How do I explain why the Tibetan plateau is colder than lowlands at similar latitudes?

Question

A common layman explanation for why does it get colder to higher elevations (considering only the troposphere here) qualitatively boils down to The Sun heats the Earth's surface and the Earth's surface heats the atmosphere. I remember that, every time I heard or read this explanation as a child, I thought: then why is the Tibetan plateau still colder than lowlands at the same latitude? After all, the solar energy reaching the surface in Tibet is no less. Clearly there is more to it.

Considering a remark from a quantum physics lecturer that you only really understand something after you've taught the subject twice, I'm phrasing my question as such: How does one explain, in layman terms, why the Tibetan plateau is colder than lowlands at similar latitudes?

Answer 1

The answer is because the Earth is not a static system.

Due to the ideal gas law, air cools as it rises. This is referred to as the dry adiabatic lapse rate. However, you are curious why every location on earth is not the same temperature at the same latitude. We know this is not true. But why is not true? Weather.

The earth, as with most natural phenomenon, attempts to achieve static equilibrium. The differences in incoming and outgoing radiation leave the earth with net energy reserves that need to be balanced out. Weather is the mechanism that the Earth uses to attempt to achieve balance.

Okay, so we know that weather is disrupting static equilibrium on earth. But how does that work? Wind. Or better yet, advection. Advection is simply the transport of an atmospheric parameter (such as moisture, temperature, or rotation) from one location to another. So, wind can 'transport' temperature, moisture, and rotation.

You don't necessarily need a storm to advect any of these parameters. In fact, they are always in motion. Cold air is always moving away from the poles.

Still, that doesn't quite answer it, does it? Well, imagine you are hundreds or thousands of miles away from Tibet. You aren't in a mountainous region anymore but instead a flatlands at sea level. The environmental lapse rate is 6.5°C/1000m above this location (because we are not dealing with an air parcel). Tibet is 4500m - 8850m high. That would be around approximately 32.5°C of cooling at 5000m. The air that advects into Tibet from elsewhere is on average of 32.5-50°C cooler than at sea level.

So, it isn't so much that Tibet is receiving less radiation than other points at the same latitude (its not). Its more that, the air at other locations at the elevation of Tibet is much cooler than the air at sea level. Remember, we measure air temperature!

Answer 2

You are correct that solar input on the Tibetan plateau will be the same as a location at sea level at the same latitude. You are also correct that the Sun heats the Earths surface and that in turn heats the atmosphere. Now for the rest of the details.

• Albedo

  Albedo is a measure of "whiteness" and gives us an idea of how solar irradiance interacts with the Earth. You probably know that dark colored things tend to get really hot when left in the sun, while light colored objects do not get as hot. This is because when a photon from the Sun hits the Earth, it is either absorbed or scattered. Without getting into the dynamics of radiation, I'll just leave it that dark colors tend to absorb and light colors tend toward scattering. Now we translate this to albedo -- a high value "white" means solar input is being reflected back into the atmosphere; a low value means solar input is being absorbed. A forest or ocean will have low albedo. Snowpack and ice will have high albedo.

  However, the albedo in Tibet on average is only slightly higher than average (~0.35 as cited in the comments below) and so this effect is limited with only a slight decrease in absorbed solar radiation than average.

• Absorption

  The solar input that is absorbed in Tibet will excite the molecules in the ground and they will respond by warming up (and emitting longwave radiation). The emitted radiation upward does nothing to the air temperature, only the greenhouse gases are good at absorbing these wavelengths, regular dry air is not. It is notable that at these altitudes will also be less water vapor, which is probably the best greenhouse gas. If radiation isn't going to warm the air, that leaves conduction and convection.

• Conduction

  The Earth heats the atmosphere via conduction with the lowest level of molecules in the atmosphere. For conduction to happen, molecules need to touch. At the lower pressures at high altitude, there are less molecules of air and conduction will be less efficient than at lower altitudes.

• Atmosphere

  Air getting up to the Tibetan Plateau will have blown in, already at high altitude, or it will be blown upslope from lower elevations. In either case, this air will tend to be cold and dry. Upslope flow will tend to cool around 6C/km until it dries out (rain, clouds) and then cool at 10C/km. For the air already at altitude, the average elevation of Tibet is around 580 hPa and the standard temperature at that pressure is -14 C. Of these two effects, cool air advection of air already at this altitude dominates over upslope adiabatic flow . This air at altitude tends to move faster than at the surface and you will have a constant flow of new incoming, cold air blowing over the surface of Tibet.

Putting it all together:

The slightly higher albedo is our first suspect as to why Tibet is colder than lower elevations at the same latitude -- more of the sunlight is reflected to space and does not contribute to warming the ground. The next suspect is the less efficient heating of the atmosphere by the surface. This is followed up by the atmosphere that is already much colder than the surface at sea level (at similar latitudes). Take a much colder airmass than sea level, heat it up less than at sea level and you end up colder than you would be at sea level.

Answer 3

Temperature is a measure of kinetic energy which, in the atmosphere, is due to the number of molecules and their speed. There is less atmosphere at higher altitudes (e.g. lower pressure, less molecules), so a measure of temperature will be lower there because less molecules are moving around. That is the fundamental/basic reason that temperature is lower at higher altitudes in the troposphere. Furthermore, since there are a smaller number of greenhouse gas molecules trapping energy, more heat is lost to space at higher altitudes.