Monterey Bay’s Crazy Labor Day Horizon

A superior mirage along the coast of Monterey Bay as seen from Twin Lakes beach in Santa Cruz, California. Mid-afternoon, September 7, 2015.
A superior mirage along the coast of Monterey Bay as seen from Twin Lakes beach in Santa Cruz, California, mid-afternoon on September 7, 2015.

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?”

A close up of Moss Landing power plant during the visual distortion. September 7, 2015.
A close up of Moss Landing power plant during the visual distortion. September 7, 2015.
Moss Landing from Seabright Beach, Santa Cruz, California, with no optical distortion present.
Moss Landing from Seabright Beach, Santa Cruz, California, with no optical distortion present.

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.

An inferior mirage on the Mojave Desert in April, 2007. Photo by Broken Inaglory.
An inferior mirage on the Mojave Desert in April, 2007. Photo by Broken Inaglory.

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.

Basic diagram of the two mirage types. Light refracting upwards results in an inferior mirage. Light refracting downwards results in a superior mirage. Image by   DensityDesign Research Lab. CC-BY-SA
Basic diagram of the two mirage types. Light refracting upwards results in an inferior mirage. Light refracting downwards results in a superior mirage. Image by DensityDesign Research Lab.

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.

A Fata Morgana (superior mirage) over the Farallon Islands as seen from San Francisco. Photos by  Brocken Inaglory.
A Fata Morgana (superior mirage) over the Farallon Islands as seen from San Francisco. Photos by Brocken Inaglory.

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.

  1. Sources Used

    • Mirage. Wikipedia.
    • Santa Cruz Superior Mirage. Atmospheric Optics website.
    • Personal Communication with Peter Weiss, Atmospheric Scientist and Associate Researcher at the University of California, Santa Cruz, Department of Microbiology and Environmental Toxicology. September 9th, 2015.

About The Author

I really enjoy field trips. I love being in a cool place and having someone tell me about it. The problem is, you can’t always find a professor or park ranger-type to tell you all they know about the local rocks, plants, and history. So I decided to combine my love of things natural with mobile technology.

Related posts

12 Comments

    1. Mobile Ranger

      Once I clicked on the photo to expand I could see what looks like some inverted mirror images – so even though its sunset, with less of a temperature differential- I would say yes just to a smaller degree. Thank you so much for asking and posting that awesome photo!!

      Reply
  1. Austin McCann

    I saw this all the time when I worked at the boardwalk and when tourists asked
    About it I acted shocked and said “you have never heard of the Giant Cliffs of California”. The aww and excitement in their faces… Good times

    Reply

Leave a Reply

Your email address will not be published. Required fields are marked *