# What is atmospheric stratification?

In the context of atmospheric stability, what are the meanings of stable or unstable stratification? What is stratification?

• Have you tried some basic Google searches? Oct 1, 2014 at 14:23
• You're right. I'm not a native speaker of English, searches in my language gave no results. Now I found a page in English, thank you. Oct 2, 2014 at 9:39
• Maybe it's simplier that I was expected: unstable stratification occurs when heavy and denser air is at the top of a layer, and lighter airis below; stable stratification is the opposite. Is it right? Oct 2, 2014 at 10:51
• I think Stratification happens mainly due to Density difference as an e.g oil on water. And gravity pulls the denser gasses down and low density gas float on it. I don't know what happens if we mix oil and water or Nitrogen and Oxygen in space. Jan 9, 2020 at 9:36

Stratification is density variations causing the fluids to separate to different layers.

As shown in the YouTube video "Stratified Flow -lecture":

• Stratification does not have to be in the form of discrete layers. Continuous stratification will even be more common. Feb 8, 2021 at 20:21
• @VladimirF True, Looked your profile and want to share you this;researchgate.net/publication/… Feb 9, 2021 at 17:55

You could use the buoyancy frequency, $N$, as a measure of stratification and stability. $N$ is defined as:$$N^2 = \frac{g}{\theta} \frac{\mathrm{d}\theta}{\mathrm{d}z} \,,$$where $\theta$ is the potential temperature.

With some variations in answers here and there. The term $\frac{\mathrm{d}\theta}{\mathrm{d}z}$ is a measure of convective stability with negative values representing convective instability for which a buoyancy frequency does not exist as an air parcel will not oscillate but instead will ascend/descend in order to produce a stable stratification.

I perhaps wonder if the moist potential temperature should be used rather than the dry potential temperature when investigating moist atmospheric processes such as cyclogenesis.

Stratification means that density changes with height. If it is statically stable, the density decreases with height. However, it is so simple only for incompressible fluids. For example, for laboratory flows in water.

For compressible fluids, such as the atmosphere, but to some extent even the oceans, we have to take into account, that when a fluid parcel moves into a region with different pressure, its temperature and density changes due to adiabatic expansion and compression. That's why potential temperature and potential density are used. They express the temperature or density of various air parcels when adiabatically expanded/compressed the same reference pressure.

If the potential density decreases with height or the potential temperature rises with height, the atmospheric stratification is statically stable. If it is constant, it is statically neutral. If potential temperature decreases with height, the stratification is statically unstable.

However, again, it is so simple only when the gradient is the same everywhere. Otherwise one has to examine the complete potential temperature profiles and see whether there is any location, where air with higher potential temperature is located under air of lower potential temperature. That would indicate static instability for a larger layer (the nonlocal static stability).