Adiabatics and Stability — Page 98, Lesson 83

I want to walk you through the next two important stability states in the atmosphere: absolute stability and conditional instability. These build directly on the idea of a parcel of air being forced up a hill, cooling at a known rate, and then comparing its temperature to the surrounding environment. Let's start with absolute stability. Imagine we have the same setup as before — a parcel of dry air is blown up a hill. But this time, the radiosonde ascent — that's the instrument package measuring the actual temperature of the surrounding air at different heights — shows a lapse rate of 0.4°C per 100 metres. That means for every 100 metres you go up, the environmental temperature drops by only 0.4 degrees. So at a height of 300 metres, the environmental temperature is 18.8°C. Now, our parcel of dry air is blown up the hill. As it rises, it cools adiabatically — that means it cools without exchanging heat with the surroundings, purely because it expands as pressure decreases. The dry adiabatic lapse rate is about 1°C per 100 metres, so after rising 300 metres, that dry parcel has cooled to 17°C. Compare that to the environment at 18.8°C — the parcel is colder, and therefore denser, than the air around it. Because it's denser, it will sink back down. It descends on the opposite side of the hill to its starting position. That's a stable situation — the parcel wants to return to where it came from. Now consider the saturated air — air that is holding as much water vapour as it can. When saturated air rises, it cools at the saturated adiabatic lapse rate, or SALR, which is slower — in this example, about 0.6°C per 100 metres. So the saturated parcel cools to 18.2°C at 300 metres. That's still colder than the environmental temperature of 18.8°C, so it too is denser and will roll down the other side of the hill. Both the dry parcel and the saturated parcel are stable. When the environmental lapse rate is less than the saturated adiabatic lapse rate — in other words, ELR is less than SALR — we have absolute stability. The atmosphere is stable for both dry and saturated air. The key relationship to remember is: ELR < SALR gives absolute stability. Now let's move to conditional instability. This time, the radiosonde ascent shows an average lapse rate of 0.8°C per 100 metres over the first few hundred metres. At 300 metres, the environmental temperature is 17.6°C. Again, the dry parcel is blown up the hill and cools adiabatically to 17°C. That's colder than the environment at 17.6°C, so the dry parcel is stable — it descends back down. That's the stable condition for dry air. But look at the saturated air. It cools to 18.2°C as it rises. Now, 18.2°C is warmer than the environmental temperature of 17.6°C. That means the saturated parcel is less dense than the surrounding air, so it will continue to rise — that's the unstable condition. So we have a split: the same environmental lapse rate produces stability for dry air but instability for saturated air. This state is called conditional instability. The stability of the air depends on whether the air is saturated or unsaturated. The relationship here is: DALR > ELR > SALR — the dry adiabatic lapse rate is greater than the environmental lapse rate, which is greater than the saturated adiabatic lapse rate. That's the condition for conditional instability. One important note from the book: the term 'conditional stability' is not a proper meteorological term. If you ever see it in an examination question, you can confidently delete it as an incorrect answer. The correct term is conditional instability. Let me show you the diagrams that illustrate these concepts. So to summarise: absolute stability occurs when the environmental lapse rate is less than the saturated adiabatic lapse rate — both dry and saturated parcels are stable. Conditional instability occurs when the environmental lapse rate lies between the dry and saturated adiabatic lapse rates — dry air is stable, but saturated air is unstable.