Relative Humidity: Understanding the Effects on Air Density and Volume

by Joost Nusselder | Updated on:  June 22, 2022
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Relative humidity (abbreviated RH) is the ratio of the partial pressure of water vapor to the equilibrium vapor pressure of water at the same temperature. Relative humidity depends on temperature and the pressure of the system of interest.

What is relative humidity

Measuring Relative Humidity: The Essential Tool for Understanding the Air Around You

Measuring relative humidity is the way to find out how much water vapor is in the air compared to the maximum amount possible at a given temperature. It is a way to understand the quality of the air around you and how it can affect your health and comfort.

How to Use a Hygrometer?

Using a hygrometer is simple and requires little effort. Here are the basic steps:

  • Find the right hygrometer for your needs.
  • Follow the manufacturer’s instructions to set up the hygrometer.
  • Place the hygrometer in the area where you want to measure the relative humidity.
  • Wait for the hygrometer to stabilize and give a reading.
  • Take note of the reading and compare it to the range of relative humidity that is suitable for the area you are in.
  • If needed, adjust the humidity level by using fans, cool or warm air, or by adding or removing moisture from the air.

What Are Some Additional Tips for Measuring Relative Humidity?

Here are some additional tips to help you measure relative humidity accurately:

  • Make sure the hygrometer is properly calibrated before use.
  • Place the hygrometer away from direct sunlight, drafts, and sources of heat or moisture.
  • Take multiple readings at different times of the day to get a better understanding of the relative humidity in the area.
  • Knowing the temperature of the air is essential for proper understanding of the relative humidity. Use a thermometer to measure the temperature.

Measuring relative humidity is an easy way to help you understand the air around you and how it can affect your health and comfort. By using the right tools and following the proper steps, you can get an accurate reading of the relative humidity and take the necessary steps to improve the quality of the air.

Air Density and Volume: Understanding the Science Behind Relative Humidity

Air is a material that contains particles such as molecules, which are constantly in motion. The number of particles in a given volume of air is known as air density. When water vapor is added to the air, it creates a change in the air density and volume. This change in air density is what we know as relative humidity.

The Role of Pressure in Measuring Relative Humidity

The scientific instrument used to measure relative humidity is known as a hygrometer. This instrument works by measuring the partial pressure of water vapor in the air. The hygrometer is calibrated to a specific temperature and pressure, usually at sea level, which is known as the standard state. When the pressure changes, the resulting change in air density affects the measurement of relative humidity. Therefore, it is essential to check the instrument regularly to ensure accurate readings.

The Influence of Ideal Gas Law on Relative Humidity

The ideal gas law is a scientific principle that describes the relationship between pressure, volume, and temperature of a gas. This law can be applied to the air, which is a gas mixture. The ideal gas law states that as the volume of a gas increases, the pressure decreases, and vice versa. This means that changes in air volume can affect the relative humidity.

Examples of How Relative Humidity Affects Our Daily Lives

The following are some examples of how relative humidity affects our daily lives:

  • High relative humidity can make us feel hot and sticky, while low relative humidity can make us feel dry and itchy.
  • Relative humidity affects the drying time of paint, which is why it is essential to know the relative humidity before starting any painting work.
  • Relative humidity affects the performance of musical instruments made of wood, such as guitars and violins. High relative humidity can cause the wood to swell, while low relative humidity can cause the wood to shrink and crack.
  • Relative humidity affects the growth of plants, as plants require a specific level of humidity to thrive.

How Pressure Affects Relative Humidity

When a system is isobarically heated, meaning that it is heated with no change in system pressure, the relative humidity of the system decreases. This is because the equilibrium vapor pressure of water increases with increasing temperature. As a result, the ratio of the partial pressure of water vapor to the equilibrium vapor pressure of pure water decreases, causing the relative humidity to decrease as well.

On the other hand, when a system is compressed isothermally, meaning that it is compressed with no change in temperature, the relative humidity of the system increases. This is because the volume of the system decreases, causing the partial pressure of water vapor to increase. As a result, the ratio of the partial pressure of water vapor to the equilibrium vapor pressure of pure water increases, causing the relative humidity to increase as well.

Understanding the Complex Factors That Affect Relative Humidity

While the pressure dependence of relative humidity is a well-established empirical relationship, it is important to note that the interaction between pressure, temperature, and the properties of the gas mixture can be quite complicated. In particular, the enhancement factor, which is a function of the properties of the gas mixture, can significantly affect the relative humidity of a system.

To compute the relative humidity of a system, the easiest way is to use a dew point hygrometer, which is a device that measures the temperature at which dew begins to form on a cooled surface. The dew point temperature is then used to estimate the relative humidity of the system using an equation that is dependent on the properties of the gas mixture.

Effects of Humidity on the Environment and Health

  • High humidity levels can cause excess moisture, leading to mold growth and damage to building materials.
  • Extremely dry air can cause materials to become brittle and crack.
  • Humidity can affect the thermal properties of materials, making them less effective at providing insulation or cooling.
  • Humidity can also affect the lifetime of sensitive materials, such as electronics or artwork.

Impact on Climate and Seasons

  • Humidity affects the average temperature of a region, with wetter regions generally experiencing cooler temperatures and drier regions experiencing hotter temperatures.
  • Humidity affects the radiative warming of the earth’s surface, contributing to the greenhouse effect and climate change.
  • Humidity affects the seasons, with summer typically being the most humid season in many places.
  • Dew point, which is the point at which water vapor in the air begins to condense, is a measure of humidity and can be used to predict weather patterns.

Impact on Health and Cooling

  • High humidity levels can make it feel hotter outside, as the combined effect of air temperature and humidity creates the heat index.
  • Humidity affects the body’s ability to cool itself through sweating, making it feel more uncomfortable on hot and humid days.
  • Humidity can also affect indoor air quality and the growth of mold, which can have negative impacts on respiratory health.
  • Humidity affects the effectiveness of cooling systems, with higher humidity levels making it harder to cool a space.

Impact on Energy and Environmental Control

  • Humidity affects the energy required to cool or heat a space, with higher humidity levels requiring more energy to achieve the same level of comfort.
  • Humidity affects the energy required for industrial processes, such as drying or curing materials.
  • Humidity affects the ability to control environmental conditions in spaces such as greenhouses or data centers.
  • Humidity is a popular topic in technical journals and is often applied in the design of HVAC systems and other environmental control systems.

Overall, humidity has a significant impact on the environment, health, and energy usage in various parts of the world. It is important to understand the effects of humidity and how to control it to maintain a comfortable and healthy living and working environment.

Fun Facts About Relative Humidity

When the relative humidity is high, you may feel hotter than the actual temperature because your body cannot efficiently cool down through sweating. On the other hand, when the relative humidity is low, you may feel cooler than the actual temperature because sweat evaporates more quickly, making you feel dry and cool.

Indoor Humidity Levels Should Be Controlled

Maintaining a relative humidity level between 30% to 50% indoors is ideal for comfort and health. If the humidity is too low, it can cause dry skin, static electricity, and damage to wooden furniture. If it is too high, it can promote the growth of mold and dust mites (here’s how to take care of them), which can trigger allergies and respiratory problems.

Water Vapor is Lighter Than Air

Water vapor is lighter than dry air, which means that humid air is less dense than dry air. This is why humid air rises and why clouds and fog can form in the atmosphere.

Supersaturation Can Result in Clouds and Fog

When the air is cooled, the relative humidity increases. If the air becomes saturated, excess water vapor will condense into tiny liquid droplets or ice crystals, forming clouds or fog. In the absence of particles called nuclei, which can act as a surface for water vapor to condense on, the air can become supersaturated, resulting in the formation of fog.

The Wilson Cloud Chamber Explains the Formation of Clouds

Albeit not directly related to relative humidity, the Wilson cloud chamber, designed by physicist Charles Wilson, consists of a sealed container filled with a supersaturated vapor of alcohol and water. When a charged particle passes through the container, it ionizes the vapor, resulting in the formation of visible droplets that grow into cloud-like formations. This principle is analogous to the formation of clouds in the atmosphere.

Humidity Can Affect Sea Levels

As the temperature of the ocean increases, the water molecules gain kinetic energy and evaporate, increasing the water vapor content in the air above the sea. This results in an increase in atmospheric pressure, which can cause sea levels to rise. Additionally, excess water vapor in the atmosphere can result in more precipitation, which can also contribute to sea level rise.

Humidity Can Affect the Mass of Objects

When an object absorbs water vapor from the air, its mass increases. This can be a concern in industries that require precise measurements, such as pharmaceuticals or electronics. Additionally, humidity can affect the weight of food products, resulting in inaccurate measurements in recipes.

In conclusion, relative humidity is a fascinating topic that affects our daily lives in more ways than we may realize. From affecting our comfort levels to contributing to sea level rise, it is important to understand the components and principles of humidity to maintain a healthy and safe environment.


So, that’s relative humidity in a nutshell. It’s a measurement of the amount of water vapor in the air compared to the maximum possible given the temperature. You need to know the relative humidity to understand air quality and comfort, and it’s an easy way to do that. So, don’t be afraid to use a hygrometer and measure it!

I'm Joost Nusselder, the founder of Tools Doctor, content marketer, and dad. I love trying out new equipment, and together with my team I've been creating in-depth blog articles since 2016 to help loyal readers with tools & crafting tips.