What is Humidity? – Humidity Academy – Part 1 Chapter 1

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What is relative humidity? ROTRONIC has been dealing for almost 50 years with the issue of relative humidity, during this time not only the know-how but also the know-what and know-why has steadily been growing. It is now time to pass on this knowledge piece by piece.

The Humidity Academy that was setup by ROTRONIC USA is the way that we have chosen to share our knowledge. Starting with the first topic relating to the theory of relative humidity. We will work chapter through chapter bringing light into the dark or in our language humidity to the drought.

Back to school


  1. What is Humidity?
  2. Relative Humidity, Pressure and Temperature
  3. The Capacitive Sensor
  4. The Wet Bulb/Dry Bulb Technology
  5. Summary of Other Technologies
  6. Choosing the Right Humidity Measurement Technology

What Is Humidity?

Humidity is defined as some measure of the water vapor content of air (or other gas). The term “humidity” is a general term to quantify the amount of water vapor in the gas.

The term “humidity” is often interchangeable with “relative humidity”, but there’s a significant difference between these two terms in the context of precise measurement. This chapter explains why the difference between these two terms matters for people who are in the business of measuring water vapor in sensitive environments, and it covers the terms and definitions used to quantify the amount of water vapor in the gas.

Water vapor plays a critical role in maintaining quality and efficacy for products that improve our daily lives – products we consumers trust are manufactured to specification. It is essential for manufacturers in a variety of industries to understand how accurate humidity measurements work, and the role your measurement instrument plays.

The Importance of Accuracy

Humidity is measured using a hygrometer, a tool that utilizes various materials and measurements to gauge a room or space’s level of water vapor. While no scientific measurement is absolutely true, reaching humidity measurements that are as accurate as possible is crucial across industries. Since water vapor above certain levels can lead to condensation and eventually corrosion or mold, highly accurate humidity measurements are vital to preventing the degradation of everything from wooden construction materials, food products, pharmaceuticals, fuels, paper, electronic components and many other materials. Humidity measurements help maintain optimal environmental conditions for products and prevent costly damage to valuable goods.

Technical Information

In this section, we’ll uncover the basic laws of physics that govern relative humidity. Within a temperature range of -50 to 150°C and at pressures not in excess of 1000 kPa, water vapor practically behaves like an ideal gas. We’ll use examples to illustrate the influence of temperature and pressure on relative humidity, and how to convert relative humidity into dew point and absolute humidity.

First let’s cover the basics with a review of the general properties of water vapor in a moist gas.

Properties of Water Vapor in a Moist Gas


When a water molecule leaves a surface and takes gas form, it has evaporated. Through the absorption or release of kinetic energy a water molecule transitions from the liquid state to the vapor state. Liquid water that becomes water vapor takes a parcel of heat with it in a process called evaporative cooling.

Evaporative cooling is restricted by atmospheric conditions. The evaporation process consumes more heat when the air is very hot and dry, which makes the cooling effect more pronounced compared to evaporative cooling in hot and moist air.


The conversion of water vapor to a liquid is called condensation. Water vapor will only condense on a surface when the surface is cooler than the dew point temperature or when the water vapor equilibrium in the air has been exceeded. When water vapor condenses on a surface, a net warming occurs. The water molecule releases heat, and in turn, the temperature of the atmosphere slightly rises.


A “Swamp Cooler” conditions air in hot weather through the evaporation of water. They work well in dry climates like Denver or Phoenix, but would not work well in Houston or Boston, where there is more moisture in the air.

Chemical Reactions

Many chemical reactions yield water as a product. If the reactions take place at temperatures higher than the dew point of the surrounding air, the water will be formed as vapor and increase the amount of water vapor in the gas. If they take place at temperatures lower than the dew point, condensation will occur, and water vapor will leave the gas.

Other chemical reactions take place in the presence of water vapor, resulting in new chemicals forming, such as rust on iron or steel.

Download our Humidity Theory eBook right now:



Permanent link to this article: http://blog.rotronic.com/en/2014/11/humidity-academy-part-1-chapter-1-what-is-humidity/

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