Lecture 22 Atmospheric Optical Phenomena Rainbows Halos Sundogs

Lecture 22 Atmospheric Optical Phenomena (Rainbows, Halos, Sundogs and Sun Pillars!)

Learning Goals for Part 2 of Chapter 15 1. Be able to explain how and why a SINGLE and DOUBLE RAINBOW forms. 2. Be able to describe the different types of HALOS (22 deg and 46 deg) and how they form. 3. Be able to describe how SUN DOGS and SUN PILLARS are formed. 2

Rainbows

Cloud drops scatter white light

Rainbows • Clarity of color varies • Outermost band is always RED • Innermost band is always VIOLET • Usually see SIX Colors • Red, Orange, Yellow, Green, Blue, Violet • Usually seen when the observer has the Sun on one side and a rain shower on the opposite side. • i. e. Sun at your back, facing the rain • Fine mists from waterfalls and sprinklers can generate mini-rainbows.

Rainbow Formation • Need three things: • 1) Sunlight • 2) Water droplets • 3) An observer in between the rain and the sun • Refraction • As light travels through water it is bent • Different colors travel at different speeds in water (different wavelengths of light) • • • Each color is then bent at a different angle Violet is refracted and bent the most Red is refracted and bent the least

Rainbow Formation • The angle between sunlight and the dispersed color is always: • 42° for red • 40° for violet • The curved shape results from the fact that the light always travels at 42° from the path of sunlight • An observer will only see one color from each raindrop • Each observer sees their OWN rainbow!

Rainbow Formation

• Dimmer Double Rainbows • Visible above the primary bow • Makes a larger arch (by 8 deg) • Narrow band • COLORS are REVERSED! • Outermost = Violet • Innermost = Red

Secondary Rainbow PRIMARY RAINBOW • Forms the same way EXCEPT the dispersed light is refracted twice • Reverses the colors! • Results in a 50° angle for the color red…. 8° above the primary rainbow’s red. • Extra refraction also makes it dimmer SECONDARY RAINBOW

Double Rainbows

Halos • Narrow whitish ring around the sun. • Look for halos on days when the sky is covered with a thin layer of cirrus clouds • Why are Halos White? • Raindrops are almost always spherical. • Ice crystals vary in shape and size • Thus, they are “imperfect” • The colors overlap and “wash” each other out • If you do see a color, it’s usually reddish on the inside of the halo

Halos • 22° halo • Most Common type • Angle of 22° from the observer • 46° halo • Less frequently observed • Larger (just like the second rainbow)

Halo Formation Column • Similar to a rainbow • Formed by dispersion of light • ICE CRYSTALS instead of water drops • • Plate Column Capped Column Bullet Plate

Paths taken by light to produce 22° halo

Halo Time-lapse

Differences between 22° and 46° halos • The path the light takes differs • For 22° halo light strikes one side of the crystal and passes through the other side. • For 46° halo light strikes the side of a crystal and then passes out either the top or bottom

Sun Dogs or Parhelia • Two bright regions on either side of the sun • Usually associated with a 22° halo

Sun Dogs or Parhelia

Sun Pillars • Usually seen at sunrise or sunset • Sunlight is reflected from the lower sides of falling plate crystals and capped columns • Usually reddish in color • Direct sunlight at sunrise and sunset is usually reddish in color

Optical Display in the Alps

Key Information 1 1. Be able to explain how and why a SINGLE and DOUBLE RAINBOW forms. • Usually seen when the observer has the Sun on one side and a rain shower on the opposite side. (i. e. Sun at your back, facing the rain) • An observer will only see ONE color from each raindrop • SINGLE RAINBOWS: • Clarity of color varies but the outermost band is always RED the innermost band is always VIOLET • The angle between sunlight and the dispersed color is always: 42° for red and 40° for violet • There is Refraction as the sunlight enters the drop, a single internal reflection and refraction as the light leaves the drop. • Usually see SIX Colors: Red, Orange, Yellow, Green, Blue, Violet • DOUBLE RAINBOWS: • Dimmer, visible above the primary bow, Makes a larger arch (by 8 deg) and is narrow. • COLORS are REVERSED Outermost = Violet, Innermost = Red • Forms the same way EXCEPT the dispersed light is refracted twice 23

Key Information 2 2. Be able to describe the different types of HALOS (22 deg and 46 deg) and how they form. • HALOS • Similar to a rainbow since it is formed by dispersion of light and refraction • ICE CRYSTALS instead of water drops • Plate, Column, Capped Column, Bullet • The path the light takes differs depending on the HALO • For 22° halo light strikes one side of the crystal and passes through the other side. • For 46° halo light strikes the side of a crystal and then passes out either the top or bottom

Key Information 3 3. Be able to describe how SUN DOGS and SUN PILLARS are formed. • SUN DOGS or PARHELIA • Two bright regions on either side of the sun • Usually associated with a 22° halo • The high intensity spots of light at the horizontal points of the halo are attributed to the HORIZONTAL orientation of the falling ice crystals. • SUN PILLARS • Usually seen at sunrise or sunset • Sunlight is reflected from the lower sides of falling plate crystals and capped columns • Usually reddish in color • Direct sunlight at sunrise and sunset is usually reddish in color