Folks waking up in Surrey, British Columbia, this past Sunday were treated to a rather unusual weather phenomenon that looked like something out of a science fiction movie.
In what is a bizarre coincidence of two atmospheric events, an elongated cloud formation known as a “fall streak” or “hole punch” cloud appeared above the southern section of the province in the early morning hours. Photos of the rare appearance, along with speculations, have since gone viral. But in this case neither Hollywood nor aliens had a hand in this event. It was all mother nature’s doing.
As the name suggests, the unusual sky formation is actually a hole in a cloud that has part of it falling out in the form of localized snowfall. Clouds are made of water droplets and, during wintertime, ice crystals and snowflakes are formed when the drops adhere to particles, otherwise known as cloud condensation nuclei. These particles can be anything from plain-old dust, flower pollen, or even particulate matter from factories.
At times, however, ice crystals and even super-cooled water can form in a much more direct manner — by simply having a rapid drop in temperature to about minus 40 degrees Celcius. At such low temperatures water molecules slow down their movements until they freeze spontaneously.
In hole punch clouds, this very same process can happen in a short time, but in only a small portion of the cloud. And in the case of the one that we saw form above Surrey, a passing plane causes these crystals to rapidly form in its wake.
Research has shown that plane wings and propellers can expand the surrounding air so that super-cooled water chills to the point of freezing. In turn, surrounding water droplets in the cloud quickly get drawn to these newly formed ice crystals and form larger particles that quickly drop out of the sky.
In essence a miniature, localized snowstorm develops that basically has all the moisture falling out, leaving behind the very distinctive and eerie looking hole at the cloud’s centre. Planes passing by — even those that have punched through the cloud hours before any formation becomes visible — can directly cause these holes to grow to gargantuan sizes, some the size of an entire city.
In some instances there have been distinctive curtains or rods of shimmering light, even rainbow-like streaks at the heart of these hole punch clouds.
The brilliant spots are caused by the refraction of sunlight on any remaining ice crystals still floating in the atmosphere. These light patches are known as “sun dogs” and “solar halos.”
At this point researchers are not sure if these fall-streak or punch hole clouds have any connection with global climate, nor are they sure if the clouds have a measurable effect on localized weather. But one thing is certain — they sure do catch the eye.