness temperature would be higher. Why?
For instance, the theoretical global mean value of OLR should be close to 239 Wm-2. If you treat the planet with the atmosphere as a blackbody, then the brightness temperature, or the emission temperature, would be close to 255K, which happens to be the kinetic temperature of the atmosphere at altitude 5 km above the sea level, which is called “emission altitude” by many. .