Toronto experienced a brief blast of cold leading into the work week, priming the skies for an interesting optical phenomenon — one of the few signs of cold weather so far this season as the city reaches the home stretch of its warmest winter on record.

Anyone scanning the skies on Monday afternoon may have spotted a strange halo of light framing the sun. This atmospheric optical phenomenon, known as a sun dog (also known as mock sun or a parhelion in meteorological terms), had some commenters on social media confused as to what they were seeing.

What is going on with the sky/sun?
byu/poohbear1011 intoronto

People unfamiliar with the phenomenon took to platforms like Facebook and Reddit for answers.

Facebook

For anyone still wondering, a sun dog occurs when sunlight is refracted by ice crystals suspended in the atmosphere. While they can be seen year-round in any season, sun dogs require the presence of these ice crystals, which bend light rays to form effects ranging from faint-coloured halos to vibrant solar duplicates flanking the sun, depending on conditions.

These ice crystals float down through the atmosphere, exposing their flat faces between the sun and the ground and acting as lenses that refract the light horizontally.

Sun dogs are best seen when the sun is near the horizon, at sunrise and sunset, while sightings at midday, like those witnessed in Toronto on Monday, are less common.

In Toronto, sun dogs are most visible amid cold temperatures like on Monday afternoon, when temperatures hovered around 0 C. A similar cold-weather phenomenon experienced in Toronto in recent years is known as light pillars.

Rare winter phenomenon appears above Ontario skies during extreme cold https://t.co/x0H8aWfeMY #Ontario #Winter

— blogTO (@blogTO) February 6, 2023

As with sun dogs, these vertical beams of light are an atmospheric optical phenomenon caused by the reflection of light through ice crystals suspended in the sub-zero air.