Definition
An optical illusion on approach in which rain or moisture on the windscreen bends incoming light, causing the runway and horizon to appear lower than they actually are, which can lead the pilot to fly an approach higher than intended.
Plain English
Water on the windshield bends the light coming through it, so the runway looks lower than it really is. A pilot reacting to that view may fly the approach too high without realizing it.
Context Anchor
Seen in instrument flying and landing discussions, especially when breaking out of clouds or continuing an approach in rain.
Derivation
Refraction comes from the Latin 'refractus', meaning 'broken back' -- light is 'broken' off its straight path when it passes through water. That bending of light is exactly what causes the visual error here.
Why Pilots Care
It can cause a pilot to misjudge altitude or distance, leading to unsafe approach paths or terrain conflicts during night or instrument flight over large bodies of water.
Analogy
A straw in a glass of water can look bent even though the straw is straight. Water on a windshield can bend the pilot’s view in a similar way.
Grounding Statement
Picture rain streaking across the windshield as you line up with the runway: the runway may seem lower than it actually is, making the airplane feel higher than it is.
Intuition Check
Water refraction does not mean the runway is wet or that water is reflecting light like a mirror. It means water on the windshield is bending the light and shifting the way the runway appears to your eyes.
Example Sentence 1
During the rainy approach into the field, the instructor reminded the student that water refraction can make the runway appear lower than it is.
Example Sentence 2
During the night instrument approach over the lake, the crew accounted for water refraction before relying on the visual glide path.