A weather thermometer measure the surrounding outside or inside air temperature. The thermometer, which is a device composed of a long tube with a fluid in it, is enclosed in a glass rod. The fluid inside the tube can be either colored alcohol or mercury.
The thermometer can use either a Fahrenheit or a Celsius scale to measure temperature. As the temperature rises, the liquid moves up the tube and markings on the glass tube of the thermometer gives the temperature reading. When it is cold, the liquid inside the tube moves down accordingly.
Although most people are familiar with the enclosed tube thermometer, there are other types of thermometers like the U-shaped one that measures the maximum and minimum temperatures over a period of time.