As the air temperature rises, the individual particles that make up the air molecules, like nitrogen and oxygen atoms, gain additional kinetic energy. These particles start moving faster and with greater vigor, colliding and bouncing off each other more vigorously.
At higher temperatures, the particles have greater thermal energy, so they spread apart further and the air expands. The hot air particles spread out and fill a larger volume of space. Some of this thermal energy is also transferred to the thermometer, causing it to expand and the liquid inside to rise up the graduated scale.
The hot air particles collide more frequently and forcefully with the molecules in the thermometer, transferring their heat and causing the whole instrument to increase in temperature. As more and more hot air particles surround the thermometer, its temperature continues to rise until an equilibrium is reached. A steady temperature is maintained as the hot air particles continue their chaotic motion, constantly exchanging heat with the thermometer.
[A diagram shows air particles at different temperatures, with lower temperature particles as dots close together, and higher temperature particles as dots spread further apart, with arrows showing their increased motion and collisions. Another diagram shows the thermometer surrounded by lower temperature air particles on one side and higher temperature air particles on the other side, with arrows showing the heat transfer causing the thermometer to expand.]
The increase in air temperature, represented by the rising thermometer, occurs due to the gain in thermal energy of the air particles which then spread apart, expand the volume of the air, and transfer their heat and increased motion to the thermometer. A balance is eventually achieved as heat continues to flow between the air and the thermometer.