When your goggles fog up, you can’t see where you’re going—which puts a quick end to a flight. Why does this happen and how can we prevent it?
Google fog occurs when warmer, humid air meets a colder surface. If the difference is enough to create condensation, tiny water droplets will adhere to the lens and prevent you from seeing through it. When you place goggles on your face, your body heat warms the air near your eyes, creating this temperature difference that causes fogging. Since this is only a problem on colder or humid days, some parts of the world never worry about it at all.
To prevent fogging, you can eliminate one of the conditions that create it. Less humidity, cooler air, or a warmer lens surface will all help to prevent fogging. You can also alter your lenses so condensation can’t form on them.
Cool the Air
You might associate fog with cold, but it’s warm, moist air that causes it—against the cold surface. Cooling the air near the lenses can prevent fogging. Goggle manufacturers rely on this when they include fans. These simply exchange the air heated by your face with ambient-temperature air from outside. Other methods of cooling or increasing the air flow can help as well, such as venting. My old Fat Shark Predators with rubber eye cups would fog constantly compared to my foam-lined Commanders. Improved airflow through the foam no doubt contributes to this. Therefore, if your goggles do not have fans, all hope is not lost: you can modify your goggles with different materials, such as switching to foam from rubber eye cups. If you can stand a little light leak, simply cutting more vents may solve your fog problems, allowing wind or natural air flow to do the same job.
Warm Your Lenses
Since warm air fogs on a cold surface, you can prevent fog by keeping your lenses warm. Some days, it’s enough simply to keep your goggles from getting cold before you use them. You can keep them warm in a pocket, or leave them on your car’s dashboard with the heater running. One local pilot simply leaves his powered up and on his head the entire time he goes out, even between flights. Be careful setting your gear in the sun to warm up— direct sunlight can damage the screens of some goggles (Fat Sharks and Commanders, for example).
This approach is common with waterproof housings for GoPro cameras. A small absorptive insert is placed inside the housing with the camera, sucking up the moisture before it can turn into condensation. You can buy these inserts and stick them into your goggles, but they probably won’t perform well. Each insert can only absorb so much moisture. Since goggles aren’t sealed to your eyes, that air is frequently replaced—and will be replaced even faster if you have a fan or if you take your goggles off. These inserts might work for a single flight, but will quickly lose their effectiveness. The only other viable options to remove humidity are to move to a different location or wait for the weather to change.
Using a Barrier
Preventing condensation from forming can be another effective solution, and there are a few different approaches to this.
One method is to add more water (commonly, your own saliva). This makes the droplets adhere to each other instead of sitting separately on the lens. It’s a quick fix, but it degrades over time as the water you added drains or evaporates away. Mostly, it’s easily repeatable and always available. If water on your lens pools together, though, these larger droplets can distort your view.
Other barriers repel water, preventing condensation for forming. Some common products with this property are baby shampoo and toothpaste. Commercial products like Cat Crap also exist for this purpose and are popular with skiers and other winter sport enthusiasts. You might not like the idea of smearing something on your lens, since you risk scratching them. Dust may stick to something you’ve put on them, or you might smudge if you touch it accidentally—not to mention discoloring or distorting your view somewhat. But this method may also last for a long time if you’re careful.