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Why does air cool? And how is this related to cloud formation? This lesson will explore the adiabatic and diabatic processes, which are a couple of ways the air can cool, and then relate this to cloud formation.

Cloud Formation

Well, hi there! My name is Claudette Cloud, but my friends just call me Claudette.

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I’m here to tell you all about how I formed. Now, there are a couple of processes involved in cloud formation, specifically the adiabatic process and the diabatic process, which are sometimes referred to as adiabatic or diabatic cooling or warming. Before we delve into each process, though, let me just give you a brief overview on how I formed!I’ll start from the beginning.

If you check out that puddle over there, you might see some water evaporating, or changing from a liquid to a gas. Or if you check out those leaves over there, you might see some dew, which is the result of condensation, or changing from a gas to a liquid.Water is continually evaporating and condensing.

At some point, the air becomes saturated, meaning the air is full and can’t hold any more water vapor molecules, and for every water vapor molecule that evaporates, one must condense. This coincides with the dew point temperature, which is the temperature where the air is saturated. As the air cools, it can hold less water vapor before it becomes saturated, so as the air cools, it becomes saturated. This whole process is a little more complicated, and air isn’t holding water vapor, but it’s important to know that colder air has less water vapor than warm air.Okay, back to that dew point temperature and saturation. When the air is saturated, water vapor will condense on condensation nuclei, or tiny particles like dust, ash, pollen or bacteria that are floating around in the air. This condensed water creates water droplets suspended in the air, and when you get a bunch of these droplets together, you get me, Claudette the Cloud!So, we know the air cooled and became saturated, but what caused the air to cool in the first place? Let’s talk about those two processes I mentioned at the beginning of the lesson, starting with the adiabatic process.

Adiabatic Process

Let’s follow the adiabatic process to see how I was formed! Look over there – I see a parcel of air, which is just a clump of air that shares similar properties. Now that parcel of air is ascending into the atmosphere.There is less pressure the higher you get in the atmosphere, so there will be less pressure exerted on the parcel as it ascends. It may be hard to visualize this, so let me give you an example. Pretend you let go of a helium balloon and it starts to float away.

As it gets higher in the atmosphere, there is less pressure, so the air inside of the balloon isn’t contained by the pressure. The higher it goes, the bigger the balloon will get, and eventually it will pop!The same thing is going to happen to that parcel of air. As it goes up, less pressure is exerted on it, and the parcel expands, taking up more room.

Of course, the parcel won’t ‘pop’ since it isn’t surrounded by plastic like the air inside of the balloon. Now, that parcel is made up of molecules that are bouncing around. As the parcel ascends and spreads out, the molecules are less likely to collide with one another.Since they don’t collide as often, there is less energy (believe it or not, when those molecules collide, energy is exchanged). So, if they aren’t colliding as often, it’ll cause a temperature drop. This temperature drop causes the parcel of air to reach the dew point temperature, and you know what happens from there, right? Yep, water vapor condenses onto condensation nuclei and you get.

…me!So, to sum up, the adiabatic process is the cooling of a parcel of air as it ascends due to a change in pressure. It’s important to note that the adiabatic process deals with a pressure change, not heat entering or leaving the parcel, or the parcel mixing with another parcel. And one final note before we move on to the diabatic process – the adiabatic process works in reverse, too.

Meaning, as air descends, the pressure is greater and the parcel warms!

Diabatic Process

Now, let’s look at the other process, the diabatic process. Unlike the adiabatic process, this process does involve our air mixing with substances outside of the parcel. Let’s follow the diabatic process for the formation of my friend, Clarence Cloud.Clarence started as a parcel of air that floated over a cold body of water. Because of its proximity to the cold water, this parcel cooled to its dew point temperature, water vapor condensed on condensation nuclei and Clarence was born!So, in a nutshell, the diabatic process is the heating or cooling of a parcel of air due to a heat exchange. You’ll notice I said ‘heating’ in that definition.

Yep, if that parcel of air had blown over a warm surface, the parcel would have heated due to a diabatic process. But if that happened, it probably wouldn’t have reached its dew point temperature and Clarence wouldn’t have been formed!Before Clarence and I go, there are a few more things to discuss. Let’s take a moment to contrast the adiabatic and diabatic processes. First, diabatic processes are the result of a heat exchange, whereas adiabatic processes are the result of contraction (from a lot of pressure being exerted on an air parcel) and expansion (from less pressure being exerted on an air parcel).

Second, diabatic cooling or heating is not reversible. For example, the parcel that created Clarence will not re-warm unless it comes into contact with something to warm it back up, like the warm ground or a warm body of water. However, adiabatic cooling or heating is reversible, because if the parcel ascends or descends, it cools or warms. Finally, diabatic is the result from heat leaving or entering the parcel, whereas in adiabatic, no heat leaves or enters.

Lesson Summary

I’ve sure enjoyed sharing my life story with you, and I know Clarence feels the same way. Before you run off, let me give you some reminders so you won’t forget us!

  • Clouds form when air reaches its dew point temperature and water vapor condenses on condensation nuclei.

  • Air can cool through adiabatic and diabatic processes.
  • Adiabatic processes are due to the expansion and contraction of an air parcel as it ascends or descends in the atmosphere. As the parcel ascends, it expands, and the molecules collide less and less, thus reducing the amount of heat energy and resulting in a cooler parcel of air.
  • Diabatic processes are due to a heat exchange. For example, a parcel of air might blow over a cold body of water, which causes the parcel to cool.

Okay, well it looks like Clarence and I are starting to warm up, so we might dissipate soon (but that’s another lesson!). Bye now!

Learning Outcomes

Once you’ve completed this lesson, you should be able to:

  • Explain how clouds form
  • Describe the adiabatic and diabatic processes

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