Weather Watch

Mirages may play tricks on even seasoned sailors

This photograph was taken on a hot September day at Port San Luis looking southeast toward Guadalupe-Nipomo Dunes and Point Sal.

In the photo, the dunes and Point Sal seem to tower hundreds of feet into the sky. This optical phenomenon is classified as a superior Fata Morgana mirage.

According to Merriam-Webster dictionary, the word mirage means, “That is seen and appears to be real but that is not actually there.”

Mirages are caused when light rays are bent or refracted when they travel through mediums with different densities. For example, a pencil in a half-filled glass jar of water appears to be bent. In the case of the atmosphere, light can be bent by thermal variations with changes in altitude.

Mirages can be categorized as inferior and superior.

Inferior mirages are the most commonly seen. They bend light downward and produce the illusion of small bodies of water in the distance. An inferior mirage is called "inferior" because the mirage is located under the real object.

Superior mirages bend light rays upward and produce the illusion of towering boats or beaches in the distance. A superior mirage is called "superior" because the mirage is located over the real object.

One kind of superior mirage is the Fata Morgana, which distorts vertically as well as elongates far away objects. For example, the sloping Guadalupe-Nipomo dunes in the photo appear to have lofty and elongated cliffs.

The day this photo was taken, the Santa Lucia (offshore) winds had driven the marine stratus clouds out to sea in the morning. In the afternoon, northwesterly (onshore) winds made their presence known.

This condition caused the cold and denser Pacific air to accumulate near the ocean’s surface, while the hot, dry and less dense air from the land skipped along on top. This formed a strong temperature inversion layer, creating the Fata Morgana mirage.

In conditions like these, radar operators on boats or flying in aircraft below the temperature inversion layer can experience very long detection ranges as the radio waves are channeled in this temperature inversion layer.

In the early 1900s, Navy Admiral and North Pole explorer Robert Peary thought he saw a large landmass of tall mountains and deep valleys off Ellesmere Island in far northern Canada. He named it Crocker Land.

Years later, Navy veteran and explorer Donald MacMillan and his men conducted an expedition to explore Crocker Land. After days of traveling across the frozen ocean to the whitecapped peaks of Crocker Land, MacMillan realized that he was actually seeing, as Peary had seen, just a superior mirage.

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