The thermometer measures temperature in a hot air balloon through the process of thermal expansion. Here's how it works:

* Thermal Expansion: Most substances, including the materials in a thermometer, expand when heated and contract when cooled.

* Thermometer Design: Thermometers typically contain a liquid (like mercury or alcohol) inside a sealed glass tube. The liquid expands more than the glass when heated.

* Measurement: As the air inside the balloon heats up, the thermometer's liquid expands and rises up the tube. The higher the liquid rises, the hotter the temperature.

* Calibration: Thermometers have a scale that is calibrated to correlate the liquid's height with a specific temperature value (e.g., degrees Celsius or Fahrenheit).

In summary: The rising hot air balloon's temperature causes the thermometer's liquid to expand, indicating the temperature change through its position on the calibrated scale.