Marie Johnson woke up one foggy morning on her historic farm in Cobar, outback New South Wales, and had a feeling the weather would be eye-catching. Heading into town for work, she quickly stopped to photograph the fog with an old house. But when she turned around, she saw an ethereal archway. 'Oh my God, look at that. What is this? It's like a rainbow, but it's white,' she said.
What is a fogbow?
A fogbow is a rare optical illusion, similar to a rainbow but with muted colors. According to Ilana Cherny, a senior meteorologist at the Bureau of Meteorology (BoM), a fogbow is visible when the sun (or sometimes the moon) is behind you and the fog is ahead. Light rays shine on water droplets in the fog, bending and reflecting them back toward you. The fog must be 'right in the Goldilocks spot'—dense enough for refraction and reflection to occur, but not so dense that it blocks visibility. Also known as a mistbow, fogbows have less pronounced colors than rainbows because the water droplets are smaller.
Where and why do fogbows form?
Fogbows require fog, which occurs in south-east Australia, particularly during autumn and winter. Fog usually forms overnight and into the morning when tiny water droplets are suspended in the air near the Earth's surface, creating an 'eerie atmosphere,' says Cherny. The most common formation is through radiation cooling: overnight, the surface loses heat to space, cooling the air closest to the ground until it becomes saturated with moisture, which condenses into water droplets. Dr. Kimberley Reid, an atmospheric scientist at the University of Melbourne, notes that fog often goes unnoticed but can have major impacts, such as flight delays and car accidents. For example, in December 2023, more than 20 cars were involved in a multi-vehicle crash northwest of Melbourne due to thick fog.
What about cloudbows and other optical illusions?
Sunlight and water in the sky can produce many striking effects. A cloudbow is similar to a fogbow but appears higher up, says associate professor Martin Singh from Monash University. To see one, the best viewpoint is from an airplane looking down with the sun behind you. 'For a rainbow or a cloudbow, you need something for the light to reflect off. With a rainbow, it's raindrops; with a fogbow or a cloudbow, it's the tiny droplets of fog or clouds,' he explains. Glories are another illusion, appearing rainbow-like but much smaller and more circular. Diamond dust, a shimmering effect, can occur in alpine areas when the weather is clear, calm, and very cold (usually below -10°C), according to the BoM. Halos are bright rings of light in the sky caused by light moving through ice crystals in high clouds. And double rainbows, made famous by a 2010 viral video, occur when two reflections inside each raindrop create an additional arch with reversed colors.
Rarity of fogbows in Australia
Fog and fogbows are quite rare in Australia, says Reid, and difficult to observe because they are so localized. 'One of the challenges with fog is that you need people there to observe it. I'm sure we get lots more fogbows that only the kangaroos get to see. So it's great that we got one where someone was able to take a photo.'
