The dry-bulb temperature (DBT) is the temperature of air measured by a thermometer freely exposed to the air, but shielded from radiation. The dry-bulb temperature is the temperature that is usually thought of as air temperature, and it is the true thermodynamic temperature. It is directly proportional to the mean kinetic energy of the air molecules. Temperature is usually measured in degrees Celsius (°C), Kelvin (K), or degrees Fahrenheit (°F). If expressed in kelvins, then the symbol is T<sub>a</sub>, if expressed in Celsius or Fahrenheit, then the symbol is t<sub>a</sub>. When measuring dry-bulb temperature, care should be taken to prevent the sensor from being subjected to radiation from neighbouring heat sources.