by the Boom


We shall use two variations of adiabatic temperature change to produce a cloud in the jug. Adiabatic expansion by pump, and adiabatic compression using Boyles Law with a water piston.

Take a clear glass 4-liter wine jug (you may have to dispose of its contents on a previous weekend), fit it with a one-hole rubber stopper with a glass bend therein. Attach it with a rubber tubing to an aspirator or vacuum pump.

Give it a safety test run in the sink covered with a towel to be sure there are no flaws in the glass.

Discuss adiabatic temperature changes, capacity of air, absolute humidity, relative humidity, saturation of air, condensation nuclei, and in general why a cloud forms. The capacity of air at 20 deg C is about 5 grams of water vapor per cubic meter, and increases with temperature.

Introduce a few ml of water into the jug, insert stopper, and shake it for a few seconds. Point out that the air will be saturated with vapor, that is at 100% relative humidity.

Now attach aspirator (or suck on it with your mouth. That works greatly!) and lower the pressure somewhat (if you are worried about an implosion, don't pump it all the way. I've done this demo five times a year for the past 30 years with no mishaps).

Not much fog forms. Oooh, we need some condensation nuclei. Remove the stopper and allow some match smoke into the jug. Pump again and, nice, there's the fog. Perhaps most noticeable when it disappears when air is re-admitted. Repeat this a couple of times.

Point out that when air is blown up a hill, the lower atmospheric pressure allows the air to expand, cool adiabatically, and upslope fog may form.

Also explain that fog and clouds are not water vapor, which is invisible, but tiny droplets of liquid water. They can be seen in a beam of light.

Now try this for even better fog in the jug. Add some more smoke, attach the rubber tubing to the water jet. Holding the stopper on the jug to prevent its blowing out, force in enough water to fill the jug one-third full. (You would have to fill it half-full to acquire the same pressure differential that you had when it was evacuated).

By Boyle's Law you have increased the pressure in the jug, this will adiabatically warm the air therein, increase its capacity, and cause it to dissolve more vapor and reach a new saturation equilibrium.

After a few seconds, pop the top. The sudden drop of pressure inside will give a quick adiabatic cooling, and, WOW, great fog. Some will be pouring out of the neck of the jug. Four liters of London Fog!

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