How does a thunderstorm form?

Many people have been afraid for them when they were a child: thunderstorms. Roaring clouds, thunder and lightning, and lots and lots of rain – typically in the summer months, after extreme heat and moist have found their way into your area.

But why and how do thunderstorms form? What are the stages of a thunderstorm? And how does one die? Those are all valid questions that we will investigate in this article. We’ll first cover convection, which is a more general term to characterize the cloud movements of thunderstorms. Then, we’ll look at the basic process of convective cloud formation.

Once we’re familiar with these basics, we’ll take a look at the lifecycle of a thunderstorm: i.e., how a regular cumulus cloud forms, why and how it grows into a thunderstorm, and what happens next.

Let’s dive into this very interesting topic. Are you ready? Let’s take a look!

Characterizing thunderstorms through the lens of convection

When you take a look at weather forums and weather blogs, you’ll often see people shout things like severe convective storms are likely, or there won’t be any convective activity today.

Obviously, they are talking about storms and most likely thunderstorms, but what is thing called ‘convection’ that they are talking about? And what does it have to do with storms?

Let’s first take a look at the Wikipedia definition of convection:

Convection is the transfer of heat due to the bulk movement of molecules within fluids (gases and liquids), including molten rock (rheid).

Wikipedia

That’s quite a mouth full of words, isn’t it?

Not surprising, given the fact that convection is related to fluid mechanics, and hence to physics. But we don’t give up, as it’s perfectly possible to explain atmospheric convection in layman’s terms.

Let’s break it apart into two parts:

  • The transfer of heat
  • (due to the) bulk movement of molecules within fluids, which can be gases and liquids.
Although we cannot see it with our eyes, air contains quite a bit of water – as a gas. Photographer: K at Pexels.

Air is a gas and hence susceptible to molecule movement

If you look at the Wikipedia page that explains our atmosphere, you will find that it contains 78.09% nitrogen, 20.95% oxygen, 0.93% argon, 0.04% carbon dioxide, and small amounts of other gases.

Gas molecules are molecules too and can hence be moved around. By consequence, heat can be transfered within the fluid – i.e., convection. But how does this happen in our atmosphere? And what does this have to do with thunderstorms?

Atmospheric convection: the relationship between heat movement and thunderstorms

If we want to establish convection in the atmosphere, we’ll need to make sure that heat is transported. If we want to understand how heat is transported, it’s important that you understand that there are temperature differences in the atmosphere. In the lower areas, near the ground, it can be quite hot – with temperatures of up to 60 degrees Celcius, or 140 degrees Fahrenheit, in some remote areas. This occurs due to the heat of the Earth as well as sun rays that convert into heat upon hitting the Earth’s surface.

However, in the upper levels of the atmosphere – this is not the case. Here, air is very cool, often many degrees below freezing level.

This difference establishes an environment where heat can flow from the lower levels of the atmosphere into the higher levels. However, air does not rise spontaneously – an air particle needs to be triggered if it is to rise.

This can happen in multiple ways, which includes these:

  • Thermically, because the sun heats a part of a local surface more extensively than another part of that surface. This is often why small cumulus clouds form. This happens because hotter air tends to be less dense than colder air, and hence the hotter particles rise – transferring heat in the process.
  • Dynamically, for example because of a front between two air masses. The front ensures that air rises dynamically, because it is pushed upwards.
  • Orographically, i.e., due to mountainous terrain – the air simply has to rise because it is pushed over a mountain.
Around mountains, clouds often form. Photo: Pexels.

A rising air particle cools and can contain less humid

An air particle that is transported into higher levels of the atmosphere does cool down adiabatically. That’s a difficult English word for the observation that while the particle cools, it does not do so because it exchanges heat with its environment. Although the topic of adiabatic cooling is too difficult to explain in this article, it’s important to understand that because of changing circumstances (such as pressure and volume of the air), it needs to adapt – and does so by taking energy from itself, in the form of heat.

As we saw too, air contains water. However, as a function of among others the current temperature, air can contain a particular amount of water – called the humidity of the air. When the air is very dry, humidity is low. When it’s very humid, humidity is high – obviously.

However, when the rising air particle cools, it can contain less and less humid. At one point in time, the humidity is 100% – the maximum amount of water that can be present in the air in the form of gas. When it rises any further, cooling down, something must happen with the excess water. At that particular level in the atmosphere, condensation will occur, and a cloud is born.

Thunderstorm lifecycle: from birth to death

Now that we understand how a cloud is born through rising air, adiabatic cooling and eventually condensation, we can take a look at the various stages of a thunderstorm lifecycle. While the previous parts of this article explained how a thunderstorm forms, we’ll now take a look at what happens when one has formed. Here are the stages, which we’ll cover in more detail next:

  1. The regular cumulus stage (cumulus humilis and cumulus mediocris);
  2. The towering cumulus stage (cumulus congestus);
  3. The conversion into a storm (cumulonimbus calvus);
  4. The conversion into a more severe storm (cumulonimbus capillatus);
  5. A thunderstorm at its peak (cumulonimbus incus);
  6. The dissipation of a thunderstorm.

The regular cumulus stage

When condensation occurs as a result of convection, regular cumulus clouds will start to appear.

These clouds, which are called cumulus humilis (from ‘gentle’), are really what their name suggests: gentle clouds, oriented relatively horizontally, and they show no significant vertical movement.

We often see those cumulus clouds on summer days, when the sun heats the surface – after which thermal bubbles result in cumulus clouds.

Color-enhanced photo of cumulus humilis clouds above a golden meadow. PiccoloNamek at the English language Wikipedia. CC BY-SA 3.0.

In dynamic situations, and in situations where strong thermal bubbles are present, cumulus clouds can show some vertical movement. In those cases, we call them cumulus mediocris – from medium-sized cumulus clouds.

As you can see in the image below, these cumulus clouds are more vertically oriented than the humilis ones. They suggest that the convective flow is stronger, and that in some cases additional growth is possible.

Some cumulus mediocris clouds. Kr-val.

The towering cumulus stage

And with additional growth, I mean the next and final stage of a cumulus cloud: the congestus stage. In this stage, cumulus clouds are quite big, and often cauliflower-style.

The picture below shows a very clear cumulus congestus: clear vertical movement and relatively intense cloud structure. A storm may now emerge any time soon, as we will see next.

A cumulus congestus.

The cumulonimbus calvus stage

In the previous cases, a so-called updraft was present in the cloud: it’s the area under the cloud where the vertical air movements happen. Those vertical cloud movements suck more and more hot and humid air into the cloud, and therefore represent its motor.

Thanks to the updraft of the cumulus cloud, it can grow further into what is known as a storm – or a cumulonimbus cloud.

As condensation means that water is converted from gas into liquid shape, those liquid water droplets smash into each other – or, more formally, coalesce. This coalescation ensures that smaller droplets merge together into bigger ones. Simple rules of gravity tell us that when a downward force becomes bigger than an upward oriented one, a certain object onto which this force applies falls down.

The same is true for a droplet of water. When it’s small, its mass is small too, and the upward oriented movement of the updraft keeps it in the sky. When it’s bigger, gravity working on the droplet can become bigger than the updraft. In those cases, it will fall down towards the ground – a phenomenon we know as rain.

In this stage, we can classify the cloud as a cumulonimbus – a merger of ‘cumulus’ and ‘nimbus’, the latter meaning rainy. The area where rain falls is known as the downdraft. It’s often a very cooled area due to evaporation of the falling rain, and all cold winds that blow into your direction from the direction of a storm are caused by this downdraft.

A cumulonimbus calvus. Photo taken by Bidgee. CC BY-SA 2.5.

The cumulonimbus capillatus stage

However, after the downdraft emerges, the storm can continue growing, for example due to the fact that the updraft is really strong. This often happens during summer. In those cases, the cumulonimbus cloud may grow into another stage, which is called cumulonimbus capillatus.

Entry to this stage means that the cloud is attempting to reach maturity. This is clearly visible from the anvil-like top, which can be observed when you compare the image below with the calvus one above.

Anvils start to appear because updrafts eventually reach the so-called Equilibrum Level. At this level, rising air becomes cooler than the surrounding air, and hence cannot rise any further. As the updraft keeps pushing new air towards this level from the ground upward, air at the EL must spread – horizontally. This results in the anvil-like structure visible in the image.

A cumulonimbus capillatus. Photo taken by Pitero 86. CC BY-SA 4.0.

The cumulonimbus incus stage

When a cloud is at full maturity, we often call it cumulonimbus incus. It can be recognized by a very strong anvil structure; for instance, compare the incus below with the capillatus above.

Storms like those can often produce severe weather phenomena like large amounts of rain, severe wind gusts and large hail. Intense lightning can also be seen with these storms.

A cumulonimbus incus. Photo by Hussein Kefel. CC BY-SA 3.0.

The dissipation stage of a thunderstorm

However, every storm comes to an end. Viewing one as a machine, it will come to a halt when its energy is depleted – and the same is true for a thunderstorm:

When it runs out of energy, it will die.

And thunderstorms run out of energy because they can no longer suck hot and humid air from their surroundings into the storm.

Also known as: the death of an updraft.

Interestingly enough, the death of an updraft is caused by the storm itself.

Here’s why: when the cloud reaches the cumulonimbus stage, rain falls and a downdraft emerges. The cold air that is part of this downdraft will eventually reach the ground and, like the rising air at the Equilibrium Level, must spread horizontally – for exactly the opposite reasons.

In those cases, air will flow over Earth’s surface (causing the wind gusts upon arrival of a thunderstorm) but will also cut off the updraft, and hence the energy source for the thunderstorm.

While an existing updraft doesn’t die immediately when the updraft is cut off, the storm will eventually die off. Rain will continue to fall, and eventually the skies will clear again.

Conclusion

This article focused on the lifecycle of a thunderstorm. Why does it form in Earth’s atmosphere, and how does this happen? We explained it by taking a look at the various stages: the regular cumulus stages, the towering cumulus stage, and eventually the birth and death of a cumulonimbus cloud.

We explained this lifecycle through the lens of convection, which is the transport of heating that can occur due to thermal effects, but also due to dynamic weather phenomena such as weather fronts that push air upwards. Adiabatic cooling and eventualy condensation kick off that first cumulus stage mentioned above.

I hope this article was useful to you and has helped you understand how thunderstorms form. If you have any questions, remarks or other comments, please feel free to leave a comment below. I’d love to hear from you! 😊

References

Cumulonimbus calvus. (2006, March 7). Wikipedia, the free encyclopedia. Retrieved August 19, 2020, from https://en.wikipedia.org/wiki/Cumulonimbus_calvus

Cumulonimbus capillatus. (2015, April 20). Wikipedia, the free encyclopedia. Retrieved August 19, 2020, from https://en.wikipedia.org/wiki/Cumulonimbus_capillatus

Cumulonimbus incus. (2006, March 7). Wikipedia, the free encyclopedia. Retrieved August 19, 2020, from https://en.wikipedia.org/wiki/Cumulonimbus_incus

Cumulus congestus cloud. (2005, August 3). Wikipedia, the free encyclopedia. Retrieved August 19, 2020, from https://en.wikipedia.org/wiki/Cumulus_congestus_cloud

Convection: Weather. (2002, April 3). Wikipedia, the free encyclopedia. Retrieved August 19, 2020, from https://en.wikipedia.org/wiki/Convection#Weather

Cumulus cloud. (2002, April 3). Wikipedia, the free encyclopedia. Retrieved August 19, 2020, from https://en.wikipedia.org/wiki/Cumulus_cloud