Relative Humidity Calculator
Relative Humidity: –%
Understanding Relative Humidity
Relative humidity (RH) is a crucial metric in meteorology, HVAC, agriculture, and human comfort. It expresses the amount of water vapor currently in the air as a percentage of the maximum amount of water vapor the air can hold at that specific temperature. When the air reaches 100% relative humidity, it is saturated, and condensation or precipitation is likely to occur.
Key Terms Explained:
- Dry-bulb Temperature: This is the standard air temperature measured by a regular thermometer. It's the temperature you typically see reported in weather forecasts.
- Wet-bulb Temperature: This temperature is measured by a thermometer that has its bulb wrapped in a wet cloth (or wick) and is exposed to airflow. As water evaporates from the wet cloth, it cools the thermometer. The amount of cooling depends on how dry the air is; drier air allows more evaporation and thus more cooling, resulting in a lower wet-bulb temperature. If the air is saturated (100% RH), no evaporation occurs, and the wet-bulb temperature will be equal to the dry-bulb temperature.
- Atmospheric Pressure: The force exerted by the weight of the air in the atmosphere. While often assumed to be standard (around 1013.25 hPa at sea level), variations in atmospheric pressure can slightly influence the psychrometric calculations, especially at higher altitudes.
How the Calculator Works
This calculator determines relative humidity using the dry-bulb temperature, wet-bulb temperature, and atmospheric pressure. It employs psychrometric principles, which relate these three variables to the moisture content of the air. The core of the calculation involves:
- Calculating Saturation Vapor Pressure: This is the maximum amount of water vapor the air can hold at a given temperature. It's calculated for both the dry-bulb and wet-bulb temperatures using a widely accepted approximation formula (Magnus-Tetens equation).
- Determining Actual Vapor Pressure: Using the saturation vapor pressure at the wet-bulb temperature, the dry-bulb and wet-bulb temperature difference (known as the wet-bulb depression), and the atmospheric pressure, the actual vapor pressure present in the air is estimated. This step utilizes a psychrometric constant.
- Calculating Relative Humidity: Finally, relative humidity is found by dividing the actual vapor pressure by the saturation vapor pressure at the dry-bulb temperature and multiplying by 100 to express it as a percentage.
Why is Relative Humidity Important?
- Human Comfort and Health: High RH can make hot temperatures feel even hotter (due to reduced evaporative cooling from the skin), while very low RH can lead to dry skin, irritated airways, and static electricity. Optimal indoor RH is generally considered to be between 40-60%.
- Agriculture: RH affects plant transpiration, crop growth, and the storage of produce.
- Industrial Processes: Many manufacturing and storage processes require precise humidity control to prevent material degradation, corrosion, or to ensure product quality.
- Weather Forecasting: RH is a key factor in predicting fog, dew, and precipitation.
Example Calculation:
Let's say you measure a dry-bulb temperature of 25°C, a wet-bulb temperature of 20°C, and the atmospheric pressure is 1013.25 hPa.
- Dry-bulb Temperature: 25°C
- Wet-bulb Temperature: 20°C
- Atmospheric Pressure: 1013.25 hPa
Plugging these values into the calculator will yield a relative humidity of approximately 61.6%.
Limitations and Assumptions:
This calculator uses standard psychrometric equations and constants, which are approximations. It assumes:
- The psychrometric constant used is for water (not ice), meaning the wet-bulb temperature is assumed to be above 0°C.
- The air is at standard atmospheric conditions, and the psychrometric constant is fixed.
- The formulas provide a good approximation for typical atmospheric conditions but may have slight deviations in extreme environments.