Expert Article Relative humidity – How is it defined and calculated? Joni Partanen Product Manager Vaisala Industrial Manufacturing and Processes Industrial Measurements The first step on the journey to mastering humidity parameters is to understand partial pressures. We’ll begin our journey by the sea, with a one-meter square drawn in the sand. Now, imagine that there is an air column going up through the atmosphere and into space, with an air weight of about 10,300 kg. This air mass generates hydrostatic pressure with a force of 101,325 Newtons per square meter. This defines a pressure unit called Pascal [Pa]: Newtons per square meter. That is the total pressure. The air we breathe is a mixture of gases, where the main components are nitrogen (N2), oxygen (O2), water vapor (H2O), argon (Ar), and carbon dioxide (CO2). These components can be expressed using Dalton’s law of partial pressures: The component with the concentration that varies the most is water vapor, and it plays an important role in affecting weather and climate, as well as numerous industrial processes and other aspects of our everyday lives. On earth, water exists in three different forms: ice, water, and gas. The maximum amount of gaseous water is dictated by the temperature: the higher the temperature, the higher the partial pressure of water vapor, i.e. there is more dissolved water in the air. The maximum water vapor pressure at a specific temperature is called water vapor saturation pressure. Water vapor pressure cannot exceed this limit, and a simple way to prove this fact is to look at the clouds in the sky. Clouds are formed when the water vapor saturation pressure has been reached and the environment can no longer hold the water in its vaporous phase, meaning some of it condenses into tiny droplets that may eventually fall on us as rain.Relative humidity (%RH) is a concept that defines what percentage of water vapor is present at that temperature relative to the saturation point. When it’s raining, it’s obviously 100 %RH up there in the clouds where the droplets are forming. While the instruments and technologies available to us have greatly improved over the years, the weather can still sometimes surprise us. We humans tend to try to control our environment, and this has led to great strides forward in technological and industrial development. We can create controlled environments depending on our need, whether it’s for laundry drying or manufacturing cutting-edge technology from highly sensitive materials. What do a wet cotton shirt, conducting salts used for lithium battery manufacturing, and a HUMICAP® thin-film polymer sensor have in common? The answer is they are all hygroscopic materials, which means that they attract water molecules from their surrounding environment until they reach an equilibrium state. Here, relative humidity plays an important role because the equilibrium moisture content of a material is closely related to the relative humidity level. Luckily, I can trust that the battery in my mobile phone has been manufactured in a controlled environment and protected in a way that it tolerates occasional splashes. What about the cotton shirt that got wet in the rain? It doesn’t dry that well outdoors while it’s raining – even if I protect it from the rain. However, I can greatly enhance the shirt’s drying rate by putting it into a clothes dryer. Why does the shirt dry faster at a high temperature? Is there less humidity in the drying cabinet? The short answer is that the air is relatively drier at elevated temperatures. This means that the relative humidity inside the dryer is lower and the cotton shirt therefore seeks to achieve equilibrium with its surroundings and eventually becomes drier. Relative humidity is the ratio of the water vapor pressure to the saturation vapor pressure at a given temperature. If you want to deepen your knowledge of how to calculate relative humidity, related quantities, and many other humidity parameters, download our Conversion Formulas technical eBook. Joni Partanen Product Manager Vaisala Joni Partanen is a Product Manager at Vaisala. He is responsible for the development of humidity and vaporized hydrogen peroxide measuring products. He has over 16 years of experience in process industry measurement technology, industrial engineering, and instrumentation. Joni holds a Bachelor of Engineering degree in Automation Technology. Vaisala Humidity Calculator Relative humidity, absolute humidity, wet-bulb temperature, enthalpy, water concentration, and many more. Humidity calculations and conversions made easy. Humidity Calculator Relative humidity related products Hand-Held Humidity and Temperature Meter HM40 for quick inspections and spot-checking Compact, portable, and easy-to-use Vaisala HUMICAP® HM40 handheld humidity meter is designed for... Read more Handheld Humidity and Temperature Meter HM70 for demanding spot-checking applications Vaisala HUMICAP® HM70 Handheld is designed for demanding humidity measurement in spot-checking... Read more Humidity and Temperature Transmitter Series HMDW110 ±2%RH transmitters with a certificate for demanding HVAC Vaisala HUMICAP® Humidity and Temperature Transmitter Series HMDW110 is suitable... Read more Humidity and Temperature Transmitter Series HMDW80 ±3%RH low maintenance transmitters for standard HVAC The HMDW80 series is versatile with transmitters for wall and duct mounting for... Read more Humidity Module HMM100 for environmental chambers The Vaisala HUMICAP® Humidity Module HMM100 is an open frame module for integration into environmental chambers. The... Read more Humidity and Temperature Probe HMP110 with ±1.5%RH accuracy for demanding volume applications The Vaisala HUMICAP® Humidity and Temperature Probe HMP110 is a trouble-free and... Read more Digital Open Frame Humidity Module HMM105 for OEM applications An open frame module for integration into environmental chambers and incubators. The module incorporates the Vaisala HUMICAP®... Read more HUMICAP® Humidity and Temperature Probe HMP113 The Vaisala HUMICAP® Humidity and Temperature Probe HMP113 is a highly accurate and cost-effective humidity probe with a plastic enclosure. It has... Read more HMP60 Humidity and Temperature Probe for volume applications HMP60 is a simple, durable and cost-effective humidity probe suitable for volume applications, integration into other... Read more Relative Humidity and Temperature Probe HMP7 for high humidities Vaisala HUMICAP® Humidity and Temperature Probe HMP7 is designed for applications which involve constant high humidity or... Read more INTERCAP® Humidity and Temperature Probe HMP63 The Vaisala INTERCAP® Humidity and Temperature Probe HMP63 is a cost-effective humidity probe with plastic enclosure. It is designed for... Read more Relative Humidity and Temperature Probe HMP4 for high pressures Vaisala HUMICAP® Humidity and Temperature Probe HMP4 is designed for high-pressure applications such as compressed air... Read more Relative Humidity and Temperature Probe HMP5 for high temperatures Vaisala HUMICAP® Humidity and Temperature Probe HMP5 is designed for high temperature applications such as baking ovens... Read more Relative Humidity and Temperature Probe HMP8 for pressurized processes Vaisala HUMICAP® Humidity and Temperature Probe HMP8 is designed for pressurized applications in compressed air... Read more Compact Humidity and Temperature Probe HMP9 for rapidly changing environments The Vaisala HUMICAP® Humidity and Temperature Probe HMP9 is designed for easy installation in rapidly changing... Read more Humidity and Temperature Transmitters HMT120/130 ±1.5%RH transmitters with exchangeable probes for cleanrooms and light industrial applications The Vaisala HUMICAP® HMT120 and HMT130 transmitters... Read more Wi-Fi Data Logger HMT140 for monitoring temperature, relative humidity, current, voltage & door contacts The Vaisala HMT140 Wi-Fi data logger is designed for... Read more Humidity and Temperature Transmitter Series HMT310 for demanding industrial applications The Vaisala HUMICAP® HMT310 series is for demanding industrial applications where compact product design... Read more Moisture and Temperature Meter Series MMT310 for transformer oil, hydraulic and lubrication systems in marine and paper industry Vaisala HUMICAP® Moisture and Temperature Transmitter Series... Read more
Relative humidity related products Hand-Held Humidity and Temperature Meter HM40 for quick inspections and spot-checking Compact, portable, and easy-to-use Vaisala HUMICAP® HM40 handheld humidity meter is designed for... Read more Handheld Humidity and Temperature Meter HM70 for demanding spot-checking applications Vaisala HUMICAP® HM70 Handheld is designed for demanding humidity measurement in spot-checking... Read more Humidity and Temperature Transmitter Series HMDW110 ±2%RH transmitters with a certificate for demanding HVAC Vaisala HUMICAP® Humidity and Temperature Transmitter Series HMDW110 is suitable... Read more Humidity and Temperature Transmitter Series HMDW80 ±3%RH low maintenance transmitters for standard HVAC The HMDW80 series is versatile with transmitters for wall and duct mounting for... Read more Humidity Module HMM100 for environmental chambers The Vaisala HUMICAP® Humidity Module HMM100 is an open frame module for integration into environmental chambers. The... Read more Humidity and Temperature Probe HMP110 with ±1.5%RH accuracy for demanding volume applications The Vaisala HUMICAP® Humidity and Temperature Probe HMP110 is a trouble-free and... Read more Digital Open Frame Humidity Module HMM105 for OEM applications An open frame module for integration into environmental chambers and incubators. The module incorporates the Vaisala HUMICAP®... Read more HUMICAP® Humidity and Temperature Probe HMP113 The Vaisala HUMICAP® Humidity and Temperature Probe HMP113 is a highly accurate and cost-effective humidity probe with a plastic enclosure. It has... Read more HMP60 Humidity and Temperature Probe for volume applications HMP60 is a simple, durable and cost-effective humidity probe suitable for volume applications, integration into other... Read more Relative Humidity and Temperature Probe HMP7 for high humidities Vaisala HUMICAP® Humidity and Temperature Probe HMP7 is designed for applications which involve constant high humidity or... Read more INTERCAP® Humidity and Temperature Probe HMP63 The Vaisala INTERCAP® Humidity and Temperature Probe HMP63 is a cost-effective humidity probe with plastic enclosure. It is designed for... Read more Relative Humidity and Temperature Probe HMP4 for high pressures Vaisala HUMICAP® Humidity and Temperature Probe HMP4 is designed for high-pressure applications such as compressed air... Read more Relative Humidity and Temperature Probe HMP5 for high temperatures Vaisala HUMICAP® Humidity and Temperature Probe HMP5 is designed for high temperature applications such as baking ovens... Read more Relative Humidity and Temperature Probe HMP8 for pressurized processes Vaisala HUMICAP® Humidity and Temperature Probe HMP8 is designed for pressurized applications in compressed air... Read more Compact Humidity and Temperature Probe HMP9 for rapidly changing environments The Vaisala HUMICAP® Humidity and Temperature Probe HMP9 is designed for easy installation in rapidly changing... Read more Humidity and Temperature Transmitters HMT120/130 ±1.5%RH transmitters with exchangeable probes for cleanrooms and light industrial applications The Vaisala HUMICAP® HMT120 and HMT130 transmitters... Read more Wi-Fi Data Logger HMT140 for monitoring temperature, relative humidity, current, voltage & door contacts The Vaisala HMT140 Wi-Fi data logger is designed for... Read more Humidity and Temperature Transmitter Series HMT310 for demanding industrial applications The Vaisala HUMICAP® HMT310 series is for demanding industrial applications where compact product design... Read more Moisture and Temperature Meter Series MMT310 for transformer oil, hydraulic and lubrication systems in marine and paper industry Vaisala HUMICAP® Moisture and Temperature Transmitter Series... Read more
Hand-Held Humidity and Temperature Meter HM40 for quick inspections and spot-checking Compact, portable, and easy-to-use Vaisala HUMICAP® HM40 handheld humidity meter is designed for... Read more
Handheld Humidity and Temperature Meter HM70 for demanding spot-checking applications Vaisala HUMICAP® HM70 Handheld is designed for demanding humidity measurement in spot-checking... Read more
Humidity and Temperature Transmitter Series HMDW110 ±2%RH transmitters with a certificate for demanding HVAC Vaisala HUMICAP® Humidity and Temperature Transmitter Series HMDW110 is suitable... Read more
Humidity and Temperature Transmitter Series HMDW80 ±3%RH low maintenance transmitters for standard HVAC The HMDW80 series is versatile with transmitters for wall and duct mounting for... Read more
Humidity Module HMM100 for environmental chambers The Vaisala HUMICAP® Humidity Module HMM100 is an open frame module for integration into environmental chambers. The... Read more
Humidity and Temperature Probe HMP110 with ±1.5%RH accuracy for demanding volume applications The Vaisala HUMICAP® Humidity and Temperature Probe HMP110 is a trouble-free and... Read more
Digital Open Frame Humidity Module HMM105 for OEM applications An open frame module for integration into environmental chambers and incubators. The module incorporates the Vaisala HUMICAP®... Read more
HUMICAP® Humidity and Temperature Probe HMP113 The Vaisala HUMICAP® Humidity and Temperature Probe HMP113 is a highly accurate and cost-effective humidity probe with a plastic enclosure. It has... Read more
HMP60 Humidity and Temperature Probe for volume applications HMP60 is a simple, durable and cost-effective humidity probe suitable for volume applications, integration into other... Read more
Relative Humidity and Temperature Probe HMP7 for high humidities Vaisala HUMICAP® Humidity and Temperature Probe HMP7 is designed for applications which involve constant high humidity or... Read more
INTERCAP® Humidity and Temperature Probe HMP63 The Vaisala INTERCAP® Humidity and Temperature Probe HMP63 is a cost-effective humidity probe with plastic enclosure. It is designed for... Read more
Relative Humidity and Temperature Probe HMP4 for high pressures Vaisala HUMICAP® Humidity and Temperature Probe HMP4 is designed for high-pressure applications such as compressed air... Read more
Relative Humidity and Temperature Probe HMP5 for high temperatures Vaisala HUMICAP® Humidity and Temperature Probe HMP5 is designed for high temperature applications such as baking ovens... Read more
Relative Humidity and Temperature Probe HMP8 for pressurized processes Vaisala HUMICAP® Humidity and Temperature Probe HMP8 is designed for pressurized applications in compressed air... Read more
Compact Humidity and Temperature Probe HMP9 for rapidly changing environments The Vaisala HUMICAP® Humidity and Temperature Probe HMP9 is designed for easy installation in rapidly changing... Read more
Humidity and Temperature Transmitters HMT120/130 ±1.5%RH transmitters with exchangeable probes for cleanrooms and light industrial applications The Vaisala HUMICAP® HMT120 and HMT130 transmitters... Read more
Wi-Fi Data Logger HMT140 for monitoring temperature, relative humidity, current, voltage & door contacts The Vaisala HMT140 Wi-Fi data logger is designed for... Read more
Humidity and Temperature Transmitter Series HMT310 for demanding industrial applications The Vaisala HUMICAP® HMT310 series is for demanding industrial applications where compact product design... Read more
Moisture and Temperature Meter Series MMT310 for transformer oil, hydraulic and lubrication systems in marine and paper industry Vaisala HUMICAP® Moisture and Temperature Transmitter Series... Read more