Understand Correctly the Dry-bulb Temperature, Dew-point Temperature and Wet-bulb Temperature

Dew Point (or Frost Point) Temperature: dew temperature (dp). The dew point temperature refers to the temperature at which air cools to saturation under the conditions that the water vapor content and air pressure remain unchanged.

Figuratively speaking, the temperature at which the water vapor in the air turns into dewdrops is called the dew point temperature. The dew point temperature is originally a temperature value, but why is it used to represent humidity? This is because when the water vapor in the air has reached saturation, the air temperature is the same as the dew point temperature; when the water vapor has not reached saturation, the air temperature must be higher than the dew point temperature. Therefore, the difference between the dew point and the air temperature can indicate the degree to which the water vapor in the air is away from saturation.
Explanation 1: Under a certain air pressure, the air temperature is gradually lowered. When the water vapor contained in the air reaches a saturated state and begins to condense to form water droplets, the temperature at this time is called the dew point temperature of the air under this air pressure. That is, when the temperature drops below the dew point temperature, water droplets will precipitate from the moist air. The principle of removing moisture from moist air by the cooling method is based on this principle.
Explanation 2: In daily life, we can observe that when the air temperature drops at night, a part of the moisture in the air will precipitate to form dew or frost. This shows that when the water vapor content remains unchanged, due to the decrease in temperature, the water vapor in the air that was originally unsaturated can become saturated vapor, and the excess moisture will precipitate. The temperature at which the water vapor reaches saturation is called the "dew point". By measuring the dew point temperature, the water vapor content can be found from the saturated water vapor content table. Since the water vapor content does not change during the process of temperature decrease, measuring the dew point actually means measuring the absolute humidity in the air. If the dew point is lower, it indicates that the moisture content in the air is less. The dew point can be measured by a special dew point meter.
Dry Bulb Temperature (dry bulb temperature) (db)
The value read on the dry bulb thermometer that is exposed to the air and not directly irradiated by the sun.
The temperature measured by the dry bulb thermometer is the temperature of the air measured by the thermometer, which is freely exposed to the air but protected from radiation and moisture. The dry bulb thermometer temperature is usually regarded as the air temperature, and it is the true thermodynamic temperature. It is the temperature measured by a regular thermometer that is exposed to the air flow. Unlike the wet bulb temperature, the dry bulb thermometer temperature does not indicate the amount of moisture in the air. In architecture, when designing a building for a certain climate, it is an important factor to consider. Nall calls it one of "the most important climatic variables for human comfort and energy conservation in buildings."
The dry bulb temperature is the actual temperature of the air in contact with the surface of the sphere. The wet bulb temperature is the temperature of the sphere when water adheres to the surface of the sphere and the heat is carried away by the evaporation of water. The amount of water evaporation is related to the humidity of the air. The greater the air humidity, the smaller the amount of evaporation, the less heat is carried away, and the smaller the difference between the dry and wet bulb temperatures; the smaller the air humidity, the greater the amount of water evaporation, the more heat is carried away, and the greater the temperature difference between the dry and wet bulbs. Therefore, the current air humidity condition can be reflected by the change law of the temperature difference between the dry and wet bulbs.
Wet Bulb Temperature —— wet - bulb temperature, also known as the thermodynamic wet bulb temperature. The wet bulb temperature is a means of calibrating the relative humidity of the air.
It is difficult to give a rigorous and precise definition of the wet bulb temperature in a few words. The wet bulb temperature is a means of calibrating the relative humidity of the air. Its meaning is that the air in a certain state comes into contact with the wet bulb of the wet bulb thermometer, and adiabatic heat and mass exchange occurs until it reaches a saturated state. This temperature is the temperature measured on the surface of the wet gauze by a thermometer with the wet gauze wrapped around its bulb, in the air with a flow rate greater than 2.5m/s and without direct radiation, and it is used as a measure of the degree of air approaching saturation. The greater the saturation deficit of the surrounding air, the stronger the evaporation that occurs on the wet bulb thermometer, and the lower its humidity. According to the difference between the dry and wet bulb temperatures, the relative humidity of the air can be determined.
Principle: Usually, the relative humidity can be obtained by measuring the dry and wet bulb temperatures with a dry-wet bulb thermometer. The dry-wet bulb thermometer contains two ordinary thermometers. The bulb of one of them is in direct contact with the moist air, and the temperature it measures is called the dry bulb temperature; the bulb of the other one is wrapped with a wet gauze that is kept moist, and the measured temperature is called the wet bulb temperature. If the incoming air is unsaturated, the moisture on the surface of the wet gauze will continue to evaporate. Since water absorbs heat during evaporation, the temperature of the layer of air close to the gauze will decrease. As a temperature difference is established with the main air flow, the main air flow transfers heat to the gauze. When the temperature drops to a certain extent, the heat transferred to the gauze is exactly equal to the heat required for water evaporation. At this time, the temperature remains unchanged, and this temperature is the wet bulb temperature. The smaller the relative humidity of the air, the lower the wet bulb temperature is compared to the dry bulb temperature.

If the air is saturated, since the air cannot accommodate more vapor, the water on the gauze will not evaporate, and at this time, the wet bulb temperature and the dry bulb temperature are the same. Therefore, there is a certain functional relationship between the temperature difference between the dry and wet bulbs and the relative humidity.
Based on the discussion of the dew point temperature and the wet bulb temperature, the relationship among the dry bulb temperature, the wet bulb temperature, and the dew point temperature is as follows:
For unsaturated air: Dew point temperature < Wet bulb temperature < Dry bulb temperature
For saturated air: Dew point temperature = Wet bulb temperature = Dry bulb temperature
It should be pointed out that the reading of the wet bulb thermometer has a certain relationship with the wind speed passing over the wet bulb thermometer, and the wet bulb temperature is not a thermodynamic state parameter. In strict situations, the adiabatic saturation temperature should be used as a parameter to determine the state of the moist air, but it is very difficult to determine the adiabatic saturation temperature. Studies have shown that within a wide range of wind speeds from more than 2m/s to below 40m/s, the reading of the wet bulb thermometer changes very little. Therefore, in engineering, the wet bulb temperature is approximately used to replace the adiabatic saturation temperature as a state parameter representing the moist air. When looking up charts or calculating to obtain the relative humidity, etc. based on the dry and wet bulb temperatures, the readings of the ventilated dry-wet bulb thermometer should be taken as the standard.