Have can’t hold anymore. And for every molecule

Have you ever wondered why a cloud forms? Well, you’ve come to the right place! This lesson will explain why clouds form and will also examine why they disappear.

What’s a Cloud Droplet?

Wally Water Molecule is back to teach you all about cloud droplets. You don’t remember Wally, you say? Don’t worry. I’ll introduce you. Like his name implies, Wally Water Molecule is a water molecule, and he can exist as liquid water, solid water (aka ice) and a gas (or, water vapor). He is quite the world traveler and can be found in lakes and oceans, in the air, in clouds, in rain and even in snow! In fact, you might even see him change from a liquid to a gas, through evaporation, or change from a gas to a liquid through condensation.

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Today you’re in for a special treat: Wally is going to become a cloud droplet! A cloud droplet is water vapor that has condensed onto condensation nuclei. When you get a bunch of cloud droplets together, you have a cloud! But what are condensation nuclei? Well, let’s start from the beginning.

Formation of Cloud Droplets

Once upon a time, Wally was hanging out in a beautiful blue lake. But, like all water molecules, Wally wasn’t content just sitting around, so as soon as he had enough energy, he evaporated and became water vapor. Meanwhile, some of Wally’s fellow water molecules were condensing. You might see some of them as dew on plants or as water droplets forming on a cold glass on a hot day.

Okay, back to Wally. The air can only hold so many of Wally and his buddies before it becomes saturated, which means the air is full of water vapor and can’t hold anymore. And for every molecule of water that evaporates, one must condense.

But what causes the air to become saturated, or full of Wally and his friends? Let’s take a look.

  1. Evaporation – If the rate of evaporation has increased, there is more water vapor in the air; therefore, the air can become saturated.
  2. The air is cooled – As air is cooled, it can hold less water vapor before it becomes saturated. Technically it’s a little more complicated, but the basic explanation is that more water vapor is found in warm air than in cold air.

Okay, so now you know what saturated means (and a couple of ways saturation can happen), so let’s check back with Wally, who’s still bouncing around in the atmosphere. Wally is now in saturated air, so he really wants to condense, but there is nothing to condense on in the atmosphere – or is there? Uh oh, it looks like Wally is about to bump into something.

Look out!Wally just collided with condensation nuclei, or tiny particles like dust, pollen, ash or bacteria that are floating in the atmosphere. And when Wally collides with these tiny particles, he’s able to condense, turning into liquid water that is suspended in the air, otherwise known as a cloud droplet! So, cloud droplet formation just means water vapor condensed on condensation nuclei. If Wally and all of his friends become cloud droplets, they will collectively become a cloud.

Dissipation of Cloud Droplets

I’d like to say Wally and his friends lived happily ever after in that cloud, but all good things must come to an end.

Unfortunately for that cloud, it’s becoming a bright, sunny day. Wally and his friends are about to disappear and the cloud is going to dissipate. When the cloud droplet dissipates, it just means the cloud droplets evaporate. But what causes a cloud droplet to evaporate? I’m so glad you asked!

  1. The cloud’s temperature increases – When the temperature of the cloud increases, water evaporates at a faster rate, so Wally is more likely to evaporate. Many things can increase the temperature of the cloud, like the sun, or warm air radiating off of the ground. That’s why low fog and clouds tend to disappear as it warms up during the day.
  2. The cloud and the drier air mix together – This drier air causes evaporation to increase, thus causing the droplets to dissipate.

    You can see where this type of mixing has occurred with a cloud because the outer edges are wispy.

  3. The air within the cloud descends – When the air descends, it warms. This is due to the adiabatic process, which means as an air parcel, like a cloud, descends, there is greater pressure being exerted on it. This increase in pressure causes the molecules to bump into each other more often, which results in an increase in heat energy, which warms the cloud. This increase in temperature results in evaporation.

Back to our story. Today is turning out to be really hot, so the temperature inside of the cloud is increasing.

Wally and his friends are starting to get really warm, and, oh no, some of Wally’s friends are disappearing! And boom! There goes Wally!Enough cloud droplets evaporated that the cloud has dissipated. But don’t worry, Wally isn’t gone forever. By now, you should know that Wally just changes forms a lot, so he’s still in the atmosphere. But now he’s water vapor again and not liquid water. So, for now, that’s the end of this Wally Water Molecule tale. However, I’m sure you’ll see Wally again really soon! Before we go, let’s take a moment to review some important parts of our story.

Lesson Summary

  • Water is continually changing states, or forms. In this story, Wally evaporated, condensed and then evaporated again.
  • When the air can’t hold any more water vapor it is saturated, and for every water molecule that evaporates, one must condense. Water can become saturated in several ways, including evaporation and when air cools.
  • When water vapor condenses in the atmosphere, it does so on tiny particles called condensation nuclei.
  • Liquid water that has condensed on condensation nuclei can form cloud droplets, and when you have enough cloud droplets, you get a cloud.

  • Clouds dissipate when the cloud droplets evaporate, which can happen for several reasons, such as a temperature increase in the cloud, the cloud mixing with dry air or the cloud descending.
Graphic of water molecules

Take a look at that! I think I see Wally, and he has formed another cloud droplet! Wait, there are some of his friends. Wow, they’re forming a giant cloud! Go Wally go!

Learning Outcomes

After you are finished with this lesson, you should be able to:

  • Explain the saturation process
  • Describe how cloud droplets form
  • Recall what makes a cloud droplet evaporate

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