What are the updraft and downdraft of a thunderstorm?
At every moment in time, at least one thunderstorm – and often multiple more – are active somewhere on Earth. What’s more, it’s very likely that the country you live in faces a set of severe thunderstorms every now and then during summer. Thunderstorms are therefore very common appearances in the world.
But did you know that every thunderstorm, while producing very different weather phenomena ranging from severe rain, extreme wind gusts or even large hail – is the same, if you look at it from a very high level?
In this article, we’ll cover the mechanics behind a thunderstorm: the updraft of the storm, as well as the downdraft of the storm. We’ll look at how updrafts emerge and what kind of situations can trigger the emergence of updrafts – and we will do the same for a storm’s downdraft.
The emergence of an updraft: thunderstorm formation
Convective clouds (among which, thunderstorms) can form when parcels of air rise and keep rising with respect to the surrounding atmosphere. These rising air parcels will become saturated with moisture at one particular point in time, and all excess moisture will condensate – a cloud will form. This cloud, which we’ll also call a cumulus, signals rising air. Air can begin to rise because of a variety of reasons:
- Heat. Because the sun heats the surface, air can begin to rise. This happens because warmer air is less dense than colder air, and locally heated air can therefore become less dense than its environment – and thus rise with respect to this environment. If this happens at much larger scale, a low pressure area can form in the lower levels of the atmosphere. Such thermal lows can produce very severe thunderstorms sometimes!
- A dynamic trigger. A front, such as a cold front, does the same, but then at much larger scale. Especially cold fronts, which signal that a colder air mass is pushing a warmer air mass away, literally ‘push’ the cold air under the warm air, making it rise. Another forms of dynamic triggers can be the dry line in the United States – where hot, humid air meets colder, dryer air, as well as extensions of low pressure areas – called throughs or vores.
- Orography. If you let air flow over mountainous terrain, you literally force it to rise.
The rising air, in effect, lies at the basis of what is called an updraft. Let’s take a look at this in more detail. When a cumulus cloud is born, it’s a cumulus humilis – from ‘gentle’. It’s a really small cumulus cloud and it suggests gentle weather, especially when you see a field of those small cumulus clouds.
However, when the forces that make air rise are really strong – they can push the air beyond what is called the Level of Free Convection (LFC). Let’s also dive into this a bit more. When air rises, it does so because it is colder than the environment – or because it is pushed upwards. This means that air does not rise naturally. However, with height, the temperature of the environment – i.e. the atmosphere – itself falls too. Often, this means that from a particular level – especially after condensation, because that produces a bit of extra heat – rising air will remain warmer than the environment, up to a particular level (which we’ll call Equilibrium Level, or EL).
This level is the LFC. From this point until the EL, rising air does rise naturally, because it’s warmer than the surrounding air. The trigger (heat, a dynamic one or orography) is no longer necessary for air to rise. From this point, a cumulus cloud can grow rapidly into a thunderstorm. It first goes through the cumulus mediocris and cumulus congestus stages, before becoming a cumulonimbus. The rising flow of air, which provides the energy for the cloud to keep growing – that is, warm and humid air – is what we call updraft.
If it’s a thunderstorm, there must be a downdraft too
Within convective clouds, there is a lot of motion. As we know, when condensation occurs in an updraft, we’ll see a cloud. This cloud is essentially a collection of water droplets. Since updraft motion can be strong (with strong updrafts, the upward motion can be tens of kilometres an hour!), a lot of activity happens in the cloud.
In fact, water droplets collide with each other quite frequently. The effect is that two water droplets essentially merge – or coalesce. When this happens, the mass of the water droplets increases. If we look back at some of our high school physics lessons, we know that an object moves into the direction of net force. That’s why everything falls back to Earth: very often, gravity is stronger than other forces working on a particular object. What’s more, the force increases with mass – increasing the pull to Earth.
Now, the updraft’s upward motion essentially occurs because a force is working on those air particles – an upward force. While gravity also works on the water droplets in the cloud, the upward motion is stronger than gravity working on those droplets. However, when two droplets coalesce – which we described just now – the droplet becomes bigger, and mass increases. By consequence, the same happens for gravity.
And sometimes, this means that gravity working on the water droplet becomes stronger than the upward motion. When this happens, a droplet starts falling towards Earth.
This process is made stronger because updrafts are never entirely ‘straight’ – they always bend a little with height in the direction of where the storm is heading, because winds often become faster with height. This is known as wind shear.
When droplets start falling, a downward flow of air appears by consequence. Often, many droplets falling to Earth evaporate, which cools down air, making the flow go even faster. That’s why storms often come with large amounts of rain and with severe wind gusts happening at the same time. This downward flow of rain and air is what we know as a downdraft.
Impact of downdraft on the storm lifecycle
Funnily, the emergence of a downdraft can mean the end of a thunderstorm. Understanding why requires seeing the updraft and the downdraft of a storm as a system. The cell as a whole, that is the system, needs energy in order to flourish. This energy is provided by the updraft. The output of the storm is rain as well as the colder air mentioned before.
Now, recall that cold air tends to push warm air upwards.
Focus on the downdraft now. What happens when cold air from the storm reaches the ground? As it cannot sink into the ground, it will spread horizontally – across the Earth’s surface. This also means that, once the downdraft has been there for a while, it will extend beyond the edge of the storm, pushing all warmer air it encounters upwards.
Unfortunately for the storm, once it passes the edge, the updraft will be cut off. This happens because the storm can no longer source hot and humid air from the surroundings, because it has been pushed away by the downdraft. When this happens, the storm’s energy source has been cut off and it will die, slowly but surely.
Now, in situations where a lot of wind shear is present – so-called multicell thunderstorms will form. In more extreme cases, these can even grow into superstorms. For both, you will see that the updraft will remain intact for much longer, because of the fact that the downdraft is blown away from the updraft. What’s more, it’s likely that the downdraft triggers new growth of cells downstream the original thunderstorm because there it also pushes air upwards.
A brief recap on updrafts and downdrafts
In this article, we have looked at two of the main concepts of a thunderstorm – its updraft and its downdraft. Looking at what these are started with rising air, which can occur because of heat, a dynamic trigger and orography – a.k.a. mountainous terrain.
When this happens, and when it reaches the Level of Free Convection or LFC, air can rise freely until it reaches the Equilibrium Level, or EL. This pillar of rising air is what we call the updraft. Once rain starts to fall, which happens after a while because the violent flow in thunderstorms ensures that water droplets in the cloud collide into each other, after they merge, a so-called downdraft appears. Often, but not in all cases, the emergence of a downdraft predicts the end of a storm.
I hope that this article was useful for your understanding of weather in general and storm clouds in particular. If you have learnt something, I’d love to hear from you – so please feel free to leave a comment in the comments section below. Please do the same if you have any questions or other remarks. I’ll happily make sure to answer if possible.
Updraft and downdraft. (n.d.). Encyclopedia Britannica. https://www.britannica.com/science/updraft