If you happened to be looking out across Monterey Bay over Labor Day weekend you might have noticed that the coastline looked weirdly distorted. It was as if someone had superimposed a strange, hallucination inspired 50-foot vertical bar code along the coastline. The Moss Landing smoke stacks appeared extremely tall and skinny with a lot of wavy lines floating around them. Everyone I pointed it out to said, “Wow, what is that?”
It’s a Mirage
A mirage is a naturally occurring optical phenomenon in which light rays are bent to produce a displaced image of distant objects or sky. They are caused when two horizontal air masses with large differences in temperature are right on top of each other. Cold air is denser than warm air and hence has a greater ability to refract (bend) light. If a viewer is observing light traveling at a shallow angle between air masses with different temperatures, the light rays bend towards the colder air.
Human eyes are used to intercepting straight rays of light back to their source. When the source ray is bent from its original source we can’t trace it back to the right spot. Instead we trace the light back along a perfectly straight line of sight to somewhere the light didn’t actually come from. If the light was bent upward your eye traces it downward and produces an “inferior image” of the sky that appears to be on the ground. If the air near the ground is colder than that higher up, the light ray bends downward just above the ground and your eye traces it upward. This is a “superior” image that appears to be coming from above the ground surface and above the original source of the light.
Mirages at the earth’s surface do not happen often because the temperature gradients between air masses aren’t usually strong enough. On average, temperatures decrease upward at a rate of 0.007 °F per vertical foot. Mirages require a much steeper gradient to start, about 1.3 °F for each vertical foot. A strong mirage needs about 2.75 °F per vertical foot. These conditions occur only with strong heating at ground level, for example when the sun shines on sand or asphalt for several hours.
A Superior Mirage Over the Bay
The optical phenomenon over Monterey Bay this weekend was a superior mirage with multiple vertically stacked mirror images. Peter Weiss, an atmospheric scientist at the University of California, Santa Cruz, explains what was going on: “Usually, the air at the ocean surface is cooled by the ocean creating a layer of cool about a thousand or two feet thick. Over the weekend, the air above this layer was warmer than normal because it was compressed by a very large and strong high pressure system, causing already warm air to sink and heat more than usual. This meant the cold marine air got very compressed at the surface instead of mixing upward.” Thus, there was an unusually steep temperature gradient very close to the ocean surface. The refraction of light in these temperature stratified layers caused the strong and complicated mirage.
These types of superior mirages are often called Fata Morganas. The name derives from Morgan le Fay, the generally evil, shape shifting, half sister of King Arthur. Below is a time series of a Fata Morgana over the Farallon Islands, California. The last two frames were photographed a few hours after the first frames, around sunset. The mirage in these two frames is not as complex as the others because the temperature difference between the air and water has decreased. The warm air is getting cooler, while the cold ocean might be a little bit warmer after a hot day.
Superior mirages are less common than inferior ones, but when they do occur they tend to be fairly stable and last longer. This is due to the warm air being on top and capping the cold air, which tends to want to sink and not rise anyway. The mirages over Labor Day weekend lasted for hours.
Check out Dr. Peter Weiss’s FogNet site for the latest in local fog water sampling by a collective of local scientific organizations.