High-up Halos

What creates Sun Halos?

Figure 1

"A ring around the sun or moon means rain or snow is coming". Folklore had soon caught on to the meaning of sun halos (Figure 1), but how are they actually created? The answer lies in an object as simple as a prism (Figure 2).

Let's take a closer look at this prism. We observe that the light changes directions as it enters and leaves the prism. This is a concept known as refraction. Refraction occurs when light changes mediums, as the light changes its speed. (Kind of like how you run slower in water than in air).

Figure 2

Figure 3

So how does the speed change actually change the direction of the light? The process is described in Figure 3. The red highlighted section is the particular "ray" of light that we'll be following. It hits the boundary between mediums at an angle, so the bottom end of the light ray will become slower first. As the top end of the ray continues to move with a faster speed for a tiny instant of time, the direction of the ray turns. This makes intuitive sense when you imagine a singular vertical line moving to the right. If the bottom end of the vertical line begins moving slower, the line will start rotating clockwise. Now we've understood a half of it. We get why the light changes directions, but why the change in color? Well, the exact amount that the light's path is changed is partially defined by a number called the index of refraction. The index of refraction is different for different wavelengths of light. Different wavelengths of light also define the different colors, which combine to make white light. So, when white light hits a prism, the different colors of light that are contained in the white light all change paths by different angles, as defined by their indices of refraction.

At this point, we understand how prisms work. They have the capability to redirect light by refracting it to a different angle. Also, they can make white light disperse into its components, the color spectrum that we're already familiar with. Let's return to our story in the sky. As dry air rises to colder temperatures, water vapor turns into ice. Once enough of these ice crystals accumulate, we can observe white wisps in the air: cirrus clouds. Cirrus clouds act as a layer of ice crystals in the atmosphere, in which each ice crystal can be simplified as being a hexagonal prism. As light from the sun passes through each prism, the light is redirected by an angle. When this angle is exactly 22 degrees, it happens that the refracted light rays all converge at a single point on the Earth's surface (Figure 5 shows two of these points at the bottom and top of the halo). From these points, we can look up to observe the halo.

Figure 5

Figure 4

"A ring around the sun or moon means rain or snow is coming". Before finishing up this article, let's discuss how we began it. From our exploration of sun halos, we've understood that they appear from sunlight passing through cirrus clouds. Cirrus clouds only form when air currents and masses get to very high levels. As a result, when you see a cirrus cloud, and sun halos, you can expect a weather change to follow soon.