Eye on the Sky

Optics in the Tropics

Awe-inspiring sights in the sky.

By Paul Wilkerson ~ Photos By Marta Morton

Have you ever been outside on a late summer afternoon watching passing storms cruise by, only to see rays of sunshine raining down in mesmerizing streaks of light on the turquoise water below? How about staring at a towering cumulus cloud in the distance at sunset and being suddenly aware of a fan of brilliant rays? Or perhaps you are looking at the sun just as it sets and you glimpse some sort of green light in the moments just before the sun disappears? That all has to do with meteorological optics.

Meteorological optics are patterns in the sky that are observable to the naked eye. Specifically, the interaction of the visible light we see with water vapor and other particulates in the atmosphere. Sun angle has to do with the varying types of phenomena that we see in any given scenario as well.

Crepuscular rays can be seen throughout the year, late in the day towards sunset.

You will likely find that the majority of these optics tend to occur when the sun is lower in the sky during the mid-morning and late afternoon/early evening hours.  Lower sun angles allow for a longer stream of light to be seen by the human eye. Think of it like a flashlight. When a flashlight shines at an object that is close and at a 90º angle to the flashlight, the light will be focused intensely on that one spot with a short beam of light visible. Move the flashlight double/triple the distance away, and turn the object at a 45º angle to the flashlight and you will discover more of the surface of the object is illuminated, but with a softer focus, and a longer beam length that is visible to the eye. It is in similar setups where we get to see some of our coolest weather phenomena.

During our time in Oklahoma, we lived in a community surrounded by winter wheat fields. We also sat in the middle of “tornado alley,” which meant thunderstorms were a good bet on many days of the Spring and early Summer. I remember being outside and watching these behemoths swallow the rolling hills in their dark, foreboding embrace. But on occasion, especially late in the afternoon, I would be treated to a burst of light through the darkness thanks to holes in the cloud deck around the thunderstorm. These are sunbeams. Thanks to varying amounts of water vapor in the atmosphere and the angle of the sun and depending on the time of day, these rays pour down on the landscape below with varying intensity and beauty. They will appear quite wide at the base and narrow toward the source they are focused through in the cloud. When occurring late in the afternoon, these beams fill a much larger area over the surface, providing for a more spectacular effect. Be on the lookout for these in the Turks & Caicos Islands, as we have seen these on North Caicos during periods of thunderstorms.

More common throughout the Islands during the year are crepuscular rays. The word “crepuscular” comes from the Latin word crepusculum which means twilight.  As the Latin meaning implies, these rays tend to occur late in the day before sunset while the sun is low on the horizon. In order for these rays to be really noticeable, there needs to be intermittent clouds between the viewer and the sun. How dramatic the effect is will be based on how close the clouds are to the observer. When clouds are quite close, the effect occurs nearly overhead in many cases and makes it harder to see/view. The best optics occur when the clouds are well out to sea with the sun setting behind them. This is the time rays can be quite spectacular. As the sun hits these clouds, shadows develop in the foreground of the cloud, blocking out sections of light which allows for the appearance of streaks, or rays of light. If the clouds are of a smooth nature (stratus clouds), the effect is minimized, while cumuliform clouds tend to have more peaks and valleys at their tops, which maximizes the light/shadow contrast. Further adding to this effect is the fact that sunlight is traveling through a much larger section of air, where the wavelengths of light are being scattered more diffusely, resulting in more yellow and orange tones.

Spotting the elusive Green Flash as the sun sets over the ocean is a wonderous event.

Folks in the Caribbean and other areas with shoreline could be treated to another phenomenon that is quite rare—the Green Flash. This is likely one of the most elusive of all of the optics we see in the weather world. If you have ever seen one, you are among the elite. The green flash occurs both at sunrise as well as sunset. It will always occur mere moments before the sun emerges from the horizon, or right as the top of the sun disappears beyond the horizon. It is important that the horizon be nearly completely flat. That is why people with large bodies of water to view across stand the greatest chance of seeing this phenomenon.

This green flash occurs due to the refraction of sunlight at sunset where the light is passing through a much larger volume of atmosphere. The atmosphere bends the sunlight passing through it, breaking it out into its different colors, much like a prism. The different colors of light refract differently based on their wavelengths. The darker colors such as blue, green and violet are shorter wavelengths and refract more strongly than orange, yellow and red which are longer wavelengths. As blue and violet light are scattered, the red, yellow and orange are absorbed by the atmosphere. This leaves the green light as the most visible light for mere seconds as the sun disappears. When the sky is clear, there is little haze, and good visibility, it is possible to see this wondrous event.

Have you experienced any of these phenomena on your outings in the Islands? The next time you are taking in a sunset on Grace Bay in Providenciales or Whitby Beach on North Caicos, try to make it a point to watch for the Green Flash or crepuscular rays. Break out your camera and see if you can capture it. Cloud watching and optical phenomenon-watching are activities that can be enjoyed safely all year. Take the opportunity to enjoy everything we have been gifted with in the heavens!

Paul Wilkerson is an American meteorologist and tourist who frequents the Turks & Caicos Islands. Along with his wife and two daughters, the Wilkersons stay actively engaged with Islanders throughout the year with his Facebook page Turks and Caicos Islands Weather Info.



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On the Cover

Photographer Marta Morton was enjoying another spectacular sunset when she spotted this lovely scene—a picture-perfect clump of Old Man Cacti and the pastel colours of what she later learned were crepuscular rays (see page 18). For more of Marta’s images, turn the pages of this issue and visit www.harbourclubvillas.com.

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