# What size does a subterranean base have to be for “natural” weather?

Many planets don't have habitable surfaces, though at least Mercury has good temperatures underground (source: http://www.einstein-schrodinger.com/Mercury_temperature.pdf). So I'm wondering how large would a subterannean nature sanctuary building/dome height have to be, in order for it to have clouds and rain?

I'm thinking of NASA's Vehicle Assembly Building which purportedly develops rain clouds on humid days if the dehumidifiers aren't on, it is 160m tall, by 157x218m, and is 8 acres.
Particularly I'm wondering what the minimum height would be, and how that may relate to both gravity and air pressure. I.E. if it was 0.8atm and gravity is 3.7m/s^2 (0.38g), would that change the required dimensions than if it was a 1atm and 9.8m/s^2 (1g) ?

In terms of more naturalistic weather, I was wondering if it would make sense to have multiple such domes somewhat connected. And then could for instance raise the pressure in one, and lower it in another for wind. I don't know if using these pressure gradients it might be possible to form weather "fronts".

Also somewhat related is the worry that maybe it would use a lot of water, and those clouds might not "come down", Mercury is rather water scarce, so perhaps it would make sense to have an alternative irrigation method for the "nature sanctuaries". (I presume farming would be mostly stacked aeroponic, but humans would need nature sanctuaries to avoid cabin fever).

• "..is 8 acres." On a side note: If the water vapor comes from within the assembly building it is not natural (e.g. a water cooled drill tip, or dousing a fire with water). If it comes from outside the building (e.g. a breeze blowing off the ocean) that would imply a significantly larger area. – Andrew Thompson Jun 17 '16 at 4:34

## 1 Answer

The airship hangar at NAS Tillamook (in Tillamook, Oregon) was big enough to generate its own internal "weather" - in the sense that the internal weather was not always the same as the external. It has, at times, generated condensation inside on cold days sufficient to result in apparent rain, and it occasionally is foggy inside while not outside. 1050 feet long by 296 feet wide, 175 feet high.① Calling it weather, however, is a stretch. ②

By the time one gets to the Disneyworld Complex, one has sufficient topographical features to change extant weather patterns; it is a safe presumption that a cavern of similar footprint would generate some form of weather-like effect in a quarter-sphere or larger arched dome. The stability, however, may not be seen as weather as much as simply internal thermal features.

To get genuine weather - as in moving air masses generating chaotic events, we're probably not yet to the actual scale yet. A typical air-mass in meteorological terms is over a hundred miles across, usually in the 400 to 1000 mile range, and it's the motion of these that triggers what we call weather; local interactions down to the 100m square size can create localized phenomenon, such as the intense thermals generated by large parking lots on sunny days; while not themselves weather, they can create sufficient updrafts to create visible effects.

So, the functional point is where the system is large enough to be chaotic; microenvironments down to a few hundred meters can be visibly different, and the rough minimum size for such is illustrated by NAS Tillamook's Hangar B.

• ① National Park Service NAS Tillamook, www.nps.gov/nr/travel/aviation/usb.htm
• ②: NAS Tillamook website, www.nastillamook.org/faqs/hangars/weather.htm