SCIN 137 AMU week 8 lesson LIGHT, COLOR, AND ATMOSPHERIC OPTICSIntroduction to Meteorology American Military university
- Lesson Overview
Students will be able to:LO-51. Explain the basic properties of light. LO-52. Discuss how the atmosphere can affect light from the Sun and the Moon. LO-53. Identify some unusual atmospheric optical phenomena
The following activities and assessments need to be completed this week:
- Read Barry and Chorley: Chapter 13
- Week 8 Lesson
- Week 8 Forum
- Week 8 Lab
- Week 8 Quiz
- COMET Modules (Optional):
- Coastal Climate Change
- Climate Change and Regional Impacts
- Climate Change and Extreme Weather
- Climate Change and Sea Level Rise
- Climate Change: Fitting the Pieces Together
How many times have you told a friend they had to come see the sunset? The sky is often the most beautiful thing you see in a day and can, within one day, be many different colors. We will start by looking at the physical properties of light, and then see how they produce blue skies, murky clouds, the perceived color of the sun, moon, and stars, and other colorful atmospheric events. Why do we sometimes see rays of the sun coming down through the clouds? Why do we have rainbows? Does a sun dog bite? You will soon find the answer to all these questions. Topics to be covered include:
- Physical properties of light
- Refraction and scattering effects
- Unique Visual Events
Physical Properties of Light
While the sun emits the full spectrum, much is either reflected or absorbed before it reaches the surface. In the first lesson you saw that visible light accounts for half the incoming electromagnetic (EM) energy from the sun. The rest that reaches the surface is mostly infrared (longwave) and ultraviolet (shortwave).
When you were a child drawing pictures with crayons, you probably thought of white as the absence of color. Instead, white light is a combination of all visible colors. Each color has a unique wavelength. Violet, with the shortest wavelength, has the most energy. Because the sun emits all the colors in nearly equal intensity, the sun at midday appears nearly white.
Our eyes are more sensitive to yellow than other colors in the daytime (when the cones of our eyes are most at play) so yellow objects are perceived to be brighter than other colors of equal intensity, hence the tendency for the sun to appear yellowish to us. At night our eyes depend on the rods more than the cones, which have a greater sensitivity to the blues and greens. Many of the stars seem to be bluish to us.
A hotter sun would emit a greater proportion of energy at higher wavelengths, and would appear blue. The same as true of other objects. If you look at a candle flame, you see blue at the hottest gases near the wick, then reds and yellows when you get to cooler ga
When light strikes an object, it is either reflected back, absorbed, or refracted. What is refraction?
Think of a marching band headed across the dry field in the picture – starting on the upper left of the diagram. Suppose you marked the position of the line every second. Unfortunately they have a muddy field in their path that will slow them down. The first band member enters the muddy field at A, and for a brief moment is traveling slower than the rest of the band, so that first person doesn’t travel as far in that second. As another person moves into the muddy field, that person travels slower. Eventually the last person enters the muddy field at point B. Now everyone is moving slower. The fact that each person entered the muddy field at a different time bends the direction the band is traveling. This bending due to a different speed of travel is called refraction.
Note that refraction only occurs if the marching band’s direction makes a less than 90 degree angle with the boundary. If the band hits the boundary head on, everyone slows down together and the direction does not change.
Light does not move at the same speed all the time. When light passes through an optically dense material, it changes speed. If the angle is less than 90 degrees, the light is refracted (bent). The higher energy wavelengths (violent end) bend more than the lower energy wavelengths (red end).
The atmosphere refracts light from the sun after it sets, making the sun (and the moon) appear to set later than it actually does. When the sky is clear, refraction bends light and the atmosphere scatters it, giving us the extended light even after the sun has set called twilight. Similar refraction and scattering at sunrise gives a period of twilight before dawn. Twilight is longer at higher altitudes and longer in summer. There is very little twilight at the equator.
“Scattering occurs when light bounces off an object in a variety of directions” (NASA, 2010). The shorter wavelengths, blue and violet, scatter the easiest when they bounce of the nitrogen and oxygen in the atmosphere. While the sky is more violet than blue, the sky appears blue to us because our eyes are more sensitive to blue.
The Blue Mountains in Australia and the Blue Ridge Mountains in Virginia are blue because of the same selective scattering of the blue light. The blue haze in the Smoky Mountains is thought to be the result of light scattering from tiny particles formed from a combination of plant byproducts interacting with small particulate matter pollution.
When the sun is near the horizon, it passes through more of the atmosphere, scattering even more of the blue and violet out, leaving the yellows and red to which are eyes are most sensitive. The reddest areas are low in the sky, with both the sun and sky around it appearing red.
A halo is a white or slightly colored ring, usually seen around the sun but at times around the moon. Ice crystals in either a high cirrus cloud refract the sunlight (or moonlight).
Most ice crystals are hexagonal with most having a near 22 degrees angle of refraction. Their associated halo is called a 22 degrees halo and is the most common type. The ring of light is 22 degrees from the sun. Crystals with differing structures make different halos, from nine to 46 degrees.
Before modern forecasting developed, halos were seen as a sign of approaching fronts and impending precipitation. Ice-crystal laden high cirrus clouds often form in advance of an oncoming front, but the front may change direction or not have the right conditions to produce precipitation.