Improving vision when riding or driving in fog

Fog: what it is

Fog and mist are along a continuum. Fog is often defined as visibility of 1 km (.6 mile) or less. The basics that apply to general safe riding or driving also apply when encountering fog. This includes adjusting your speed so as to not out-drive your line-of-sight. In heavy fog, this may require slowing down to 20 mph or less. In extreme situations, stopping well off the side of the road and turning off all your lights may be the safest approach.

Why you can’t penetrate fog with normal headlights

Fog results from a mantle of air moisture attaining 100% relative humidity that condenses when it encounters cool conditions near the ground. It is composed of extremely small water droplets. Each water droplet acts as a reflector or disperser for light, much like a suspended mirror. Together, these water droplets bounce light off each other, magnifying the effect. So light, no matter how powerful, does not effectively penetrate heavy fog. Most bounces off and scatters. The more powerful the light you shine on fog, the more intense the back-reflectivity created. In some cases, the reflection can be near-blinding in intensity.

Based on this, some <lighting experts> will sometimes state that there is no such thing as a fog light. They are correct in that light from a fog lamp does not penetrate fog. But they probably have little real-life experience riding or driving in heavy fog.

Given light scatter and back-reflectivity, the purpose of a good fog lamp is not to shine through fog but to shine under it in order to illuminate the road surface. The strong horizontal cut-off found in good auxiliary fog lamps allows this. Mounting a fog light low increases its effectiveness. Higher mounting means more light reflected off the fog.

Placing fog lamps low also enables more light to be cast closer to the motorcycle. This is of critical importance when dealing with denser fog.

Correct mounting height and aiming are important for good results. Fog lamps that are placed high and then aimed downwards will result in bouncing light off the road and into the fog.


Ok, but if a little is good, isn't more better?

The goal is to light up the road under the fog with the least amount of light intensity that will still allow you to see your way. If you use too high a light intensity, even with a strong horizontal cut-off, the light will reflect off the road surface back onto the fog, creating back-reflectivity and light scatter. For this reason, installing higher wattage bulbs in fog lights is often counterproductive.

Light-type matters

A very cool and white type of light, such as that produced by HID lights, is closer to the blue end of the color temperature spectrum and is beneficial in improving night vision. Unfortunately, the color temperature that makes HID lights improve night vision also means that they create more glare. Couple this to the higher intensity produced and you have increased back-reflectivity and light scatter when encountering fog using HID lights. In addition, there is more reflectivity off the road surface. Even when using a strong and low horizontal cut-off. This is good for regular night-riding conditions. Bad for fog.


Sometimes, less is more

Warmer light, whether a normal halogen white or an amber color typically used for auxiliary fog lights, tends to produce less glare and reduces back-reflectivity. In heavy fog conditions, a relatively low-intensity amber bulb (about 35 watts), coupled to a fog lens and placed as low as possible to the road surface will probably provide the highest amount of usable light.


Why?

When you think about it, how far you need light to shine under the fog is limited by how far you can see through the fog. If you can only see through the fog for 100 feet, then having a light that can shine under the fog for 3,000 feet is useless. And probably counterproductive.

In addition, how far you need to see mainly depends on how fast you are going. In fog, my speed is usually way down. Sometimes to 20 mph or under. I don't need to see for miles ahead at that speed. But I do want to see as best I can for the 30 to 200 feet in front of me. For me, that means with the lowest back-reflectivity and light scatter possible as well as the lowest reflectivity off the road surface. The relatively warmer light normally found in good fog lamps helps in this. So do lower-intensity lightbulbs.

This becomes more important in denser fog where there is a greater quantity of suspended water droplets in the air. Think of this as a higher number of suspended reflective surfaces. So the denser the fog, the lower the light intensity you want to use.


Benefits of auxiliary fog lamps when driving in fog

When compared to a relatively narrow driving light beam that projects further ahead, a good fog lamp beam is broad in scope. This illuminates the sides and shoulder of the road, enhancing peripheral perception. What you give up in depth, you gain in breadth.

Purpose Rules

Fog lamps can also be very effective when used as cornering lights. A higher
placement, along with a higher wattage white halogen bulb and lens or a HID
light, will enhance illumination into the the corner and to the sides of the road.
In this application, a slight re-aiming is beneficial to ensure effective beam-
spread when leaned.

Sumary:

Within reason, a good fog lamp is not principally about the color of the bulb or lens. More important, its ability to enhance visibility when riding in heavy fog depends on the following three points:

- How effective the lens or the Free Form reflector is at directing the light output.
._This is where a quality fog lamp makes all the difference.
._
- How well the fog lamp is aimed. The best lamps will give poor results if aimed
._improperly.


- Using a lower-wattage bulb of a warmer light when riding in denser fog. This will
._reduce glare.