Atmospheric Optics Bend, Bounce, and Scatter

Why is the sky blue? First, remember that visible light is made up of a spectrum of colors.

We see objects as a certain color because that color is either reflected from an object or scattered by the object (scattered means it was absorbed and then re-radiated) RED GREEN BLUE

The atmosphere is made up of mostly nitrogen and oxygen. They are selective scatterers, meaning they scatter short wavelengths best (violet, blue, and green). Our eyes are most sensitive to blue light, so the sky appears blue to us!

Why do clouds look white and sometimes dark? Water vapor (clouds) scatter all wavelengths equally. The result is white. When clouds are thick (like thunderclouds) they absorb much of the light. Water drops also tend to absorb light. The result is a darker cloud.

Why are sunsets red?

The Green Flash Green flash is an atmospheric phenomenon observed occassionally at sunset. Remember at sunset that the light travels through a much greater amount of atmosphere—this bends the light from the setting sun so that we see the sun for a short while after it has actually set. Blule light bends the most so we should see some blue light at the top of the setting sun. However because the blue light is scattered out the most very little reaches us and we see green light instead. Usually the green light is too faint to see with the human eye, but under certain conditions (hot surface air or an upper level inversion) it can be seen for about a second or so. (polar regions can see it for longer

Crepuscular Rays (Jacobs Ladder) Particles in the sky (dust, water droplets, or haze) scatter light in their path making that region appear bright with rays

Inferior Mirage: It looks like there’s water on the road

A layer of very warm air near the ground refracts (bends) light from the sky that is headed down toward the ground to our eyes. Our mind interprets the light rays reaching our eyes as having come along a straight path. Thus, we see that patch of sky and cloud "on the ground" and interpret the image as a surface "pool of water."

The layer of hot air above the ground causes the inferior mirage. The key is not what the temperature is, but what the temperature difference is between the layer at the ground and the air above it. Highway mirages are as common over dry pavement on sunny winter days as during the summer months. Measurements of air temperatures within 1cm of the pavement have been recorded 20 to 30 F degrees higher than the air temperature above that layer!

Any object can be seen that is located at or beyond the mirage's apparent location. If you look closely, you can often see details such as vehicles in the highway mirage. Images are often inverted in the mirage, thus giving the appearance of the object being reflected in a pool of water. Van’s refraction

How is a rainbow formed?

Remember that light is dispersed by a prism..

Sun’s rays Each raindrop acts like a prism and separates the colors. I’ve only drawn in the two extremes: violet and red. All the other colors are in between these two.

We see only one color for each drop because of it’s exit angle. The other colors from that drop are either too high or too low. They don’t hit our eyes.

Double Rainbow. Notice the secondary bow has reversed color order.

Halo around the sun Light is refracted by tiny suspended ice crystals

Sundogs (parhelia)

Halos are formed with the ice crystals are randomly oriented. Sundogs are formed when the crystals are oriented in the horizontal plane as shown here.

Tangent arc Formed when large pencil- shaped ice crystals are falling parallel to the ground

Sun Pillars are formed when sunlight reflects (bounces) off ice crystals. They may extend upward or downward from the sun.

Glitter Path

Corona Due to diffraction (bending around an objects) from tiny cloud droplets. Colors are sometimes seen when drops are of uniform size (different wavelengths are bent different amounts)

Stay Curious!

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