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It's called a fallstreak hole, so-called because the water in that region of the cloud is falling to Earth.

We explain: Strange cloud formation over British Columbia


Scott Sutherland
Meteorologist/Science Writer

Monday, February 23, 2015, 1:11 PM - If you were awake early Sunday in Surrey, B.C., you would have witnessed a bizarre phenomenon that formed in the clouds, prompting many residents of the area to take to social media for answers.

The clouds made it seem like something supernatural was going on, but in reality was just a combination of a few events that lead to what many refer to as a 'fallstreak' or 'hole punch cloud'.


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What was it, though?

Aliens?

Divine intervention?

The apocalypse?

It's certainly pretty bizarre, and definitely something you don't see every day. However, it's entirely natural. No alien or divine intervention required, and it's not harmful in the least.

It's a fallstreak hole, so-called because the water in that region of the cloud is falling to earth, producing those pronounced streaks it does, and leaving behind a hole in the cloud.

How does this happen?

If you've ever taken a close look at weather reports, there's always something included called the dewpoint temperature. This is the temperature the air needs to reach before the water vapour in the air will condense out, to form clouds, fog or dew (or all three!). However, a key 'ingredient' to this condensation is that the water vapour needs something to condense onto - dust, pollen, particulate matter, etc - which are known as cloud condensation nuclei.

So, when a cloud forms, each of the millions to billions of tiny water droplets contained within surrounds a grain of dust or pollen or particulate matter.

If you cool the droplets in a cloud to below freezing temperatures, they won't necessarily form ice crystals, though. For that, there needs to be even more bits of dust, pollen, etc (which have also been cooled to at or below freezing) for the water droplets to grab hold of. These are called ice nuclei. If there aren't any of these ice nuclei around, because the cloud droplets have 'used up' all potential nuclei in that region of the sky, then the water will remain liquid at below-freezing temperatures - supercooled!


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The thing about supercooled water, though, is that it's just waiting for a chance to freeze, and it will take the first and easiest chance it gets!

In the case of a fallstreak hole - which is also known by names like skypunch or hole punch cloud - it's usually caused by the introduction of tiny ice crystals directly into the cloud layer. This can happen when an aircraft punches through the cloud layer, and crystals form in the wake of its wings, or on the tips of its wings or propeller blades.

As soon as they show up amid all those supercooled water droplets, these ice crystals are suddenly 'mobbed' as the water droplets rush in to freeze and help form larger ice crystals. This forms the streaks in the air, and since lots of water droplets go into producing just one larger ice crystal, this whole process sucks up a lot of the surrounding droplets, tearing a hole in the cloud.

As for the rainbow hues seen spread through part of this particular fallstreak hole ... well, that's caused by sunlight being refracted as it passes through the very ice crystals causing the hole to form. It's similar to a parhelion - more commonly known as a sun dog.

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