Yeah, but How?
A brief foray into atmospheric optics


Part II
(To be read in conjunction with the
principles laid out in the last issue.)

   Mirages: Quite often during the heat of the day, it is possible to witness the appear- ance of water on roadways and in desert regions. This puddle of water doesn’t exist. No matter how far you walk, you never reach it. Objects that occur above the apparition become reflected in the substance. This is a mirage.

   There are two different types of mirages: superior and inferior. Both are affected by temperature rather than atmospheric or gravitational pressure. Inferior mirages happen in desert regions where the surface of the Earth is extremely hot. This heat expands the light waves closest to the ground and stretches them, if you will, making an inconsistent temperature/pressure zone. This creates an often inverted, distorted image. The light, as it carries the reflected image of an object to us, is bent by the expanded, hot air near the ground.

   As you can see from the figure above, we get two images: the real one we expect (saguaro right-side up), and an apparent one (saguaro upside down). We mistakenly see this apparent saguaro as reflected in a pool of water (we see both A and A1 plus B and B1). This however, is not reflection, but refraction. The same layers of pressure we employed to describe the distorted image of the sun can be used here, yet with a twist; there is a special pocket of air here where the hottest, most expanded layer is closest to the ground with successively colder layers above. Each layer, of course, has its own refractive index, so the image can be reflected entirely or partially depending on the amount of heat emanating from the ground. In our experience we only witness reflection in mirrors or water, so we project/misinterpret a water surface reflecting the image. We don’t see the curved portion of A1 or B1, we see in straight lines, hence the upside down image. This is just another optical illusion depending on heat and pressure. Occasionally, the image can be presented right side up in the reflecting portion of the mirage.

   You figure it out.
   Superior mirages happen less frequently. As inferior mirages are named because they invert and lower an image that is agreeably on the Earth’s surface and in our range of vision, superior mirages invert and raise images from above and below the horizon. This happens when there is a temperature inversion between warm air and a colder surface. Since we know that light traveling through warm pockets bends (refracts) toward colder air, we can conceptualize objects below a watery or above an earthly horizon being distorted and displaced above their true location. Sailors often used this phenomena to search for ships on their way in to port; they called it “looming”. When both superior and inferior mirages are visible over water we see images that are excessively exaggerated vertically. It stretches the objects and makes them look like towers and castles. These are known as “Fata Morgana”, after King Arthur’s sorceress sister, Morgan, who was an infamous builder of castles.

   The Green Flash: It exists. Jules Verne called it the Green Ray. Admiral Byrd saw the Green Flash for a record 35 minutes on his expedition to the South Pole, during the end of a polar night when the sun moved along the horizon for an extended period of time. The Green Flash is a split second of green light visible during the last speck of the sun at sunset. Although most often seen over water, it can be visible over a distant horizon when from a high mountain top. It is necessary for the light to travel through great distances of atmosphere. Telescopes and binoculars can help view them, but then you must wait until the sun’s rays have decreased in intensity–this way you won’t be blinded. The sun, however, needs to be bright all the way to the horizon–no clouds can be in the way. Red suns seem to obscure the effect so there cannot be any dust, smoke or pollution in the air.

   There are several explanations for the Green Flash, yet all seem to say the same thing. The green rim of the sun is produced when the atmosphere is selectively cutting out the rays of the setting sun. This allows the yellow light waves of the sun to predominate. Then, according to Robert Greenler, atmospheric refraction allows the blue color to linger as the sun sets. (He suggests we stare at a white light bulb, then look away and see the image. Blue rims the top edge of the light image and red the bottom.) The added amount of dispersion available from a low horizon (as in a great expanse of water or a mountain top) and the apparent blue top edge of the sun, coupled with yellow light waves of a setting light source, could combine to explain this effect. This flash happens in a second and is easiest to see when the last dot of light disappears behind the horizon.

   St Elmo’s Fire: In one of Captain Ahab’s most dramatic moments, an electrical current strikes the top of the masts of the Pequod and flows down them. This is St. Elmo’s Fire. It is an electrical charge created just before a lightning strike, when the electrified field- strength of a mountaintop, of a masthead on a ship at sea, in grass on a golf course, even in someone’s hair, grows and precipitates a sizzling sound. It is called a corona current and is made up of small sparks that seem to glow at night. Sailors thought it was their patron saint, Elmo. We should know it means trouble.

   Lightning and thunder: Lightning is formed by electrical charges within cumulonimbus clouds. No one truly understands lightning, or its cohort, thunder, but I will attempt to explain the current theory (Ah, hahahaha). Inside large storm clouds, positive and negative charges separate and accumulate in their respective domains: positive-top, negative-bottom.

   Graupel, a small, amorphous snow pellet, has no charge and is too small to be dropped out of the sky as rain. As it is carried by air currents to the upper part of the cumulonimbus cloud to accumulate more water (weight), it becomes positively charged. Yet as soon as it gains enough weight it drops through the cloud and to the negative region. This meeting of charges precipitates lightning within the cloud as the negative charge reacts to the positive.

   The charge soon creates an “electrical field” between the Earth and the cloud. The strength of this field escalates and as an electrical threshold is passed, there is a cloud-to-ground lightning reaction. This is not an immediate event. It proceeds by 50 meter increments. The charge moves like a ladder, piece by piece, until the ground is reached. We can’t really witness the ladder. We can feel (during the day) or see (at night) St Elmo’s fire. Yet only the “return stroke” of lightning is visible to us. This is when the positive charge of the Earth has surged up to meet the ladder, short circuits it and begins to drain the negative charges from deep within the cloud. This return stroke averages 20,000 amperes of electricity and can reach 100,000 amperes. (Our homes carry an electrical strength of 30 amperes.)

   Lightning is rarely a single stroke; quite often it is a multi-stroke event with up to 40 flashes, yet three or four strokes is the norm. Cloud-to-cloud flashes occur when the ladder is extended horizontally.

   When the air around these electric channels expands, we hear thunder. With incredible speed, the electricity flowing through the channel can be heated to 30,000 degrees Kelvin in microseconds. This can increase the pressure along the channel 10 to 100 times. A “shock wave” is created which eventually becomes a sound wave. This occurs so fast that it affects every point along the channel simultaneously, so that we cannot isolate the sound as coming from the top or the bottom of the channel. Thunder happens whenever there is a lightning event, even with heat lightning; you just can’t hear the sound because of the distance.

   It is a common fallacy that we can tell the distance of lightning by counting one second for every mile. If the speed of sound is around 700 miles per hour, and we divide this by 3600 to give us seconds, we see that one second of time is worth one fifth of a mile. Therefore five seconds of time equals one mile.

   Every few hours, as I researched and wrote this article, I had to step outside and see again. I felt elated after I recognized things I had read about and could see them properly myself for the first time. I hope this happens for you too.

Cynta de Narvaez