Adiabatics and Stability — Page 90, Lesson 78

Let’s start a new topic: Adiabatics and Stability. I want to walk you through the idea of adiabatic temperature changes first, because it’s the foundation for everything that follows. An adiabatic temperature change happens when a gas is compressed or expanded with no external exchange of heat. That means the gas changes temperature purely because of the work done on it or by it — no heat flows in from the surroundings, and no heat flows out to them. You’ve actually seen this in everyday life. Think about using a manual pump to inflate a bicycle tyre. You’ll notice the tyre valve gets hot. Why? Because the compression of the air inside the pump raises its temperature, and that heat is transferred to the valve as the air passes through it. The temperature rise comes from compression, not from an external heat source. The opposite effect happens when you discharge a carbon dioxide (CO₂) fire extinguisher. The CO₂ is stored under very high pressure in the cylinder. When you operate the release handle, the gas expands rapidly as it exits the cylinder, and it cools as it does so. In fact, the expansion is so great that the temperature drop is severe enough that you risk frost burns if you hold the horn. Again, no external heat is involved — the cooling is purely from expansion. In both cases, the temperature changed because of expansion or compression of the gas. No heat was added from or removed to external sources. That’s the core of an adiabatic process. Now, let’s connect this to the atmosphere. In the atmosphere, pressure decreases as altitude increases. So if a parcel of air is forced to rise, it will expand as it rises, and therefore it will cool by the adiabatic process. Similarly, if a parcel of air is forced to descend, it becomes compressed and heats up — again, by the adiabatic process. No heat is exchanged with the surrounding air; the temperature change is entirely due to the pressure change. That brings us to the Dry Adiabatic Lapse Rate, or DALR. The DALR is the lapse rate for rising dry air — and by "dry" we mean unsaturated air. It has a constant value of 1°C per 100 metres, which is about 3°C per 1000 feet. That’s a fixed rate, and it’s illustrated in Figure 7.2. Next is the Saturated Adiabatic Lapse Rate, or SALR. When saturated air is forced to rise, it also cools — but as it cools, condensation takes place. Condensation releases latent heat, and that released latent heat slows the rate at which the air cools. So the SALR is the lapse rate for rising air that is saturated — meaning relative humidity is 100%. In temperate latitudes near the ground, it has an average value of 0.6°C per 100 metres, which is about 1.8°C per 1000 feet. You can see that in Figure 7.3. So to summarise: the DALR is the cooling rate for unsaturated rising air — 1°C per 100 metres. The SALR is the cooling rate for saturated rising air — slower, about 0.6°C per 100 metres, because latent heat from condensation offsets some of the cooling. Both are adiabatic processes, with no external heat exchange.