Heat Humidity Factor

Overview

Engineering reference data for Heat Humidity Factor in air psychrometrics.

Key Formulas

Humidity Ratio

$\omega = 0.622 \frac{P_v}{P_a}$

Mass of water vapor per mass of dry air.

Relative Humidity

$\phi = \frac{P_v}{P_{vs}} \times 100\%$

Ratio of actual to saturation vapor pressure.

Wet Bulb Temperature

$T_{wb} = T_{db} - \frac{P_{vs} - P_v}{\gamma}$

Temperature measured by wet-bulb thermometer.

Enthalpy of Moist Air

$h = c_p T + \omega h_g$

Sensible + latent heat per unit mass of dry air.

Variables

Symbol	Description	Unit
$\omega$	Humidity ratio	kg/kg
$\phi$	Relative humidity	%
$P_v$	Vapor pressure	Pa
$P_{vs}$	Saturation vapor pressure	Pa
$T_{db}$	Dry bulb temperature	°C
$T_{wb}$	Wet bulb temperature	°C

Heat Index Table

The following table shows how apparent temperature (Heat Index) increases with both dry temperature and relative humidity:

Note: Empty cells indicate conditions beyond the standard NWS heat index chart range, where heat-related illness risk is extremely high.

Heat Index Danger Zones

Understanding the health risks associated with elevated heat index values is critical for safety:

Heat Index Range (°F)	Risk Level
90 – 104	Heat cramps or heat exhaustion possible
105 – 130	Heat cramps or heat exhaustion likely, heatstroke possible
> 130	Heatstroke highly likely

At heat index values above 130°F, the body's thermoregulation system may fail. Outdoor activity should be avoided, and immediate cooling measures should be available.

Example

From the table above: with a dry temperature of 85°F and relative humidity of 80%, the apparent temperature (Heat Index) is 97°F.

Interactive Charts

References

• Original Source