Squalls: How to predict them, recognize them, and take cover
The warm half of the year is not only about freedom and relaxation, but also about the most dangerous weather phenomena. In the southern hemisphere, this season is coming to an end, and in the northern hemisphere it is about to begin. We will tell you about squalls—strong and dangerous winds that almost every inhabitant of the Earth has felt at one point or another.
Photo: Misael Silvera / Unslash
A squall is a sudden increase in wind, right?
It’s a little more complicated than that. There are many types of strong winds, and each has a different cause. A squall is a sharp increase in wind speed by 8 m/s or more, for a duration of more than 1-2 minutes. The average wind speed for the period of a squall should be higher than 11 m/s.
You can see that the definition of a squall is very formal: just a set of numbers. But behind these numbers there are several specific weather phenomena.
What kind of squalls are there?
The causes of squalls can be divided into two groups:
- Cumulonimbus clouds
- Every other cause
The fact is that, in cumulonimbus clouds, squalls are caused by the development of the cloud itself. The appearance of a squall (albeit a weak one) is almost an obligatory part of a thunderstorm.
However, all other causes are very different and are usually related to terrain features. For example, in hilly areas, squalls occur during bora or foehn winds, which we have already written about.
Next, we will elaborate on how squalls occur in cumulonimbus clouds.
Microbursts
Cumulonimbus clouds have very strong upward air currents. Vertical wind speeds can reach tens of meters per second! This wind carries huge masses of water and ice upward. When the flow begins to weaken, and there are too many crystals and droplets, they begin to fall.
Now, let’s quickly recall two facts:
- Cold air is heavier than warm air, so it sinks downward. The colder the air, the faster it sinks.
- As the air descends, it warms up. Moisture-laden air heats up more slowly than dry air, so it ends up being colder.
And back to the cloud again. Falling ice flakes and droplets, due to the friction force, drag the air behind them, and it begins to sink. The air in the cloud is very moist, so as it sinks, it heats up slower than it «should.» Because of this, it gets colder and colder compared to the surrounding air, and sinks faster and faster. As a result, droplets and crystals fall to the surface in the form of rain and hail, and the air spreads out over the surrounding area at a tremendous rate.
The process of air sinking and spreading is called a microburst. And a sudden increase in wind on the ground occurring as a result of a microburst is a squall.
If a thunderstorm cloud has «grown a leg,» this is a sign of a microburst. Photo: Andrea Fabry / Unsplash
How to recognize that a squall is approaching
You should always be wary of a squall when a thunderstorm is approaching, because a microburst can occur in any cumulonimbus cloud. But a squall often has a beautiful and visible precursor—the thunderstorm collar, the «little brother» of the cumulus cloud.
When colder air descends from a thunderstorm cloud, it pushes warmer air upward. Moisture in the warmer air condenses, and an elongated tangle of clouds appears at the leading edge of the cumulonimbus cloud.
Squall formation. The blue arrow shows descending cold air, the red arrow shows the storm collar and rising warm air, and the yellow arrow indicates the squall zone. Photo: John Fowler / Unsplash
Interestingly, cumulonimbus clouds themselves are formed in a similar way by a cold front: a huge mass of cold air piles up on top of a huge mass of warm air. It turns out that inside the «big» cold front, «small» ones are formed. A real fractal!
These small fronts are called gust fronts, and a squall is considered one of their signs.
How dangerous are squalls?
Very dangerous! They are one of the most dangerous phenomena in cumulonimbus clouds. You should be especially wary of squalls when there is something that could fall on you. Trees, billboards, and scaffolding are your main enemies. Therefore, it is better to take shelter in a safe place before a squall approaches.
Almost always, together with a squall you will get a thunderstorm and a downpour, so it is better to take shelter from everything all at once. Read our text about lightning, there are detailed instructions. In short, your main friends are buildings and cars. And if you don’t have them, then any depressions in the terrain will do. But we recommend you watch the weather forecasts, and don’t risk being in the center of the storm!
Our app has a parameter called the CAPE index. The higher it is, the more dangerous weather events you can expect.
The CAPE index is measured in J/kg, and can provide valuable information:
- Less than 0 J/kg indicates a stable condition with no cumulus or cumulonimbus clouds, heavy rain, or thunderstorms.
- 0-1000 J/kg indicates slight instability, with possible cumulus clouds, cumulonimbus clouds, light rain, and thunderstorms if there is enough moisture in the air.
- 1000-2500 J/kg implies moderate instability, with the possibility of cumulus clouds, cumulonimbus clouds, heavy rain, and thunderstorms.
- 2500-3500 J/kg represents high instability with the potential for thunderstorms, sometimes severe, hail, and squalls.
- Greater than or equal to 3500 J/kg indicates very high instability, leading to severe or very severe thunderstorms, and even tornadoes.
Keep an eye on the forecast, and stay safe!
Text: Eugenio Monti, a meteorologist and a climatologist
Cover photo: Oliver Schwendener / Unsplash
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