Final answer:
As the surrounding air gets cooler, the liquid in a thermometer contracts, causing the level of the liquid to drop and indicating a lower temperature.
Step-by-step explanation:
When the surrounding air gets cooler, the liquid in a thermometer contracts. This process occurs because liquid molecules move closer together and take up less space as they lose thermal energy.
For instance, in an alcohol or mercury thermometer, this contraction results in a lower level of the liquid in the thermometer's tube, indicating a lower temperature. The same liquid, when exposed to heat, would expand, causing the liquid column to rise and show a higher temperature reading.