Red and blue food coloring
Step 1: A dense liquid, representing cold, heavy air, goes in one side of the plastic container. This is made with salt and water and with blue food coloring added. A lighter liquid, representing warm, less dense air goes in the other side. This is made with cooking oil and red food coloring added. The partition represents the warm/cold boundary, or the front.
Step 2: Now pull out the partition quickly. The cold, heavy (blue) liquid pushes below the lighter (red) liquid. The lighter liquid is pushed toward the top but the two hardly mix.
Step 3: After some sloshing around, the liquids settle down with the cool, heavier one on bottom and the warmer, lighter one on top.