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Is that a ferry boat in the sky? How summer mirages play tricks on your eyes

This story originally aired Aug. 23, 2019.

Most people have had that classic summer experience of driving along a warm road and seeing a shimmering patch ahead that looks like water. But when you get there, it’s gone. This is a trick of the atmosphere, caused by different densities of the air, associated with temperature.

KNKX weather expert Cliff Mass says people ask him about mirages all the time. Among the “most famous" ones, he says, is that "water on the road" mirage — a result of bent light, as the layers of air act like a lens. 

The blue light of the sky gets bent by the atmospheric lens and projected onto the road, where, from a distance, it looks like water.

“But your eyes are lying to you," Mass says. "Because that light actually started in the sky and was bent down and then up to your eyes."


This kind of optical illusion — in which the light is bent downward, causing you to think something is lower than it actually is — is called an inferior mirage.

These are common in summer, because the warmth on surfaces often causes the kind of temperature inversion that produces them. And the most common by far, Mass says, is the "water on the road" mirage.


A common mirage seen on Puget Sound, is the one called "superior." It has the opposite effect from the inferior mirage, because it is produced from a colder surface with warmth above. The cold air is dense while there is warmer, less-dense air aloft. The result is an atmospheric lens that can make things on or near the water look like they’ve been elevated, sometimes substantially.  It can create some bizarre illusions, as you gaze across the water.

“Sometimes the cliffs look like they’re mountains. Sometimes a boat or a ferry boat on the other side of the sound is lifted up into the sky in a strange way, sometimes even inverted,” Mass says.


Another very common superior mirage is the sunset, though most people have no idea that’s what they’re seeing. But most of the time, there is one at work and the orb you see on the horizon has already set, several minutes before you see it or take that snap shot.

“What’s happened is the light from the sun, which is actually below the horizon, has been bent up and makes it look like it’s higher than it actually is. So it’s really strange, but the sun has already set when you see it setting,” Mass says.

He adds that phenomenon also is at play with the stars we see in the night sky — especially those that are lower in the sky or close to the horizon. They appear higher in the sky than they actually are.

“They are not where you think they are,” Mass says, and things are not always what they seem.

Weather with Cliff Mass airs at 9:02 a.m. Friday, right after BirdNote, and twice on Friday afternoons during All Things Considered. The feature is hosted by KNKX environment reporter Bellamy Pailthorp. Cliff Mass is a University of Washington professor of atmospheric sciences, a renowned Seattle weather prognosticator, and a popular weather blogger. You can also subscribe to podcasts of Weather with Cliff Mass shows, via iTunes or Google Play.

Bellamy Pailthorp covers the environment beat for KNKX, where she has worked since 1999. From 2000-2012, she covered the business and labor beat. Bellamy has a deep interest in Indigenous affairs and the Salish Sea. She has a masters in journalism from Columbia University.