Cloud in a jar
What do we need to do to make a cloud?
We must make the air saturated and get water vapor to condense into liquid droplets. There are two ways to achieve saturation - cool the air or add moisture.
We have a jar with a little bit of water on the bottom. Some of the liquid continually evaporates, just as some vapor continually condenses onto the water surface. But that evaporation is greater than that condensation because there is not enough vapor in the air of the jar for the air to be saturated since the top is open and air much farther from saturation can mix into the jar.
We have a hand pump we use to pump up the jar - what effect does this have?
This increases the air pressure in the jar. More air is added to the jar, thus the air compresses to fit in the fixed jar volume. The compression is just like in a parcel that is sinking. A sinking, compressing parcel warms. Though it may not be obvious, the air in the jar warms also.
What effect does pumping have on the moisture content of the jar?
Warmer air has a higher saturation vapor pressure - it can potentially hold more water vapor. In this case the actual vapor pressure goes up as well (by an increase in water vapor into the air from the source of liquid water at the bottom of the jar) such the relative humidity remains about the same, almost at saturation.
What happens when the stopper is pulled out of the top of the jar?
Pressure decreases back to that of the surrounding air, and the air in the jar expands (forcing some of it out of the top). This is what happens in a rising parcel. Associated with the expansion of the air, there must be cooling of that air. The air cools back to its initial temperature.
What is the effect of the cooling on the moisture in the air?
Since the air returns to its initial temperature, the saturation vapor pressure returns to its initial value. However, when the air in the jar was pumped up more water vapor was added to the air. Thus there might be enough water vapor in the air with that addition such that when the air cools back to its initial temperature there is more vapor than needed for saturation. If the air is supersaturated, the excess moisture can condense into liquid droplets we can observe - a cloud!
The first try resulted in almost no cloud at all. Why not?
Water vapor does not spontaneously condense into liquid droplets just because the air is saturated or slightly supersaturated. The relative humidity must be much greater than 100%.
What can we do to assist condensation like in the atmosphere?
Add cloud condensation nuclei (CCN). Provide a surface for vapor to condense at relative humidities more like 100%. When we added some smoke particles to the jar and repeated the experiment a cloud was clearly visible.