Temperature sensors are generically called thermometers. Anything that changes with temperature can be used to measure temperature. Many materials expand when warm, so the size of the material can be calibrated into a temperature. Classical liquid-in-glass thermometers use either mercury or a dyed alcohol or glycol fluid that can expand from a reservoir or bulb up into a narrow tube.
House thermostats (temperature controls) often use a bimetallic strip, where two different metals are sandwiched together, and their different expansion rates with temperature causes the metal to bend as the temperature changes. Car thermostats use a wax that expands against a valve to redirect engine coolant to the radiator when hot. Some one-time use thermometers use wax that melts onto a piece of paper at a known temperature, changing its color.
Many electronic devices change with temperature, such as resistance of a wire, capacitance of a capacitor, or behavior of various transistors (thermistors). These changes can be measured electronically and displayed. Thermocouples (such as made by a junction between copper and constantan wires, where constantan is an alloy of roughly 60% copper and 40% nickel) generate a small amount of electricity that increases with temperature. Liquid crystals change their orientation with temperature, and can be designed to display temperature.
Sonic thermometers measure the speed of sound through air between closely placed transmitters and receivers of sound. Radio Acoustic Sounder Systems (RASS) transmit a loud pulse of sound upward from the ground, and then infer temperature vs. height via the speed that the sound wave propagates upward, as measured by a radio or microwave profiler.
Warmer objects emit more radiation, particularly in the infrared wavelengths. An infrared thermometer measures the intensity of these emissions to infer the temperature. Satellite remote sensors also detect emissions from the air upward into space, from which temperature profiles can be calculated (see the Satellites & Radar chapter).
Even thick layers of the atmosphere expand when they become warmer, allowing the thickness between two different atmospheric pressure levels to indicate average temperature in the layer.