The most common measurement of air temperature is known as the dry bulb temperature (DBT). This reading is the standard value people see on weather reports and digital thermometers. It is a fundamental measurement because it reflects the actual thermal state of the air without factoring in humidity or solar heating effects.
The Definition of Dry Bulb Temperature
Dry bulb temperature is defined as the temperature of the air as measured by a thermometer that is not affected by the air’s moisture content or any radiation effects. It is termed “dry bulb” to differentiate it from other temperature measurements, such as the wet bulb temperature. The DBT represents the true thermodynamic temperature of the air, indicating its sensible heat content. This measurement is typically expressed in degrees Celsius or Fahrenheit.
How Dry Bulb Temperature is Measured
Measuring the dry bulb temperature relies on using a standard temperature sensor, such as a liquid-in-glass thermometer or a digital resistance thermometer. The process requires the sensor to be freely exposed to the moving air while being carefully shielded from solar radiation and direct moisture. Shielding is necessary because direct sunlight can artificially inflate the reading, while water contact would initiate evaporative cooling.
In professional meteorological and engineering applications, instruments are often placed within a ventilated enclosure, like a Stevenson screen. This screen prevents radiative heating while allowing air to flow freely over the sensor. For precise measurements, ensuring a specific air velocity around the sensor helps guarantee proper heat transfer and accuracy.
Dry Bulb Temperature vs. Wet Bulb and Dew Point
The dry bulb temperature serves as the foundation for understanding other related temperature and humidity measures, specifically wet bulb temperature (WBT) and dew point temperature. Unlike the DBT, the WBT is obtained using a thermometer with its bulb wrapped in a water-soaked cloth. As air passes over the wet cloth, water evaporates, which draws heat away and cools the thermometer.
This evaporative cooling means the WBT is almost always lower than the DBT. The only time the two temperatures are identical is when the air is fully saturated, meaning the relative humidity is 100%. The difference between the dry bulb and wet bulb temperatures, known as the wet bulb depression, is a direct indicator of the air’s relative humidity. A larger depression signifies drier air with a greater capacity for evaporation, while a smaller depression indicates high humidity.
The dew point temperature is the temperature at which air must be cooled to become completely saturated with water vapor. At the dew point, any further cooling causes the excess moisture to condense into liquid water, forming dew or fog. When the dew point temperature is close to the DBT, the air has high humidity, indicating it is near saturation. In the specific condition of 100% relative humidity, all three values—dry bulb, wet bulb, and dew point—are exactly the same.
Importance in Weather and HVAC Systems
The dry bulb temperature is foundational to both meteorology and the design of Heating, Ventilation, and Air Conditioning (HVAC) systems. In weather forecasting, the DBT is the primary temperature value reported to the public, offering a simple metric for the sensible heat of the air. Its true value in these fields emerges when it is combined with the other psychrometric variables.
In HVAC engineering, the DBT is used to calculate the sensible heat load, which is the heat that must be added or removed to change the air’s temperature. This data is essential for properly sizing heating and cooling equipment to meet the thermal requirements of a building. Engineers rely on design-day DBT values for a location to ensure the installed system can handle the hottest and coldest expected conditions.
DBT is a major component in calculating thermal comfort indices, such as the heat index, which helps assess how the environment feels to humans. By measuring both the DBT and WBT, technicians can determine the air’s total energy content, or enthalpy, which is necessary for optimizing air conditioning processes. The DBT is also the main set point used in most thermostats to automatically adjust cooling and heating functions.