A thermometer measures temperature. Traditional liquid-in-glass thermometers use the expansion of mercury or alcohol in a narrow tube. Modern instruments include electronic sensors, bimetallic strips, and infrared devices that read surface temperature without contact.

Accurate weather thermometers are housed in ventilated shelters that shield them from direct sun and precipitation while allowing free air flow. Calibration against known standards ensures consistency across observing networks.

Maximum and minimum thermometers record the highest and lowest values reached since the last reset. Digital sensors can log temperature continuously. In research, fine-wire thermocouples and resistance thermometers capture rapid fluctuations.

The humble thermometer remains one of the most essential instruments in meteorology.