Q:

How can the frozen clouds on Dr. Mann's planet stay afloat?

When the crew is entering Mann's planet's atmosphere, their ship hits a frozen cloud. How can a frozen cloud stay afloat even though the planet contains 80% of Earth's gravity? 

A:

ScienceInsider interviewed Kip Thorne (who is a "renowned theoretical physicist at the California Institute of Technology in Pasadena and one of the world’s leading experts in the astrophysical predictions of general relativity", scientific consultant and executive producer of Interstellar, and author of The Science of Interstellar):

Q: Is there anywhere the moviemakers strayed outside your guidelines?
A: Not seriously. The one place where I am the least comfortable is on [a] planet where they have these ice clouds. These structures go beyond what I think the material strength of ice would be able to support. But I’d say if that’s the most egregious violation of physical law, they’ve done very, very well. There’s some artistic license there. Every time I watch the movie, that’s the one place where I cringe. I don’t think I’ve ever told anybody that.

(bold emphasis mine)

There’s also a lesson from Interstellar for Educators: Can Frozen Clouds Exist?

In this activity, students will experimentally determine the density of ice, then consider the air density and temperature required for a frozen cloud to remain suspended in the air. 

A:

I think gravity is not a factor because everything has mass it's really a matter of how much space that mass displaces, i.e., density.  Like, ice cubes float in your milk even though gravity is pulling on the ice cubes.  So I think the only factor that matters is that the solid clouds are less dense than the gaseous atmosphere.
In fact, stronger gravity contributes to greater atmospheric density by pulling the gases closer to the planet surface (e.g., air is thinner atop Mt. Everest).  So, if planet Mann had an atmosphere made of the same molecules as Earth's atmosphere, that atmosphere would in fact be thinner on Mann than on Earth because Mann's 80% gravity is not pulling down as much atmosphere.  Which would make floating a solid in such a thin atmosphere even harder--not easier.  But, we know Mann's atmosphere is not the same as Earth's because Mann's atmosphere is not breathable.
Possibly the clouds and the atmosphere of Mann are not composed of the same elements/compounds as each other.  The atmosphere could be chock full of dense gases.  I see that sulfur hexaflouride has a density of 6.17 g/L.  And the clouds are made of really not-dense solid.  I see that NASA aerogel is only 3 g/L.  So, maybe NASA made a sh*tload of aerogel, floated it out to Saturn where it [accidentally] got sucked through the wormhole and fell to planet Mann.  Just maybe.
Or maybe the clouds and the atmosphere of Mann are made of the same elements/compounds as each other, and the temperature is just enough to have some of it freeze up, and the solid state is less dense than the gaseous state.  Like imagine the planet is just a ball of water or whatever, where a lot of it has frozen into iceburgs but there's still some ocean left.  I don't think this is likely though, as I don't know of any compounds whose solid state is less dense than its gaseous state.

