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 insulated 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 influence the air temperature outside at all.
On this page we take a deep dive in the process or processes that heat or warm up our planet/ air/ atmosphere.
The composition of 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.
There are five processes that are involved with the transfer of heat (energy) from the Earth to the atmosphere, they are:
We will discuss these below and start with the definition of temperature.
Temperature is basically the numerical indicator of the amount of internal energy (kinetic energy of the molecules and atoms) in the air. With this we can define absolute zero as the temperature where molecular movement has stopped. It is at zero Kelvin or minus 273,15 °C.
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 with 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.
Conductivity of a substance (thermal conductivity) depends largely on its property to conduct heat, silver has the highest and air the lowest. It is expressed in Watts per meter-Kelvin (W ⋅ m-1 ⋅ K-1). More details on thermal conductivity here.
Convection occurs when air touching the radiator moves upwards (because the density of warm air is lower, molecules further apart) taking the heat away, and this motion thus basically tries to cools the radiator.
All objects emit heat as radiation as long there is a difference between it and its surroundings and the intensity diminishes with the distance squared. This emitted (IR) energy passes through air, space and water (although it penetrates water not very much). This energy transfer always occurs from a warmer body to a colder body.
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 (known as a thermal low). Air is then moving in from nearby higher pressure areas and turbulence develops when this moving air (also known as wind) collides with objects on the surface or other air masses.
Whether by radiation, conduction or convection; the temperature response of a substance (input or output) varies between substances. It is know as specific heat capacity expressed in units of Joules per gram-Kelvin (J ⋅ g-1 ⋅ K-1).
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 the 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 (through conduction and convection). It only re-radiates a miniscule bit of the outgoing infrared radiation.
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. It is on average, when the sun is directly overhead some 1000 W/m2. Conduction and convection then and only then warms the atmosphere, moving the warmth away from the planet into space (which is around 0 K).
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 that of 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. The amount of moisture in that air bubble also determines how fast it cools, more moisture means more energy in the bubble thus slower cooling while ascending.
The above process, rising clouds taking away the warmth from the surface, only serve to cool the planet. And thus forms a negative feedback system. The overall cooling effect of H2O is evidenced by the fact that the wet lapse rate is significantly less than the dry lapse rate in the troposphere.
On the other hand, within a high pressure area air descends and adiabatically increases, due to the rising pressure, in temperature whereby clouds dissolve due to lowering of relative humidity. You will notice this effect with an inversion layer at a certain altitude, where low clouds rise up this layer and above it, the descending air is clear. The conclusion one might draw is that overall air pressure also influences the temperature of the air/ atmosphere.
The gas law (Pv = nRT, for an ideal gas) explains this phenomenon perfectly: pressure is proportional to temperature, if the number of particles and the volume of the container are constant. For an indepth presentation about gas laws and gas properties, we have a document by Dr. Pedro Julio Villegas Aguilar about "General Chemistry, Gas Laws".
As the air is warmed by the above processes and pressure, the so called 'greenhouse gases' also warm up and these molecules in the air re-radiate at lower infrared frequencies. But this lower temperature, longer wavelength radiation does not get very far as the molecules are very tiny and radiation is governed by the Inverse Square Law.
And since CO2 radiates at a longer wavelength (mostly around 13 µm, with about 2 W/m2) than the Earth and has lower density, it can not warm the planet. With a lower temperature than the planet, and according to the laws of thermodynamics, a cooler object (or molecule) can not heat up a warmer object.
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 thermodynamically impossible. Due to the differences in temperature and densities. Also known as heat capacity, where the capacity of air is order of magnitudes less than that of the planet.
Should you wish to get up to speed regarding the laws of thermodynamics, follow the next link: Thermodynamic Laws.