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Humidity on Earth and on the Space Station

Condensation on a window pane

Condensation on a window pane (Paul Maguire, iStockphoto)

Condensation on a window pane

Condensation on a window pane (Paul Maguire, iStockphoto)

Let's Talk Science

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Humidity is an important part of life on Earth and life in space. This backgrounder explains what humidity is, how it is measured and how it affects us both mentally and physically.

Humidity on Earth

Most of the air around you has water in it. But the water is invisible. So how do we know it is there? We can feel it! Humidity is a measurement of how much water vapour is in the air. Water vapour is water in its gas state. We can see the water in its solid state (ice) and its liquid state. But we cannot see water vapour. 

Misconception Alert

Many people think steam coming out of a kettle is water vapour. They’re wrong! Steam is actually tiny droplets of condensed liquid water. 

There is always water vapour in the air, even when the sky is clear and blue! 

Hygrometer (Source: bong hyunjung via iStockphoto). 

How do we measure humidity?

People measure humidity with a device called a hygrometer (high-grow-meter). The most common measurement of humidity is called relative humidity (RH)

Relative humidity is a comparison of two numbers. The first number is how much water vapor the air can hold at the current temperature. The second number is the amount of water vapor in the air. This type of measurement is called a ratio.

The hygrometer gives relative humidity measurements as a percentage (%). 

Let’s see how this works with an example. Imagine that the air at a certain temperature can hold 50 g of water vapour. But there is only 25 g of water vapour. 

In this case, the relative humidity is 25 g/50 g or 50% RH.

High humidity is over 50% RH. Low humidity is under 30% RH.

What role does water vapour play in the water cycle?

Water vapour plays a big role in the water cycle. There are three important parts of the water cycle:

  • Condensation
  • Precipitation
  • Evaporation

Condensation is when water vapour in the air changes to liquid water. If you look outside the window, you might see an example of condensation. Can you guess what it is? If you said a cloud, you’re right!  Clouds are made of water that condensed from water vapour. 

But sometimes, clouds can no longer hold all of their condensed liquid water. That’s when precipitation happens.

Precipitation can be liquid or solid. Some examples of precipitation are:

  • Rain
  • Snow
  • Sleet
  • Hail
  • Drizzle 

The opposite of condensation is evaporation. Evaporation happens when water changes from a liquid to a vapour. This happens because of heat.

The main processes of the water cycle are evaporation, condensation and precipitation
The main processes of the water cycle are evaporation, condensation and precipitation (Let’s Talk Science using an image by Wasserkreislauf.png: de:Benutzer:Joooo derivative work: moyogo [CC BY-SA 3.0] via Wikimedia Commons). 

How does humidity affect our physical and mental health?

Many people are very sensitive to humidity. We sweat to keep our bodies from overheating. When we sweat, water from the sweat evaporates from our skin. As it evaporates, the water transfers heat away from our skin. The result? Our skin feels cooler.

When the relative humidity is high, we can get overheated very easily. That is because less water evaporates from our skin. Overheating can make you feel sick. This is why it’s very important to drink lots of water when the RH is high. 

High humidity can cause other problems, too. It can cause microorganisms like bacteria and mold to multiply and spread.

In the winter, the outdoor air is cooler. The relative humidity can go way down.  When the relative humidity is low, our skin can become dry and itchy. Our hair can become dry. Our throats and noses might feel scratchy. And we might get colds and flu that last for a long time! 

But what about the indoor air? What is the best range for relative humidity? It depends on the season. In the winter, the RH indoors should be around 30%. In the spring and summer, the RH indoors should be between 30% and 50%. That includes your classroom at school!

Humidity on the International Space Station

The International Space Station (ISS) is a large spacecraft that orbits the Earth. It is closed ecological system. It’s a little bit like a terrarium. It does not naturally exchange any solids, liquids or gases with anything outside of itself. Any new matter is brought in by the astronauts. Closed ecological systems are very useful for scientists. Scientists use them to study plants, humans and other animals. The ISS is also a science laboratory. Astronauts bring animals and plants on board to study them. 

Almost all of the humidity on the ISS comes from the astronauts. When the astronauts breathe or sweat, it creates humidity. The process of breathing is called respiration. The process of sweating is called perspiration

It is very important to control the humidity on the ISS. This way, the astronauts, the equipment and the ISS itself can all stay safe. 

The Water Reclamation Subsystem for the International Space Station
The Water Reclamation Subsystem for the International Space Station (Source: NASA).

How is humidity controlled on board the ISS?

The ISS has a special system for controlling humidity. It’s called the Temperature and Humidity Control (THC) subsystem. It’s part of a bigger system called the Environmental Control and Life Support System (ECLSS). The ECSS provides water and air to astronauts, animals and plants on the ISS. 

Humidity on board the ISS stays at around 60% RH. The THC makes sure that the air moves throughout the station. That way, moisture does not build up anywhere. 

The ISS uses heat exchangers to control the humidity. As air passes through the heat exchangers, it gets cooler. The water in the air condenses. The water is then collected so that it can be reused.

Recycling water on board the ISS is very important. Astronauts reuse approximately 93% of the water they produce. That includes breath, sweat and urine (pee). This keeps the humidity on the station down. It also helps maintain the station’s water supply.

The relative humidity must stay below 70%. If it gets higher, there can be problems for both the astronauts and the ISS. Remember how humidity on Earth can make microorganisms grow? This can happen on board the ISS, too. Astronauts can get sick if they breathe in these microorganisms. 

Microorganisms may also be bad for the station itself. They can make glass hard to see through. They can make rubber seals brittle. They can even clog air and water filters. 

High humidity can also lead to condensation inside the ISS. There is lots of electronic equipment inside the ISS. Can you guess what would happen if water condensed and collected on it?The equipment could short-circuit. It could possibly start a fire!

As you can see, it’s very important to control the humidity on board the ISS.



Learn More

  • The Challenges of Building Closed Ecological Systems (2015) David Russell Schilling -- A short article outlining the challenges of building closed ecological systems, including a short video about Biosphere 2
  • Water Recycling on the ISS (2013) Canadian Space Agency -- In this video (1:52 min.) Commander Chris Hadfield explains the water recycling process on the ISS
  • What Space Smells Like (2012) Megan Garber -- A short article chronicling the ways astronauts describe the “smell” of space, and a look into the way NASA is trying to recreate that “smell” 


Canada Mortgage and Housing Corporation. (2009, February 2). Measuring humidity in your home.

EDinformatics. (1999). What is humidity?

Federal Aviation Administration. (n.d.). Cabin environment and EVA environment.

Government of Canada. (2015, June 18). Reduce humidity, moisture and mould.

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