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

2. Learning Goals for Part 2 of Chapter 15
Be able to explain how and way a SINGLE and DOUBLE RAINBOW forms.
Be able to describe the different types of HALOS (22 deg and 46 deg) and how they form.
Be able to describe the how SUN DOGS and SUN PILLARS are formed.
You can just read this slide.

3. Rainbows

4. Cloud drops scatter white light

5. 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.

6. Rainbow Formation Need three things: Refraction 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 speed in water
Each color is then bent at a different angle
Violet is refracted and bent the most
Red is refracted and bent the least

7. 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!

8. Rainbow Formation

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

10. 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

11. Double Rainbows

12. Halos Narrow whitish ring around the sun.
Look for halos on days when the sky is covered with a thin layer of cirrus clouds.
Whey 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

13. Halos 22° halo 46° halo Most Common type
Subtends an angle of 22° from the observer
46° halo
Less frequently observed
Larger

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

15. Paths taken by light to produce 22° halo

16. Halo Time-lapse

17. 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

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

19. Sun Dogs or Parhelia

20. 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

21. Optical Display in the Alps

23. Key Information 1
Be able to explain how and way 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
You can just read this slide.

24. Key Information 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
You can just read this slide.

25. Key Information 3
Be able to describe the 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
You can just read this slide.