Making Martian clouds on Earth in chambers:

http://phys.org/news/2013-10-cloud-chamber-clouds-mars-humid-conditions.html

The experiments showed it takes a humidity of 190 percent to cause clouds to form on mars by direct simulation of the Martian atmosphere.

My question is: How do you get humidity to 190 percent in the first place? I thought if you get to say 110 percent humidity, rain would start falling immediately.

Originally posted by sonhouse
My question is: How do you get humidity to 190 percent in the first place? I thought if you get to say 110 percent humidity, rain would start falling immediately.

In theory, clouds can form once you exceed 100% relative humidity. Rain only forms once the cloud droplets get big enough.
However, even here on earth, you need something to trigger the water droplets. Usually small dust particles in the atmosphere is what causes cloud formation on earth. It can be encourage by 'seeding':
http://en.wikipedia.org/wiki/Cloud_seeding

This is similar to the way bubbles form in carbonated drinks, cloud chambers (nuclear research), and ice formation in supercooled water.
Here is an example of the last one, but you can find many more on youtube:
YouTube

Also, in the case of Mars, it is ice crystals that are formed and not water vapour on Earth this also happens for higher altitude clouds.

Originally posted by twhitehead
Also, in the case of Mars, it is ice crystals that are formed and not water vapour on Earth this also happens for higher altitude clouds.

But how can it physically happen to get to that near 200% humidity?

Originally posted by sonhouse
But how can it physically happen to get to that near 200% humidity?

By not having any dust or molecule in that atmosphere to act like hydrophilic nuclei to seed a condensation reaction that would lead to a droplet of water (it would have to be either super-cooled water or not liquid water but ice in this case ) forming.

Originally posted by sonhouse
But how can it physically happen to get to that near 200% humidity?

Relative humidity changes with pressure and temperature. So as air rises it cools and looses pressure and its humidity goes up. That why there is usually a cloud on the top of Table mountain. But if the air is relatively clean the relative humidity can easily go above 100%.

I have seen claims that clean air must get to 800% before water droplets form.

Originally posted by twhitehead
Relative humidity changes with pressure and temperature. So as air rises it cools and looses pressure and its humidity goes up. That why there is usually a cloud on the top of Table mountain. But if the air is relatively clean the relative humidity can easily go above 100%.

I have seen claims that clean air must get to 800% before water droplets form.

I thought that 100% humidity at any given temperature and pressure was all the air would hold before forming dripping liquid! Guess I was wrong. If that were the criteria about humidity, then maybe they should recalibrate the measurement to 800% as 100%, the point at which water will start dripping,

Originally posted by sonhouse
I thought that 100% humidity at any given temperature and pressure was all the air would hold before forming dripping liquid! Guess I was wrong.

And I thought water froze at 0 C, but apparently not. it turns out pure water will get a lot colder than that before it freezes.

If that were the criteria about humidity, then maybe they should recalibrate the measurement to 800% as 100%, the point at which water will start dripping,
At 100% humidity, open water will stop evaporating and any object, or dust will trigger condensation.
I often find condensation on my car in the evening even though there is no mist.