A fogbow is a rare optical illusion that appears as a white arch in foggy conditions, rarer than a rainbow and often mistaken for one. When Marie Johnson woke up one foggy morning in outback New South Wales, she captured this ethereal sight on her historic farm. This article explains what a fogbow is, how it forms, and where you can see one in Australia.
What Is a Fogbow?
A fogbow, also known as a mistbow, is a meteorological phenomenon visible when the sun (or sometimes the moon) is behind you and fog lies ahead. Light rays shine on tiny water droplets in the fog, bending and reflecting them back toward the observer. Unlike a rainbow, the colors in a fogbow are muted because the water droplets are much smaller—typically less than 0.05 mm in diameter. This results in a white or faintly colored arch that can seem ghostly against a foggy landscape.
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How Is a Fogbow Different from a Rainbow?
The key difference lies in droplet size and color intensity. Rainbows form from larger raindrops that refract sunlight into distinct spectral colors. Fogbows, however, involve smaller droplets that cause diffraction—light waves spread out and overlap, producing a white appearance. The conditions for a fogbow are also more specific: fog must be dense enough for refraction but not so dense as to block visibility.
| Feature | Fogbow | Rainbow |
|---|---|---|
| Appearance | White or faintly colored arch | Vivid spectrum of colors |
| Droplet size | Less than 0.05 mm | 0.5 mm or larger |
| Frequency | Rare | Common |
| Formation | Fog droplets causing diffraction | Raindrops causing refraction |
Where and Why Do Fogbows Form in Australia?
In Australia, fogbows are most commonly observed in south-eastern regions, particularly during autumn and winter. Fog forms overnight as heat radiates from the ground into space, cooling the air near the surface. When the temperature drops to the dew point, tiny water droplets become suspended—creating an eerie atmosphere perfect for a fogbow. Senior meteorologist Ilana Cherny of the Bureau of Meteorology notes that the sun, observer, and fog must align precisely for a fogbow to appear.
Key Takeaways for Spotting a Fogbow
- Look for fog in early morning, especially after cold nights in autumn or winter.
- Position yourself with the sun behind you and fog ahead.
- Check the density—fog that is too thick will block the light; too thin won't produce the arch.
- Be patient: fogbows are fleeting and require perfect alignment.
FAQ
What causes a fogbow to be white?
The tiny water droplets in fog (less than 0.05 mm) cause diffraction of light, where waves spread and overlap, canceling out individual colors and producing a white or pale arch.
Can you see a fogbow at night?
Yes, a lunar fogbow or moonbow can form when moonlight shines through fog, though it is even rarer and appears as a faint white arc.
Are fogbows common in Australia?
No, fogbows are rare anywhere in the world, including Australia. They require specific fog density, sun position, and observer alignment, making them a treat for weather enthusiasts and photographers.
Understanding the science behind fogbows not only deepens your appreciation for nature's subtle displays but also helps you recognize one of the rarest optical illusions on Earth. Next time you wake to a foggy morning in Australia, step outside with the sun at your back—you might just witness a ghostly white rainbow.