A terrifying phenomenon known as a "fire cloud" has appeared over France for the first time in recorded history, and experts say it's one of the clearest signs yet that Europe's wildfires are reaching a dangerous new level.

Officially called a pyrocumulonimbus, these towering clouds don't just hover over a fire, but actually make the blaze even more intense.

As swarms of fires continue tearing across Europe, scorching hundreds of miles and forcing thousands to evacuate, firefighters are bracing for this frightening new threat. 

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How does a 'fire cloud' form?

Massive wildfires release enormous amounts of heat, smoke and ash into the atmosphere. As that superheated air rises, it cools and condenses into a cloud.

(MORE: How the biggest wildfires can create their own weather)

If the fire is hot enough and the atmosphere is unstable, that cloud can explode upward several miles into the stratosphere.

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And that's where things get really scary.

Why are they so dangerous?

These "fire clouds" can generate their own powerful winds, throw burning embers miles ahead of the flames, spark lightning that ignites brand new fires and, in some cases, even produce spinning fire tornadoes.

It's essentially a wildfire creating the perfect conditions to keep feeding itself.

Even worse, these clouds can become even more dangerous when they run out of energy. As they collapse, they can send showers of burning debris back to the ground in what's known as a plume collapse, potentially spreading flames in multiple directions and creating hazardous conditions for firefighters.

The phenomenon is normally associated with massive fires in places like Canada and Australia. But now meteorologists have observed what appears to be France's first-ever pyrocumulonimbus over the Gironde wine region.

(MORE: Wildfire smoke could change the taste of your wine)

What this means for France

Recently, fire clouds developed over a massive wildfire already burning close to the city of Bordeaux, where more than 200,000 people were forced to evacuate.

Experts say seeing conditions like these in France is scientifically remarkable, but deeply concerning.

"Seeing pyrocumulonimbus anywhere is concerning because there must be enough heat and dry enough conditions to create fires that are big enough to change the weather around them. Those conditions are less common in Europe," explained meteorologist Jonathan Belles.

That means that as we see Europe's wildfire season growing longer and more intense, once-rare fire clouds are a reminder that the most out-of-control fires could still be ahead.