Sunstrike condensation concern

Edna Cruikshank, of Dunedin, asked:

In winter, I was driving towards the setting sun. I was familiar with the area and was expecting sunstrike. My car engine and interior was well warmed up. I noticed a small amount of condensation at the bottom of the windscreen. I flicked on the demister at a high temperature and the airconditioning. At that same moment the sunstrike occurred and my entire windscreen steamed up immediately and very densely. I was unable to see through any part of it. I have been driving for many years and have never experienced this before. I had to stop very sharply as I had no visibility at all. Can you explain what caused this and what I can do to prevent it happening again?

John Campbell, a retired physicist at the University of Canterbury, responded:

I doubt that the condensation happened over the whole windscreen instantly so I assume it was present all the time but unnoticed because of the way we see light.

We see the surrounding scenery because light from the sun is scattered by the objects it illuminates and that (weak) scattered light reaches our eyes, so we see it. We don’t notice the cleanish windscreen when it is not in direct sunlight because it is only illuminated scattered light.

The light directly from the sun is incredibly more intense, which is why we should never look directly at it.

One form of sunstrike occurs when we suddenly look at the sun, e.g.: when we round a corner or come over the crest of a hill. We quickly close our eyes and/or look away, and lower the sunvisor, thus preventing sunstrike.

A more insidious form of sunstrike is when there is condensation on a clean windscreen. The forward scattering of sunlight means we have sunstrike even when not looking directly at the sun, but the sun is illuminating a windscreen coated with dust or fine water droplets. Forward scattering of these droplets sends very bright light towards our eyes.

For an unilluminated windscreen, if these water or ice drops are large enough we see them as well as the scenery. However, if they are fine enough we look through them without mentally registering that they are there.

However, when the sunlight suddenly illuminates the windscreen covered in dust or fine droplets, we get an incredible amount of forward lightscattering, which obliterates the weak light scattered from the scenery behind, and momentarily blinds us. This causes us to close our eyes or to look away, which is quite dangerous when driving.

A similar effect occurs with people reading through notquiteclean glasses. Any air to solid surface (e.g., each surface of one’s glasses), because of the change in speed of light in air compared to the speed in glass, reflects up to 4% of the incident light. Whilst the glasses are in the shade there is no problem, we don’t register the small amount of light scattering from the glasses by weak scattered light. However, when the glasses are illuminated directly by sunlight, the forward scattering from a very bright source is brighter than that due to the weaker light illuminating the book etc., so it is harder to read with the glasses in sunlight compared to in shade.

How to prevent it? Use the car heater to warm the whole windshield (inside and out) so it cannot form condensation or ice.

Send questions to: AskAScientist,

PO Box 31035, Christchurch 8444 Or email

questions@askascientist.net