Causes of Saturation & Saturation Processes

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  • 0:01 Saturation
  • 1:47 Causes of Saturation
  • 3:30 Condensation Nuclei &…
  • 5:06 Types of Ice Nucleation
  • 6:36 Lesson Summary
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Lesson Transcript
Instructor: Julie Zundel

Julie has taught high school Zoology, Biology, Physical Science and Chem Tech. She has a Bachelor of Science in Biology and a Master of Education.

Have you ever wondered what happens to water vapor in the air around you? This lesson will explore saturation, or what happens when the air can't hold anymore water.


You're about to go on a whirlwind adventure following a water molecule around! Now, all water molecules look the same, so we'll need to keep track of your water molecule, so let's call him Wally Water Molecule, or just Wally for short. It's probably hard to believe, but Wally is going to become lots of things, from water in a lake to an ice crystal in the atmosphere!

Before we head out, let's get a few things straight about water. You probably already know that water can exist as a solid (ice), a liquid and as a gas (water vapor), but did you know that all three forms can exist in the atmosphere? I know it's strange to think of ice in the atmosphere, but it can happen, as you'll see shortly.

Let's start our tour in the lake where Wally and his friends are all liquid. It won't take us long to observe some evaporation, or the change from a liquid to a gas. And just like that, some of our lake water is water vapor, a gas. You'll notice Wally is happily bouncing around in the atmosphere now (please see the video at 01:03 for the illustration).

You might also notice some of Wally's friends undergoing condensation, or the change from a gas to a liquid. You can see condensation as dew on plant leaves or even as water droplets on the outside of a cold mug on a hot day.

Condensation and evaporation are going on all of the time, but at some point, the air becomes saturated, meaning the rate of evaporation is the same as the rate of condensation. In other words, the air can't hold any more water vapor at that temperature and pressure, so if any liquid water molecules become water vapor, the same amount of water vapor must condense and return to a liquid. As the air gets closer to becoming saturated, it feels more and more humid.

Causes of Saturation

So, what causes the air to become saturated? Good question. There are actually a few ways air can become saturated, so let's take a moment to investigate each.

1. Air becomes saturated due to evaporation.

Of course, this makes sense because in order for water vapor to enter the atmosphere, evaporation needs to occur. Evaporation is impacted by temperature, so if it's hot, the molecules move faster and are more likely to become a gas.

2. Air can also become saturated if it's cooled.

Colder air can hold less water vapor, so if you cool some hot air that is not saturated, it will eventually become saturated. Now, technically, this is a little more complicated, and it's not the air that 'holds' the water vapor, but more water vapor can be found in warmer air versus colder air, which is the take home message.

Air can cool for various reasons. For example, after the sun sets, the ground cools, which cools the air close to the earth's surface, causing it to reach saturation. Or, as a parcel of air rises, it will cool and expand as it gets higher in the atmosphere thus reaching its saturation point.

3. The mixing of two unsaturated air masses can create a saturated air parcel.

In this case, the two air masses are not saturated, but when they mix, they become saturated. For example, let's say you have cold air sitting above a warm body of water. As the water evaporates, it adds more water vapor to the cold air, causing it to become saturated. Before this mixing, the cold air was not saturated nor was the air with the water vapor from the lake. But after mixing, the cold air gained enough water vapor to reach saturation.

Condensation Nuclei and Ice Nuclei

Let's talk a little more about condensation. In order for our water vapor to condense, a couple of things need to occur.

  1. Air needs to be cooled to its saturation point where the air can't hold any more water vapor molecules.
  2. There needs to be something for the water vapor to condense on. For example, we mentioned the dew and the water forming on the mug from before. Both the plants and the mug are the things the water condensed on.

But what happens when condensation happens in the atmosphere where there are no plants or mugs? Let's check back with Wally, who is still happily bouncing around in the atmosphere.

Sometimes Wally and his friends can bump into nuclei in the atmosphere. Nuclei are tiny particles, like salt, pollen, bacteria, ash or dust, that give water vapor a surface to condense on. The nuclei acts like the plant or the mug back on the ground. There are different types of nuclei, depending on what type of particle forms.

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