Air Density from Wet and Dry Bulb Temperatures

What is Air Density?

Air density is the mass of air per unit volume, typically expressed in kilograms per cubic meter (kg/m³). It plays a key role in weather prediction, aviation, HVAC design, and industrial processes. Air density changes with temperature, humidity, and air pressure.

Role of Dry Bulb and Wet Bulb Temperatures

The dry bulb temperature is the regular air temperature measured with a standard thermometer. The wet bulb temperature is measured with a thermometer covered in a wet cloth and reflects the cooling effect of evaporation. The difference between the two helps estimate the humidity level in the air.

How to Calculate Air Density

A simplified method to estimate air density uses the ideal gas law, adjusted by the effect of humidity:

Air Density = (P / (R × T)) × (1 - 0.003 × (Dry Bulb - Wet Bulb))
  

Where:
– P is atmospheric pressure (assumed standard: 101325 Pa)
– R is the specific gas constant for dry air (287.058 J/kg·K)
– T is the absolute dry bulb temperature in Kelvin (°C + 273.15)
– The 0.003 factor adjusts for humidity effect using temperature difference

Example

If the dry bulb temperature is 30°C and the wet bulb temperature is 25°C:
T = 30 + 273.15 = 303.15 K
Base density = 101325 / (287.058 × 303.15) ≈ 1.161 kg/m³
Adjusted for humidity = 1.161 × (1 – 0.003 × (30 – 25)) ≈ 1.144 kg/m³