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Greenhouse

Air Temperature

One might wonder how the air in our atmosphere warms up as it is transparent for heat radiation coming from the sun. We feel that heat, but that is in fact infrared radiation (IR) much like when we stand in front of a heater or open fire.

As air is known to be a good isolator (home construction with double walls with a gap in between as isolation is a good example) it is interesting to find out why and how air gets its temperature so that we feel comfortable in our house and in our environment, although we can not change outside air temperature at all.

On this page we take a deep dive in the process or processes that heat or warm up our air/ atmosphere.



Transparent Air

Our atmosphere consists mainly of Nitrogen and Oxygen (99%), both of which are transparent to incoming and outgoing infrared radiation and visible light too :-). The remaining 1% consists of Argon (0,93%) and Carbon Dioxide (0,04%) plus some other trace gases; see our page about atmospheric composition for more detail on that.

Heat transfer

There are five processes that transfer heat in air, they are:

  • Conduction - transfers heat via direct molecular contact between the earth and air
  • Convection - air which is warmed up by conduction travels upward in the atmosphere, cooling the surface
  • Radiation - will not heat up the atmosphere but only objects in it (Earth by the Sun)
  • Advection - this is the horizontal movement of an air mass with different properties
  • Turbulence - this causes the mixing of air layers with different temperatures

Conduction, Radiation & Convection

These are easily explained with the following example: A radiator (in a car engine or your house) does radiate a little when hot water is pumped through it. But the air is actually heated by touching/ conduction the radiator. Try holding your hand some distance away and then touching it. At a couple of inches away the heat is barely felt but by touching it, the heat is felt almost instantly.

Convection occurs when air touching the radiator moves upwards (because its density is lower, molecules further apart) taking the heat away, and this motion thus basically cools the radiator.

Advection & turbulence

Both are a result of the movement of air after it has warmed up by conduction and moving upwards by convection, resulting in the development of a lower pressure area (thermal low). Air is then moving in from nearby higher pressure areas and turbulence occurs when this moving air (known as wind) collides with objects on the surface or other air masses.

Sun -> Earth -> Atmosphere

As such, we can see that the mantra of Hans Schreuder "Sun heats Earth and Earth heats Atmosphere" is correct, see the next link on greenhouse effect?. The atmosphere does not heat the Earth (thermodynamically impossible due to difference in mass and temperature); on the contrary we can see the atmosphere as a giant cooling system.

The radiation from the Sun passes through the atmosphere and collides with the mass of the Earth, be it rocks, sands, prairies, forests, lakes, rivers and oceans. Conduction and convection then and only then warms the atmosphere, moving the warmth away from the planet into space.

Altitude

Pressure vs Altitude

With a gain in altitude the air molecules will get farther and farther apart, and thus their temperature drops. An air bubble which has been warmed by conduction rises by convection as long as its temperature is higher than the surrounding air. During ascend the air bubble also cools, adiabatically by virtue of lower air pressure. Until an equilibrium with the surrounding air is reached and vertical movement stops.

Atmospheric pressure

On the other hand, within a high pressure area air descends and adiabatically increases, due to the rising pressure, in temperature whereby clouds disolve. You will notice this effect with an inversion layer at a certain altitude, where low clouds rise up this layer and above the descending air is clear. The conclusion one might draw is that overall air pressure also influences the temperature of the air/ atmosphere.

Warm air

co2 spectrumco2 spectrum
CO2 spectrum

As the air is warmed by conduction and pressure, the so called 'greenhouse gases' also warm up and the molecules in the air re-radiate infrared. But this lower temperature, longer wavelength radiation does not get very far as the molecules are very tiny and are governed by the Inverse Square Law. And since CO2 radiates at a longer wavelength (around 13 µm) than the Earth, it can not warm the planet.
Those thinking that only the so called 'greenhouse gases' (1% of the atmosphere) get warm, should realize that its impossible for the air to be 99% cold and 1% warm.
And then for those greenhouse gases be able to warm up the Earth, as some say, is virtually impossible.

written by EAI.