Friday, April 18, 2014

Puerto Peñasco Mirage and Fata Morgana!

On our last trip to Puerto Peñasco at Christmas, we observed several mirage effects over the Sea of Cortez, and I was looking forward to our return to see if any more could be seen.  While the sun and moon were rising and setting too far north to be seen over water, I was not disappointed!  We were able to spot several instances of Fata Morgana - a mirage effect caused by layers of air at different temperatures.  In the cases below, I believe cooler air is trapped near the Sea of Cortez or the ground with an inversion layer and warmer air above.  The effect of this is a vertical magnification or stretching caused by bending of light rays by the temperature differences.

The first instance was on our first evening there, on eclipse night.  After taking the picture mosaic of the rising full moon in our last post, I whipped over and took a shot of Bird Island to the southeast (shown at left), about 25 miles (40km distant).  So far, so good...  About 2.5 hours later, just before the eclipse started, I went looking for Bird Island again to try imaging it by moonlight (shown at right).  Wait a minute - they appeared much higher from the exact same viewpoint!  Even though they are reproduced at the exact same scale here, they appear at least twice as tall!  I'm thinking that a cool layer of air formed over the water, and magnified the height of the island vertically.  At Christmas I observed them to float over the Sea (see link above), in a more complicated mirage effect, but this version was interesting too!  Both of these pictures were taken with the William Optics 11cm diameter F/7 APO (770mm focal length), with the moonlit image a 10 second exposure.

The next night found us near downtown Puerto Peñasco, at a popular restaurant with a spectacular view of the harbor and sea.  From our vantage point, even a casual look to the north showed an interesting mountain range.  The distant mountains were stretched vertically into a nearly impossible profile.  This shot was taken with the 70-200 zoom (set to 200mm), and is another example of Fata Morgana.  Taken again, shortly after sunset, it slowly dissipated over 20 minutes.

After taking the above mountain picture, I noticed another example to the west over the Sea of Cortez.  Here, with just the horizontal line of the sea marking the horizon, the vertical displacement due to the Fata Morgana is easily identified.  I actually took a time lapse of it, but likely won't display it (I didn't get the frame very level), but at right show how it dissipated in 30 minutes time - nearly, but not totally back to normal.

The interesting thing about all these examples of Fata Morgana is that they depend on stable layers of air at different temperatures, but for nearly our entire stay in Mexico the wind was blowing pretty steadily, and I would have predicted there would be too much mixing of the air to produce these effects.  But I'm glad I was able to see and document them.

Some of the finest examples of Atmospheric optics, as well as written explanations for their cause is at the excellent website by Les Cowley - Optics Picture of the Day.  He has some great images of Fata Morgana shown here and here...

1 comment:

Andrew Cooper said...

I love Fata Morgana! I have seen them several times while piloting a boat in Alaska. The effects on small tree covered islands are fascinating.