Wet-Bulb Globe Temperature (WBGT)

$WBGT = (0.1*DB)+(0.2*GT)+(0.7*WB)$

$(DB)"Dry-Bulb air temperature"$

$(GT)"Globe Thermometer Temperature"$

$(WB)"Wet Bulb Temperature"$

The Math / Science

The Wet-Bulb Globe Temperature is a measure of heat stress which takes into account air temperature, humidity and solar radiation. The Wet-Bulb Globe Temperature equation is:

WBGT=(0.1xDB) + (0.2xGT) + (0.7xWB)

where:

WBGT = Wet Bulb Globe Temperature Index
WB = Wet-Bulb Temperature
DB = Dry-Bulb (air) Temperature
GT = Globe Thermometer Temperature

The WBGT was developed for the US Marine Corps Recruit Depot on Parris Island in the late 1950's. Humidity at Parris Island can be very high and Marines are subject to intense training in full military gear under full sun. This creates conditions where there is substantial risk of heat injury.

The WBGT was later used by researchers as an easily measured general heat-stress index. In time its use widened. Because its use is recommended in the Standard, ISO 7243, it is often used in Occupational Health and Safety guidelines for working in hot environments. It has been advocated for use in sports requiring continuous exertion, such as the marathon. It is also used for horses in equestrian events.

The WBGT is measured by a device with three temperature elements:

Black Globe Thermometer (Tg): This is measured by the six inch black globe thermometer which approximates the combined effects of radiation and wind.
Natural Wet-Bulb Temperature (Tnwb): This is a thermometer with its bulb covered with a wet cotton wick moistened with distilled water from a reservoir. Evaporation from the wet bulb cools the thermometer. The natural wet-bulb thermometer, like the black globe thermometer is not shielded from wind or radiation. This thermometer represents the integrated effect of humidity, wind and radiation.
Shade Air Temperature (Ta): This is the (shade) air temperature measured by a thermometer shielded from solar radiation.

Wet-bulb Temperature

The wet-bulb temperature is a measure of temperature that takes into account the cooling effect of evaporation. It represents the lowest temperature that air can reach through the process of evaporative cooling. This temperature is important in various fields, including meteorology, agriculture, and engineering, as it provides information about humidity, comfort levels, and the cooling potential of the air.

How It Is Measured

The wet-bulb temperature is measured using a psychrometer, which consists of two thermometers:

Dry-Bulb Thermometer: Measures the actual air temperature.
Wet-Bulb Thermometer: Has a bulb wrapped in a wet cloth. As water evaporates from the cloth, it cools the bulb, and the thermometer measures the temperature after this cooling effect.

Significance

Humidity: The wet-bulb temperature is influenced by the humidity of the air. Higher humidity levels result in a smaller difference between the dry-bulb and wet-bulb temperatures because the air is already saturated with moisture and can evaporate less water.
Evaporative Cooling: It is an essential parameter in evaporative cooling processes, such as cooling towers and some air conditioning systems.
Human Comfort: It is used in calculating heat indices and assessing thermal comfort, especially in hot and humid climates.

Practical Applications

Weather Forecasting: Used to predict dew point and potential for fog or frost.
Agriculture: Helps in planning irrigation and understanding plant stress due to heat.
HVAC Systems: Helps design systems for efficient cooling and maintaining indoor air quality.

Example

Consider a day with a dry-bulb temperature of 30°C and a wet-bulb temperature of 25°C. The 5°C difference indicates that the air can still evaporate moisture, providing a cooling effect. On a humid day, the wet-bulb temperature would be closer to the dry-bulb temperature, indicating less cooling potential.

Weather (Meteorology) Calculators

Heat Index: Approximates the heat index based on the temperature and relative humidity
Canadian Humidity Index (HUMIDEX): Computes the Canadian Humidity Index (HUMIDEX) based on the dry-bulb ambient temperature and the dew-point temperature.
Australian Apparent Temperature (AT): Computes the apparent temperature based on the dry ambient temperature, the relative humidity and the wind speed at ten meters.
Relative Humidity: Computes the relative humidity based on the actual density of vapor and the saturated density.
Dew Point from Relative Humidity: Computes the dew point based on the relative humidity and ambient temperature.
Summer Simmer Index: Computes the Summer Simmer Index based on the air temperature and relative humidity.
Wind Chill Index (North America): Computes the wind chill index based on the air temperature and the velocity of the wind.
Wet-Bulb Globe Temperature (WBGT): Computes the wet-bulb globe temperature index based on dry-bulb (air) temperature, globe thermometer temperature and the wet-bulb temperature.
Humature Index: Estimates the humidity index based on the temperature and dew point.
Antoine Equation: Computes the apparent vapor pressure of pure substances based on temperature and coefficients for the substance.
Barometric Formula (Tropospheric) calculator computes the normal barometric pressure based on the altitude (h) using the Exponential Atmosphere formula.
Planetary Temperature: Estimates a planet's temperature based on the luminosity of its star, distance to the star, average albedo and infrared optical depth of atmosphere.
Snow Water Equivalent (SWE): Computes the volume of liquid water contained in rain or snow pack defined by area, depth and snow type or rain.

References

1. UNITED STATES DEPARTMENT OF LABOR. (n.d.). Retrieved May 23, 2016, from https://www.osha.gov/dts/osta/otm/otm_iii/otm_iii_4.html#iii:4_2