Relative humidity is defined as:

Ratio of actual vapour partial pressure p_v to saturation pressure p_s at the same DBT

Mass of vapour per kg of dry air

Ratio of omega to omega_s at the same temperature

Temperature at which condensation starts

Specific humidity omega is expressed as:

kg of vapour per kg of dry air, omega = 0.622 p_v/(p - p_v)

kg of vapour per kg of moist air

Which statement is correct?

Dew point is the saturation temperature at the vapour's partial pressure

Degree of saturation equals p_v/p_s

Wet bulb can exceed DBT in unsaturated air

At saturation phi is 100% but mu is lower

Practice · 6 marks

Definitions of psychrometric terms

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%.

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.

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 $\phi = \mu = 100\%$ .

✓ Answer checked by Claude Opus 5.5 (high effort)

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Definitions of psychrometric terms

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