Rh Humidity Calculator

Calculate Relative Humidity (RH) using Dry Bulb Temperature and Wet Bulb Temperature.

Understanding Relative Humidity (RH) and How This Calculator Works

Relative Humidity (RH) is a crucial meteorological and environmental metric that describes the amount of water vapor present in the air compared to the maximum amount it can hold at a specific temperature. It's typically expressed as a percentage. An RH of 100% means the air is saturated with water vapor, and any additional vapor will condense into liquid water (like fog or dew).

Why is RH Important?

• Comfort: High RH can make warm temperatures feel hotter and cold temperatures feel colder, affecting human comfort.
• Health: Optimal RH levels (typically 40-60%) can help prevent the spread of airborne viruses, reduce mold growth, and alleviate respiratory issues.
• Materials: RH significantly impacts the preservation of goods, artifacts, electronics, and building materials, preventing degradation caused by dryness or excessive moisture.
• Agriculture: RH plays a vital role in plant growth, disease prevention, and seed germination.
• Industry: Many industrial processes, from manufacturing to food storage, rely on precise RH control.

The Science Behind the Calculation

This calculator uses the relationship between Dry Bulb Temperature (DBT), Wet Bulb Temperature (WBT), and the psychrometric chart principles to estimate Relative Humidity. The Dry Bulb Temperature is the ambient air temperature measured by a thermometer freely exposed to the air. The Wet Bulb Temperature is the lowest temperature to which air can be cooled by the evaporation of water into it, measured by a thermometer whose bulb is covered in a wet cloth.

The difference between DBT and WBT, known as the "wet-bulb depression," indicates the amount of moisture in the air. A smaller depression means the air is more humid (closer to saturation), while a larger depression indicates drier air.

The exact calculation of RH from DBT and WBT involves complex psychrometric equations that relate vapor pressure, saturation vapor pressure, and air pressure. A simplified approximation often used, especially for educational purposes and with reasonable accuracy at typical atmospheric pressures, involves iterative methods or lookup tables derived from these equations.

For this calculator, a common approach involves calculating the saturation vapor pressure at the Dry Bulb Temperature and the actual vapor pressure using empirical formulas derived from thermodynamic principles and experimental data. One such method approximates the actual vapor pressure using an equation that incorporates both DBT and WBT.

The formulas used can be complex, but a simplified representation of the process is:

1. Determine the Saturation Vapor Pressure (e_s) at the Dry Bulb Temperature (T_db). This can be approximated using the August-Roche-Magnus formula or similar empirical relations. For example: e_s(T) = 0.61094 * exp((17.625 * T) / (T + 243.04)) where T is in °C and e_s is in kPa.
2. Determine the actual Vapor Pressure (e) using the Wet Bulb Temperature (T_wb). This often involves an iterative psychrometric equation or an approximation. A common approximation relates the actual vapor pressure to the saturation vapor pressure at the wet bulb temperature and the wet bulb depression: e ≈ e_s(T_wb) - A * P * (T_db - T_wb) where:
   • e_s(T_wb) is the saturation vapor pressure at the wet bulb temperature.
   • A is the psychrometric constant (approximately 0.000665 K⁻¹ for aspirated psychrometers, but can vary).
   • P is the atmospheric pressure (standard atmospheric pressure is ~101.325 kPa, but this calculator assumes standard pressure for simplicity).
   • (T_db - T_wb) is the wet-bulb depression.
   More accurate calculations use iterative solutions of the psychrometric equation. This calculator uses a common approximation.
3. Calculate Relative Humidity (RH): RH = (e / e_s(T_db)) * 100%

Note: This calculator provides an estimate. For highly critical applications, consult professional psychrometric charts or specialized software that accounts for precise atmospheric pressure and other environmental factors.

Example Usage

Let's say you measure the following temperatures:

• Dry Bulb Temperature: 25°C
• Wet Bulb Temperature: 18°C

Inputting these values into the calculator:

Dry Bulb Temperature = 25°C
Wet Bulb Temperature = 18°C

The calculator would then estimate the Relative Humidity based on these inputs, giving you a value around 60% RH. This indicates that the air is holding about 60% of the maximum water vapor it could hold at 25°C.

% RH