They want to move close together to warm each other up. They should move back to the circle. After this exercise, the students should figure out that heavy air masses have cooler temperatures and that light air masses have warmer temperatures and that lots of sun can change a heavy mass into a light mass and vice versa. Ask the students to remember the grocery bags from the beginning of the talk. The sacks were hard to tell apart without picking them up to feel the difference.
Scientists have the same trouble with air pressure.
They can t tell whether the air is heavy or light without measuring it in some way. They measure the air pressure with a barometer. It works on the same principle as a barometer. Compare the weather that occurs 12-24 hours after the barometer readings are done. You should see a pattern of lower barometer readings equaling stormy or wet weather and higher barometer readings equaling sunny, nicer weather.