Just because it isn’t all that sunny, wearing sunglasses in the rain and fog is likely to improve visibility…

TL;DR: Wearing polarised glasses when driving in the rain, fog and snow will nearly always help as they reduce glare.

OK, IT’S not quite as straight forward an answer as that because you can’t just wear any kind of sunglasses, they must be polarised. Don a pair of normal, non-polarised sunnies and they won’t reduce road glare any more than wearing none at all.

However, polarised glasses are unique in that they block (absorb) horizontal light. Basically, when a concentration of bright light reflects off an object some of this is horizontal light. Wearing polarised glasses blocks out the horizontal light, which means the object is easier to look at and in many instances easier to distinguish.

When it’s raining or foggy, the light waves are scattered at many different angles including horizontal…you can see the trend here – polarised glasses help minimise glare caused by the horizontal light waves. Normal glasses simply minimise the light and don’t cut glare in the way polarised glasses do.

But keep in mind that not all polarised sunglasses are equal, and not all glasses are polarised.

If you want to buy a pair of polarised glasses, you’ll need to make sure they are indeed polarised, and this special coating on the lens adds to the cost of glasses. For no-name brand sunnies, it’s not much more because the coating might not be of high quality, and for some well-known brands, they can cost a lot more polarised because either the coating is of a high-quality standard and/or you’re paying for the brand name.

In any event, for driving, you want a pair of glasses designed for sports use or even specific driving glasses. These will usually provide added protection on the side without hindering peripheral vision. 

Things to keep in mind though is that horizontal light rays are absorbed from things like LCD displays and screens in the cabin when wearing polarised glasses and will become much darker or disappear altogether. And that’s an easy way to check polarised glasses are working (or are indeed polarised) – take them off and hold them over a screen while rotating the glasses around, and you’ll notice the screen almost dissapear as it becomes darker and the light is absorbed before reappearing.

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