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