How is weather formation prevented in the stratosphere?

Question

Water diffuses from an area of high concentration to an area of low concentration.

By that principle, I would expect water from the troposphere to diffuse into the stratosphere, where water content is significantly lower than in the former. However, this is not what happens; a reason given in my textbook is as follows:

Increasing temperatures stop clouds and weather systems from reaching this height: it acts like the lid on top of a boiling pan above the active weather systems in the troposphere. $^{1}$

How is it that rising temperatures play a role here? Plus, the hottest region of the stratosphere (its top) isn't much hotter/colder compared to the troposphere, so a temperature gradient wouldn't be responsible, would it?

Essentially, what role does temperature play with weather "regulation" and how does it prevent weather formation & water entering the stratosphere through diffusion?

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1. Pallister, John. IGCSE Environmental Management. Second ed., Oxford University Press, 2017.

Answer 1

By that principle, I would expect water from the troposphere to diffuse into the stratosphere, where water content is significantly lower than in the former.

You are forgetting that water is not nearly as volatile as are the long-lived gases that comprise the bulk of the Earth's atmosphere. Water's triple point temperature is 0.01° C. The triple point temperatures of nitrogen, oxygen, and argon are -209.97° C, -218.79° C, and -189.34° C. There is no point in the Earth's atmosphere where those long-lived gases can make the phase transition from gas to liquid or solid. There are plenty of places in the Earth's troposphere where water vapor can do just that. We call the result snow or rain.

How is it that rising temperatures play a role here?

It's not just rising temperatures in the stratosphere with increased altitude, although that does play a role. The decreasing temperatures in the troposphere with increased altitude also plays a role. The temperature at the top of the troposphere / bottom of the stratosphere is around -50° C. The Earth's atmosphere can sustain but a paltry amount of water vapor at the temperature and pressure at the tropopause.

Where the increasing temperatures with increased altitude in the stratosphere does play a role is that this makes the tropopause a temperature inversion layer. Inversion layers tend to stop vertical transport of air. Inversion layers in the lower atmosphere are one of the reasons pollution can be rather nasty in cities such as Los Angeles and Denver. Diffusion of water vapor from the upper troposphere into the lower stratosphere is still possible, but at -50° C, there is very little water vapor available to diffuse across the tropopause.