Adiabatics and Stability — Page 98, Lesson 85

I want to walk you through the concept of neutral stability and then the full stability summary, because this is where everything we've been discussing about lapse rates comes together into a practical picture you'll use every time you look at a TAF or a weather radar. Let's start with neutral stability. Imagine we have an environmental lapse rate, the ELR, that is exactly the same as the dry adiabatic lapse rate, the DALR. In the example on page 94, the ELR is 1°C per 100 metres. If we blow a parcel of unsaturated air up a hill to 300 metres, it cools at the DALR — also 1°C per 100 metres — so it reaches 17°C. The surrounding environment at that same altitude is also 17°C. That means the parcel and the environment have the same temperature, and therefore the same density. So the parcel has no buoyancy force pushing it up or pulling it down — it just stays at 300 metres. That condition is called neutral stability, or sometimes indifferent stability, for unsaturated or dry air. Now, a similar argument holds for saturated air. If the ELR equals the saturated adiabatic lapse rate, the SALR, then a saturated parcel would also be neutrally stable. But the book points out that this situation is less likely in practice, because the SALR itself is a function of both temperature and pressure — it's not a constant like the DALR — so the match is more complex and less common. Let me give you the shorthand you'll see in your notes: ELR equals DALR gives neutral stability for unsaturated air. ELR equals SALR gives neutral stability for saturated air. Now, let's move to the stability summary on page 95. This is the core decision tree for stability. The relationship between the ELR and the two adiabatic lapse rates — the DALR and the SALR — determines the stability state. First, when the ELR is less than the SALR, we have what's called absolute stability. That means the environment cools more slowly with height than even a saturated parcel would. In absolutely stable conditions, the weather you can expect includes clear skies, moderate to poor visibility, and light turbulence — except at any inversion and in mountain waves, which we'll cover in the turbulence chapter. You may also get stratiform cloud, possibly fog especially in winter, and continuous or intermittent light precipitation. The clouds that form in stable air tend to be small in vertical extent but large in horizontal extent — these are layer clouds. Layer clouds include stratocumulus, which is identified by its well-defined shape, and stratus, which is ill-defined in shape but can cover equally large areas. Figure 7.13 shows an example of stratocumulus. Now flip to the other end of the spectrum. When the ELR is greater than the DALR, we have absolute instability. The environment is cooling faster with height than even a dry parcel would. In absolutely unstable conditions, the weather includes cumuliform clouds, moderate to heavy showers, potential for moderate to heavy precipitation, and good visibility except in showers. The clouds that form in unstable air tend to be large in vertical extent and small in horizontal extent — these are heap clouds. Figure 7.14 shows cumulus of moderate to strong vertical development. Now, the book gives you a table of examples to work through. Let me walk you through the logic so you understand how to apply this. The table gives you temperatures at 2000 feet and 5000 feet, plus relative humidity. You're asked to determine the stability state, assuming a constant lapse rate in that layer and ignoring pressure change effects. For any row, you first calculate the ELR from the temperature difference between 2000 and 5000 feet. Then you compare that ELR to the DALR, which is about 3°C per 1000 feet, and to the SALR, which is roughly 1.5°C per 1000 feet as a typical value — though remember it varies. Let me do a couple of examples so you see the pattern. Question 1: +7° at 2000 feet, +1° at 5000 feet. That's a drop of 6° over 3000 feet, so the ELR is 2°C per 1000 feet. That's between the DALR of 3° and the SALR of about 1.5°, so with 60% relative humidity — unsaturated — we compare to the DALR. Since 2° is less than 3°, the ELR is less than the DALR, so we have conditional stability for unsaturated air. But wait — the book's summary says absolute stability when ELR is less than SALR. Here ELR is 2°, SALR is about 1.5°, so ELR is greater than SALR — that means it's not absolute stability. And ELR is less than DALR, so it's not absolute instability. This is the conditional stability region, which the book covers separately. Question 2: +15° at 2000, +9° at 5000. Drop of 6° over 3000 feet, ELR is again 2° per 1000 feet. But relative humidity is 100% — saturated air. For saturated air, we compare to the SALR. ELR of 2° is greater than SALR of about 1.5°, so the saturated parcel is unstable. That's conditional instability for saturated air. Question 11 is interesting: +10° at both 2000 and 5000 feet. That means the ELR is zero — an isothermal layer. That's much less than both the DALR and the SALR, so we have absolute stability. Question 12: +10° at 2000, +15° at 5000. That's a temperature increase with height — a temperature inversion. The ELR is negative, which is also much less than both lapse rates, so again absolute stability. The book asks what else is unusual about questions 11 and 12 — the answer is that in both cases the temperature either doesn't decrease or actually increases with height, which is the opposite of the normal decrease we expect in the troposphere. The answers to all these are on page 102 for you to check. But the key takeaway is this: you compare the ELR to the DALR for unsaturated air, and to the SALR for saturated air. If ELR is less than the relevant lapse rate, the air is stable. If greater, unstable. If equal, neutral. And when ELR is between the DALR and SALR, you have conditional stability — stable for unsaturated, unstable for saturated.