Water molecules are present in the air inside the jar, but they’re in the form of an invisible gas or vapor—moving around individually and not sticking to one another. When you pull the glove outward, you allow the air in the jar to expand. In expanding, the air must do work, which means that it loses some of its thermal energy, which in turn means that its molecules (including those of the water vapor) slow down slightly. This is a roundabout way of saying that the air becomes cooler.
When the water molecules slow down, they can stick to each other more easily, so they begin to bunch up in tiny droplets. The smoke particles from the match that you put in the jar help this process along: water molecules bunch together more easily when they find a solid particle to act as a nucleus. When you push the glove back in, you warm the air in the jar slightly, causing the tiny droplets to evaporate and again become invisible.
In the atmosphere, air expands as it rises to regions of lower pressure and cools off, forming clouds. This is why clouds often obscure mountaintops. Dust, smoke, and salt particles in the air all provide nuclei that help the droplets condense.
Meteorologists consider a falling barometer reading (low air pressure) to be a sign of an approaching storm, whereas high pressure is usually a sign of clear weather. The temperature at which water vapor begins to form droplets on a surface is called the dew point.
