---
title: "Definitions of psychrometric terms"
subject: "Applied Thermodynamics"
chapter: 4
key: "semester_3/applied_thermodynamics:psychrometric-terms-definitions"
source: https://www.bctengineer.com/q/semester_3/applied_thermodynamics/psychrometric-terms-definitions
site: BCT Engineering
---

# Definitions of psychrometric terms

Applied Thermodynamics · Chapter 4: Air-Conditioning

- Asked in: year not given, 6 marks
- Video: [Psychrometry | Psychrometric Properties](https://www.youtube.com/watch?v=V1Wrl37oZAk) — SELF ENGINEER
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_3/applied_thermodynamics/psychrometric-terms-definitions
- Whole chapter: https://www.bctengineer.com/md/semester_3/applied_thermodynamics/chapter-4

## Question

Define the following terms used in psychrometry: dry bulb temperature, wet bulb temperature, dew point temperature, relative humidity, specific humidity (humidity ratio), degree of saturation and enthalpy of moist air.

## Short answer

Dry bulb temperature (DBT) is an ordinary thermometer reading; wet bulb temperature (WBT) is from a wet wick in moving air; dew point (DPT) is the saturation temperature at the vapour partial pressure. Relative humidity is p_v/p_s; specific humidity ω = 0.622 p_v/(p-p_v) kg/kg dry air; degree of saturation is ω/ω_s. Enthalpy h = 1.005t + ω(2501+1.88t) kJ/kg dry air. Saturated air has DBT=WBT=DPT and RH=100%.

## Answer

Moist air is a mixture of dry air and water vapour; psychrometry deals with its properties.

| Term | Definition |
|---|---|
| **Dry bulb temperature (DBT)** | Temperature of the air measured by an ordinary thermometer with a dry bulb, unaffected by moisture. |
| **Wet bulb temperature (WBT)** | Temperature shown by a thermometer whose bulb is covered with a wet wick and exposed to a moving air stream; always $\le$ DBT. |
| **Dew point temperature (DPT)** | Temperature at which the water vapour in the air starts to condense when the air is cooled at constant pressure (the saturation temperature at the partial pressure of the vapour). |
| **Relative humidity (RH, $\phi$)** | Ratio of the actual partial pressure of the vapour $p_v$ to the saturation pressure $p_s$ at the same DBT: $\phi = p_v/p_s$. |
| **Specific humidity ($\omega$)** | Mass of water vapour per kg of dry air: $\omega = 0.622\,p_v/(p - p_v)$ (kg/kg dry air). |
| **Degree of saturation ($\mu$)** | Ratio of the actual specific humidity to the specific humidity of saturated air at the same temperature: $\mu = \omega/\omega_s$. |
| **Enthalpy of moist air** | Heat content per kg of dry air: $h = 1.005\,t + \omega(2501 + 1.88\,t)$ kJ/kg dry air, with $t$ in °C. |

**Relations:** For unsaturated air, DPT $<$ WBT $<$ DBT. For saturated air all three are equal and $\phi = \mu = 100\%$.

## Check yourself

The answer's key points as multiple-choice questions, with the correct option and why.

1. Relative humidity is defined as:
   A. Mass of vapour per kg of dry air
   B. Ratio of actual vapour partial pressure p_v to saturation pressure p_s at the same DBT
   C. Ratio of omega to omega_s at the same temperature
   D. Temperature at which condensation starts
   Answer: B. phi = p_v/p_s at the same dry bulb temperature. The other options describe specific humidity, degree of saturation and dew point. (Key point: Relative humidity)

2. Specific humidity omega is expressed as:
   A. kg of vapour per kg of moist air
   B. kg of vapour per kg of dry air, omega = 0.622 p_v/(p - p_v)
   C. p_v/p_s
   D. 0.622 p_v/p
   Answer: B. Specific humidity is on a dry-air basis: omega = 0.622 p_v/(p - p_v). (Key point: Specific humidity)

3. For unsaturated air, the correct ordering of temperatures is:
   A. DPT > WBT > DBT
   B. DBT < WBT < DPT
   C. DPT < WBT < DBT
   D. WBT < DPT < DBT
   Answer: C. Evaporation cools the wet bulb below DBT, and the dew point is lower still. They are equal only at saturation. (Key point: Order of temperatures)

4. Which statement is correct?
   A. Dew point is the saturation temperature at the vapour's partial pressure
   B. Degree of saturation equals p_v/p_s
   C. Wet bulb can exceed DBT in unsaturated air
   D. At saturation phi is 100% but mu is lower
   Answer: A. DPT is the saturation temperature at p_v. Degree of saturation is omega/omega_s, and at saturation both phi and mu are 100%. (Key point: Dew point and degree of saturation)

## Related in Applied Thermodynamics

- [Properties of moist air numerical](https://www.bctengineer.com/question.md?key=semester_3%2Fapplied_thermodynamics%3Apsychrometric-properties-numerical) — Applied Thermodynamics, chapter 4
- [Adiabatic saturation and wet bulb temperature](https://www.bctengineer.com/question.md?key=semester_3%2Fapplied_thermodynamics%3Aadiabatic-saturation-wet-bulb) — Applied Thermodynamics, chapter 4
- [Adiabatic mixing of two air streams](https://www.bctengineer.com/question.md?key=semester_3%2Fapplied_thermodynamics%3Amixing-of-air-streams-numerical) — Applied Thermodynamics, chapter 4

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- [Define saturation temperature, critical point, etc.](https://www.bctengineer.com/question.md?key=semester_2%2Ffundamental_of_thermodynamics_and_heat_transfer%3Asaturation-temp-critical-point-defs) — Fundamental of Thermodynamics & Heat Transfer, chapter 3
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