Adiabatics and Stability — Page 90, Lesson 80

I want to walk you through the variation of the Saturated Adiabatic Lapse Rate, or SALR, with temperature. The key idea here is that the amount of water vapour the air can hold is directly proportional to temperature. At high temperatures, the air can hold large amounts of water vapour, so when it cools, a much greater amount of that vapour condenses. That condensation releases a lot of latent heat, which slows the cooling process even more. Conversely, at low temperatures, the air holds a relatively small amount of water vapour, so little latent heat is released to slow the rate of cooling. The result is that the SALR increases as latitude and/or altitude increase, and it tends towards the Dry Adiabatic Lapse Rate, or DALR, at high altitude and high latitude. Let me be precise about what that means. The DALR is a fixed rate of 1.0°C per 100 metres for dry or unsaturated air. The SALR is variable, and the difference between them is shown in Figure 7.4. The table in that figure gives you a comparison between SALRs at different latitudes. At polar low level and high altitude at all latitudes, the DALR is 1.0°C per 100 metres, and the SALR is greater than 0.6°C per 100 metres — that's the cold zone. At mid-latitudes low level, the DALR is still 1.0, and the SALR is 0.6°C per 100 metres — that's the medium zone. At equatorial latitudes low level, the DALR is 1.0, and the SALR is less than 0.6°C per 100 metres — that's the warm zone. So the warmer the environment, the slower the saturated lapse rate. Now, let's move to the Environmental Lapse Rate, or ELR. The ELR is the actual temperature profile of the troposphere as measured by radiosonde ascents. It varies with time and position — it's not a fixed number like the DALR or SALR; it's what you actually measure in the real atmosphere on a given day at a given place. That brings us to stability. Stability can be defined as resistance to change. When we deal with atmospheric stability, we are looking at what happens to air in vertical motion. Imagine a parcel of air is forced to rise, for example over a mountain. When it gets to the top of the mountain, there are three things it can do. It may return to its original height, it may continue rising, or it may remain at the height of the summit. In the first case, in terms of vertical position, the air is where it started — before and after are the same — so we have a stable situation. In the second case, we have continual change and hence instability. The third situation is a neutral or indifferent case, since the parcel of air is remaining where it was moved to. Atmospheric stability is determined by comparing the ELR with the DALR and the SALR. That comparison tells you whether the air is stable, unstable, or neutral. Let's look at a specific scenario called absolute instability. Imagine a hill 300 metres high. A radiosonde ascent gives the ELR over the first few hundred metres as 1.2°C per 100 metres. That means the environmental temperature at a height of 300 metres is +16.4°C. Now, the wind blows a parcel of unsaturated air up the hill. That air cools adiabatically at the DALR of 1.0°C per 100 metres, so at 300 metres it has cooled to 17°C. This air is now warmer than the environment at that level — the environment is 16.4°C, the parcel is 17°C — and hence it is less dense, so it will continue to rise. That is an unstable situation. Now, the wind blows a parcel of saturated air up the same hill. That saturated air cools at the SALR of 0.6°C per 100 metres, cooling to a temperature of 18.2°C at 300 metres. This air is also warmer than the environment at 16.4°C, so it will also continue to rise and is hence unstable. In this scenario, when the ELR is greater than the DALR, the air is unstable for both dry and saturated air. We call this situation absolute instability. The shorthand is: ELR greater than DALR means absolute instability.