On overcast or rainy days around here, it’s fairly common for the bottoms of the clouds to be lower than the top of Lavender Mountain. Those low clouds become thick fog for us up here on the mountain. Sometimes the fog closes in and visibility drops to a few dozen yards. We have had that kind of weather several times over the last few days.
We have flood lights on the corners of the house and some bright low-voltage spot lights at ground level behind the house. When I walk the dogs around the house late on a foggy night, I cast a shadow on the fog itself. It’s hard to get a decent photograph of it. This is the best I could do.
I had to enhance this image in Photoshop Elements. That caused a lot of noise in the image that makes the fog seem grainy. But one thing you can see is the brightness of the fog around my shadow.
This is what you get when you use a flash. The reflected light completely washes out anything in the scene, and the resulting image is nothing but noise.
Zeke sat down to wait for me while I fiddled with the camera. I tried to get a shot of him, but, even though he was close and there did not seem to be much fog between us, this is all I could get.
I took shot of the front of the house. It was a kind of neat scene, but it was also a hard image to get, especially with the little point-and-shoot camera I was using.
All of these images illustrate some of the interesting things that happen when light goes through fog. The effects are caused by the scattering of light from water drops. Water drops tend to scatter light strongly back towards the source. That’s what happened when I used the flash; a lot of light was scattered (reflected) right back at the camera, flooding the sensor and washing out anything that otherwise have been visible in the scene.
Light is also scattered strongly into the forward direction, that is, the direction that the light was originally traveling. When you see a bright light in a foggy scene, like the floodlights in the picture of the front of our house, you will probably notice that the light itself looks bright, but there is also a lot of glare around the light. That is light being forward scattered.
The way light is scattered into all directions around a water drop is called the scattering phase function. If you could see it plotted, you would see that some light is scattered into all directions around a water drop, but there is a lot more in the backward (towards the light source) and forward directions.
All this is fairly wonky, but it leads to some really neat things, like, for example, the glory. The glory has been noted for hundreds, if not thousands of years, mainly in regions with high mountains where the clouds are sometimes lower than the tops of the mountains. If a person is on the top of a mountain, and there are clouds below the top of the mountain, and the sun is behind his back, sometimes if he looks down towards the clouds, he will see his shadow cast on the clouds, and there will be something that looks like a halo around his head. That is the glory.
If two people are standing together looking down at their shadows, each one will see a glory only around his own body. It’s easy to understand why someone who doesn’t know what causes a glory to think that it must mean that the person who sees it is special.
Probably the most common place to see glories today is from an airliner. If the sun is in the right place, and the plane is flying over clouds, and you are in the right seat to see the plane’s shadow, you will probably see a glory around the shadow of the plane.
There are several explanations for the glory, but they seem unnecessarily complicated to me. It seems to me that the scattering phase function explains it pretty well. When the light is coming from directly behind you, your head will cast a shadow, but the light that passes around your head will be scattered strongly back towards the source; in other words, directly back towards you. Thus you will see a bright area around the shadow of your head. The glory is often colored, which also doesn’t seem to require a very complicated explanation, since a similar effect can be seen in every rainbow.
You can see a more pedestrian version of this almost any time the sun is out. Just stand so that you can see your own shadow. Most of the time whatever surface you are standing on will tend to reflect light more strongly directly back towards the source than in other directions. This will cause the ground or other surface to look brighter right around the shadow of your head. This is often a subtle effect, but if you look carefully you ought to be able to see it. A roughish surface, like grass, is better than a smooth surface like a concrete patio.
So, when you’re out looking around, keep looking up, but don’t forget to look down sometimes, too.