It’s a common knowledge that as you gain altitude, pressure gets lower and so does temperature. Ever wonder why?
When I was pretty much puzzled by this fact, even when I was a kid. I remember asking my dad once, “Isn’t when you get up the mountain, it should get warmer as you are nearer to the sun?”… pretty silly question, huh? But scientist likes to ask this kind of silly questions… so probably that’s the reason why I chose physics later in my life =P
Okay, let’s analyze the first 2 parameters, namely altitude and pressure. Why should pressure decreases as altitude increases? To gain an appreciation on this subject, just recall a tradition, commonly played among college students or winning sport team in Singapore, called taupok. In this so-called tradition, the guys will start piling up, body-flat, one on top of the other. Obviously, if you are the guy at the bottom-most, you’ll experience the highest pressure that comes from everyone at the top. Gradually, as you are higher in the stack, you’ll experience less pressure. So does air column. The atmosphere pressure that we experience is a result of air molecules piling up on top of each other.
Pressure VS Altitude Profile. Notice that at the top of Everest (8848m) the pressure there is roughly 20-30% of the pressure at sea-level, which means you need to draw your breath 3-4 times to get the same amount of oxygen you breathe in once at sea-level. (Figure's taken from www.ux1.eiu.edu/~jpstimac/1400/atmos_struct.html)
After we gain a qualitative understanding how pressure’s related to altitude, let’s now examine the relation between pressure and temperature. Temperature is actually a macroscopic phenomenon, meaning that it is a resulting emergent property of the dynamics of many particles; each is behaving in its own, independent of each other, and yet put them altogether, we can measure their resultant effect. The particles I’m referring here is none other than the molecules that form up the air. What actually happens is that each molecule is floating around in the space freely. However, as they are floating freely obviously they are going to hit on each other. Hence, you can imagine billions and billion of this molecules are jiggling and bouncing around in the space, randomly moving and colliding on each other, performing a motion that physicists technically call “Brownian motion” (http://en.wikipedia.org/wiki/Brownian_motion).
The more molecules packed within a given space (i.e. the more dense the air is), the more “furious” this random Brownian motion is. As an analogy think of this: imagine if I pack-up 10 people into a hotel ball-room, and you’re one of the 10… then 100 people… and finally 1000 people (still into the ball-room of the same size), in which situation do you think you’d feel more “hyped-up”?
Interestingly, this random motion gives the microscopic picture of what temperature is all about. That’s because temperature (i.e. the unit Celcius or Fahrenheit or Kelvin) is essentially a measure of the kinetic energy of these collection of randomly moving molecules. As the molecules gets more “hyped-up” because of the frequent collision and all, the higher the thermometer reading is. Thus, increase in pressure implies essentially increase in the density of air (as the air at the bottom of the column will be pressed by the air at the top of the column… which further implies increase in the kinetic energy of the air molecules… which means increase in temperature.
So, putting everything together: as we gain altitude, pressure decreases, air density gets lesser, kinetic energy gets lesser, and all is tantamount to decrease in temperature.
Take note, however, that this concept works up to certain altitude. As one gains altitude further up, temperature actually increases, decreases, and increases again one final time. In fact, I can easily imagine that the outermost layer of the atmosphere must be very hot due to direct radiation from the sun as given by the profile below.
