What effect does rain on the windscreen have during an approach to a runway?

When rain affects the windscreen during an approach to a runway, it can create visual illusions that may cause the pilot to misinterpret the aircraft's altitude. The rain distorts visibility by creating a visual haze or blurring effect, especially when looking at runways and surrounding terrain. In this situation, the moisture and varying light conditions can lead to an optical illusion, causing the pilot to perceive the aircraft as being higher than it actually is. This is particularly critical when making descent decisions, as it could prompt the pilot to carry out an incorrect approach profile or delay necessary corrective actions to maintain the desired altitude.

Understanding these illusions is vital for pilots, as they indicate the importance of relying on instruments during approaches in inclement weather conditions, rather than solely on visual references that could be misleading. This highlights the necessity of accurate instrument proficiency, even when visual references are available.