1.
Electromagnetic spectrum
a.
Wavelength of 0.4 (violet) 0.7 (red) micrometers
(Fig. 2.8).
2.
Scattering (as opposed to Reflection)
a.
Light striking a relatively large surface, e.g., a
mirror, is reflected (more about this later).
b.
But light striking a relatively small aerosol particle
(solid or liquid) is scattered in all directions (Fig. 15.1, 15.2).
c.
Relative size of aerosol particle and wavelength of
light can dramatically affect:
i.
Preferred direction of scattering (often in the
forward direction for natural aerosols).
ii.
Preferred wavelength (color) that is scattered (e.g.,
cigarette smoke; blue haze; white haze (Fig. 15.6)).
3.
Some Effects of Scattering
a.
Aerosols are easier to see when the observer looks
towards the general direction of the Sun (e.g., morning or evening).
b.
Cloud droplets scatter all wavelengths equally well
and so appear white (Fig. 15.2).
c.
But air molecules scatter blue light preferentially
and so the sky appears blue (Fig. 15.4).
d.
Amount of scattering also dependent on amount (concentration)
of aerosols, e.g., the base of a thick cloud looks dark because so much light
has been scattered out of the sides of the cloud (Fig. 15.3).
4.
Refraction
a.
Whenever a light beam passes from a relatively low
density medium (e.g., the vacuum of space, air) to a relatively high density
medium (e.g., air) then its direction of travel will change, becoming closer to
the “normal”, i.e., vertical.
b.
The opposite is also true: passage from a relatively
high density medium into a relatively low density medium will cause the
direction of the light beam to move further from the “normal”.
c.
Blue light is refracted the most, red light the least
(Fig. 15.21).
d.
The density of the atmosphere itself varies depending
on:
i.
Temperature: cold and warm air (at the same altitude)
are high and low density, respectively (Figure 1.9).
ii.
Altitude: low
altitude air is high density and high altitude air is low density (Fig. 1.7,
1.8).
e.
Refraction also occurs when light passes from the air
(relatively low density) through an ice crystal (relatively high density), and
vice versa (Fig. 15.21).
5.
Some Effects of Refraction
a.
Twilight (Fig. 15.13)
b.
Stars twinkle (scintillate) (Fig. 15.12)
c.
GPS can be used for precipitable water vapor (PWV)
profiles (SUOMINET)
d.
Green flash (Fig. 15.14)
e.
Mirages (Fig. 15.15, 15.16, 15.17)
f.
Halos (Fig. 15.19, 15.20, 15.22)
g.
Sundogs (Fig. 15.23)
h.
Sun pillars (Fig. 15.24, 15.25)
6.
Reflection
a.