Measuring thermal comfort - the temperatures¶
Dry bulb, wet bulb and dew point temperature explained, plus the heat index. These are the measurements the ARC comfort model and its thresholds are built on.
Dry bulb temperature (Tdb)¶
The dry bulb temperature, usually referred to as "air temperature", is the air property most commonly used. When people refer to the temperature of the air, they are normally referring to the dry bulb temperature.
It is called "dry bulb" because the air temperature is indicated by a thermometer not affected by the moisture of the air.
Dry bulb temperature can be measured using a normal thermometer freely exposed to the air but shielded from radiation and moisture. It is usually given in degrees Celsius.
The dry bulb temperature is an indicator of heat content, and is shown along the bottom axis of the psychrometric chart, or along the left side of the Mollier diagram.
Wet bulb temperature (Twb)¶
The wet bulb temperature is the adiabatic saturation temperature.
It can be measured using a thermometer with the bulb wrapped in wet muslin. The adiabatic evaporation of water from the thermometer bulb, and the resulting cooling effect, is indicated by a wet bulb temperature lower than the dry bulb temperature.
The rate of evaporation from the wet bandage on the bulb - and so the temperature difference between dry bulb and wet bulb - depends on the humidity of the air. The evaporation from the wet muslin is reduced when air contains more water vapour.
The wet bulb temperature is always between the dry bulb temperature and the dew point. For the wet bulb there is a dynamic equilibrium between heat gained, because the wet bulb is cooler than the surrounding air, and heat lost because of evaporation.
By combining the dry bulb and wet bulb temperature in a psychrometric chart or Mollier diagram, the state of the humid air can be determined.
Wet bulb temperature is the metric ARC uses for the humid-climate limits in success criteria and transient space temperatures, because it captures the combined effect of heat and humidity on the body's ability to cool itself.
Dew point temperature (Tdp)¶
The dew point is the temperature at which water vapour starts to condense out of the air - the temperature at which air becomes completely saturated. Above this temperature the moisture stays in the air.
- if the dew point temperature is close to the dry air temperature, the relative humidity is high
- if the dew point is well below the dry air temperature, the relative humidity is low.
If moisture condenses on a cold bottle taken from the refrigerator, the dew point temperature of the air is above the temperature in the refrigerator.
The dew point temperature is always lower than the dry bulb temperature, and will be identical at 100% relative humidity - the air is at the saturation line. As air temperature changes, the dew point tends to remain constant unless water is added to or removed from the air.
The dew point temperature can be measured by filling a metal can with water and some ice cubes, stirring with a thermometer, and watching the outside of the can. When vapour in the air starts to condense on the outside of the can, the temperature on the thermometer is the dew point.
The dew point is given by the saturation line in the psychrometric chart.
Heat index¶
The heat index combines air temperature and relative humidity, in shaded areas, to express hot weather as a "feels like" value.
The human body normally cools itself by evaporation of sweat. High relative humidity reduces evaporation and cooling, increasing discomfort and potential heat stress.
Use with care
The heat index is a news-channel format that may cause confusion when considered alongside ARC's adopted comfort model. ARC's thresholds are expressed in dry bulb and wet bulb temperature, not heat index.
Sources¶
- Engineering ToolBox - dry, wet bulb and dew point air
- Wikipedia - wet-bulb globe temperature
- Wikipedia - heat index
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The source document contains a heading "Dangerous heat" in this appendix with no content beneath it. It has not been reproduced here.
Source: ARC Technical Criteria - Appendix 1: measuring thermal comfort
Reference · confidence high · Imported 2026-08-29, not yet reviewed · next review 2026-11-29
Tags: reference, measurement, humidity, definitions
Citation key: measuring-thermal-comfort