What illusion, if any, can rain on the windscreen create?

The illusion created by rain on the windscreen can lead a pilot to perceive that the aircraft is flying higher than its actual altitude. This phenomenon occurs because the water beads and streaks on the windscreen can distort the view of the horizon and the ground, making it difficult for pilots to gauge their altitude accurately. As rain blurs and obstructs visibility, there is a tendency for pilots to misinterpret their flight parameters.

In this scenario, the visual cues that are typically relied upon to assess the aircraft's position relative to the ground can be misleading due to the presence of rain. This optical illusion can potentially lead to a dangerous situation if the pilot overestimates altitude, as they might think they have more clearance above terrain or obstacles than is actually the case. Understanding this illusion is crucial for safe flying, as it emphasizes the need for reliance on instruments rather than visual references when conditions are poor.