Sunday was a day of flip-floppy fall weather in Southern Ontario, with periods of rainfall and blustery winds punctuated by bursts of sun — a mix that, in some parts of the province, led to an ultra-rare, seasonally perfect cloud display from Mother Nature.

Residents across the Ottawa area in particular were awestruck by sinister, wave-like patterns that took over the skies yesterday as asperitas clouds appeared overhead, brought on by the unstable atmospheric conditions.

#ONStorm #ONwx Another example of stunning asperitas clouds over Eastern Ontario - this time in Carleton Place. This photo looks so unreal it could be a painting! 😮

These clouds tend to form when there’s a disturbance in the atmosphere, often after a storm or in the presence of… pic.twitter.com/e8ynfJOmox

— Instant Weather Ontario ⚡️ (@IWeatherON) October 6, 2024

Meteorologists believe the creepy formations, which only just made it into the International Cloud Atlas in 2017, may be caused by the "turbulent air" of storm fronts, as they often (but not always) appear around the same time as convective thunderstorms.

first time seeing these! pic.twitter.com/Q6e3iuxCtl

— cornchip (@scorn_chip) October 6, 2024

The weather phenomenon was first proposed as its own cloud type in 2009, and is quite special, not only because of how infrequently it happens, but because of its eerie and dramatic look that is more akin to an oil painting or cartoon than real life.

Asperitas/Asperatus Undulatus clouds in Kanata!! Awesome! #onstorm pic.twitter.com/Ea4KSbcv3p

— Nick d'Entremont (@ThruNicksLens) October 6, 2024

As is often the case when asperitas materialize, many took to social media to buzz about the undulating veil of cloud cover, which was very evocative of the Halloween season.

Unreal. pic.twitter.com/2avaqEqVo4

— Ache (@NounouAcheson) October 7, 2024

Excited weather experts were thankfully online to explain the sight to those who were baffled by what they were looking at, giving a positive identification to inquiring minds.

😳 soooo cool

— Linda Ann Stofa (@Fallingstar6) October 6, 2024

Described as "otherworldly," "spooky," "unreal," and more, the event had everyone looking at the sky, wondering what sort of apocalypse the formations may have been heralding.

5:20 pm Sunday October 6. Wild Asperitas clouds with rolling thunder. Very cool. #ONStorm ##ONwx #Ottawa pic.twitter.com/WK4WvYOYoT

— Ian Black (@BlacksWeather) October 6, 2024

Fortunately, asperitas clouds are not dangerous and do not produce rain, though they feel extremely threatening when looming overhead.