3. Expanding and cooling

Aitken then cooled the air suddenly through expansion. Expanding the container in which the air was held caused an increase in volume and a decrease in the air pressure, accompanied by a drop in temperature. In normal air this expansion caused water vapour to condense on dust in the air, producing a cloud as before, while in filtered air no cloud was produced.

Because the air had cooled the energy of the water molecules suspended in the air had decreased. The condensation on dust occurred because the water molecules no longer have enough energy to float around independently.

Aitken's experiments showed that expansions could produce clouds in air which contained nucleating points, such as dust. Air containing a little dust produced wisps of cloud, while thick cloud could be produced in quite dusty air.

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