When air with a relative humidity (RH) of 50% passes through an air conditioning unit for cooling and dehumidification, its relative humidity jumps to 80% or even 90%. So where does the dehumidification go? Isn't relative humidity supposed to get lower?
The core point is this: Temperature is the decisive factor affecting air's capacity to hold water vapor.
Detailed Explanation of Core Concepts
1. Temperature
Definition: A measure of air's hotness or coldness, serving as the foundation for all other humidity parameters.Key Role: The saturated water vapor content of air (the maximum amount of water vapor air can hold) changes drastically with temperature. Higher temperatures mean greater molecular kinetic energy in air, enabling it to hold more water vapor; lower temperatures reduce this capacity.Units: Degrees Celsius (℃), Kelvin (K), Fahrenheit (℉).
2. Moisture Content
Definition: The mass of water vapor mixed with a unit mass of dry air. This is an absolute humidity index.Key Insights: Based on dry air as a reference, it remains constant regardless of changes in temperature or pressure (provided the total mass and water vapor mass stay unchanged). For this reason, it is the most commonly used humidity parameter in air conditioning and drying engineering, allowing precise calculation of humidification or dehumidification requirements. It eliminates interference from changes in the total mass of moist air (since the mass of water vapor itself is variable).Unit: Grams per kilogram of dry air (g/kg dry air).
3. Absolute Humidity
Definition: The mass of water vapor contained in a unit volume of moist air. This is also an absolute humidity index.Key Insights: Provides a direct measure of "how many grams of water are present in one cubic meter of air." Its main drawback is that its value changes with air volume expansion or contraction (i.e., temperature and pressure fluctuations), even if the total mass of water vapor remains constant. As temperature rises, air expands, reducing the number of water molecules per unit volume.Unit: Grams per cubic meter (g/m³).
4. Relative Humidity (RH)
Definition: The percentage ratio of the actual water vapor content in air (absolute humidity or water vapor partial pressure) to the saturated water vapor content at the same temperature (maximum possible capacity). This is a relative humidity index.Key Insights: It is the direct indicator of human perception of "humidity" or "dryness." It describes how close air is to saturation, rather than the absolute amount of water vapor present. It has an extremely close relationship with temperature: when the absolute water vapor content (moisture content) in air remains constant, higher temperatures increase air's holding capacity (saturated water vapor content), causing relative humidity to drop; lower temperatures reduce holding capacity, leading to a rise in relative humidity.Unit: Percentage (%).
Example: On a summer noon with a temperature of 35℃ and 50% RH, you may feel hot and dry. But in the evening, when the temperature drops to 25℃, the absolute water vapor content in air (moisture content) remains unchanged, but air's holding capacity decreases, potentially raising RH to 90%, leaving you feeling swelteringly humid.
5. Dew Point Temperature
Definition: The temperature to which air must be cooled (at constant moisture content and pressure) to reach saturation (RH = 100%).Key Insights: Although it is a temperature value, it essentially serves as a measure of the absolute water vapor content in air. Higher moisture content corresponds to a higher dew point temperature. Knowing the dew point temperature allows direct determination of the actual water vapor content in air.Practical Significance: When the surface temperature of an object (e.g., window panes, cold drink cans) falls below the dew point temperature of surrounding air, water vapor in air condenses into water droplets on the surface-a phenomenon known as "condensation."Impact on Relative Humidity: When air temperature approaches its dew point temperature, relative humidity nears 100%.
Comprehensive Relationship Analysis & Real-Life Examples
Core Logical Chain
Temperature ↑ → Air's saturated water vapor content (holding capacity) ↑Temperature ↓ → Air's saturated water vapor content (holding capacity) ↓
When Moisture Content/Absolute Humidity Remains Constant, Temperature and Relative Humidity Are Inversely Proportional
·Summer Day to Night: Daytime heat brings low relative humidity; nighttime cooling causes relative humidity to rise, creating a damp feeling.
·Air Conditioning Cooling: Air conditioners lower indoor temperature but do not immediately remove water vapor from air (moisture content remains unchanged). As temperature drops, air's holding capacity decreases sharply, leading to a rapid rise in indoor relative humidity and a cold, clammy sensation. Therefore, modern air conditioners must incorporate dehumidification (to reduce moisture content) to ensure comfort.
When Relative Humidity Remains Constant, Temperature and Moisture Content Are Directly Proportional
·Indoor Humidification in Winter: Cold outdoor air (e.g., 0℃, 80% RH) has very low moisture content. When heated to room temperature (e.g., 20℃), if no additional humidification is applied (moisture content stays constant), its relative humidity can plummet to below 20%, resulting in a dry feeling. To maintain a comfortable relative humidity (e.g., 50%), moisture content must be increased simultaneously with heating (the role of humidifiers).
Dew Point Is the "Thermometer" for Moisture Content
A bottle of beverage taken out of the refrigerator has a very low surface temperature. When warm, humid indoor air comes into contact with the bottle surface (which is below the air's dew point temperature), water vapor in air condenses into droplets on the bottle. In meteorology, the smaller the temperature difference between the dew point and the current air temperature, the higher the relative humidity, and the greater the likelihood of fog or rain.
Summary
·To know how much water is in the air: Look at moisture content or dew point temperature.
·To know how humid it feels: Look at relative humidity.
·To understand the driving force behind all changes: Look at temperature.
With a solid grasp of these key concepts, it becomes easy to explain why air conditioning dehumidification can increase relative humidity from 50% to 90%.
Air conditioning units achieve dehumidification by cooling air, which reduces its moisture content. For example, air at 30℃ and 50% RH, after passing through an air conditioning dehumidification system, may become 13℃ with 90% RH. While the relative humidity has risen, the actual moisture content has decreased-from 13.3 g/kg dry air to 8.39 g/kg dry air. The rise in relative humidity occurs simply because the cooled air has a much lower capacity to hold water vapor. If this dehumidified air were reheated back to 30℃, its relative humidity would drop to just 30.9%.
Suzhou Pharma Machinery Co.,Ltd.
2026/01/23
Gino
