ATM OCN (Meteorology) 100

Answers for Homework 4

Summer 2004

Date Due: Thursday, 22 July 2004

The total maximum points were 60. Point distribution for each question noted below. 


Part A.  (36 pts. or 3 pts. each item) For the following atmospheric optical phenomena, identify:

1. Mirage (Superior or inferior): 

PHYSICAL CAUSE(S): Refraction of light by density differences in the atmosphere. 
THE AGENT(S): Lower atmosphere with a strong vertical temperature gradient and, ultimately, a rapid vertical change in the density. 
WHERE (WHEN) FOUND: Large vertical temperature and density gradients, such as the hot pavement and cool air immediately above, or the cold lake and warm overlying air mass. 

2. Halo (22 deg;):

PHYSICAL CAUSE(S): Refraction (and dispersion) of light through ice crystals. 
THE AGENT(S): Airborne ice crystals. 
WHERE (WHEN) FOUND: Look toward the sun or the moon, with a thin veil of cirriform clouds (such as cirrostratus) in front of the illuminating source. 

3. Corona:

PHYSICAL CAUSE(S): Diffraction of light around liquid water droplets. 
THE AGENT(S): Airborne liquid cloud droplets (with a nearly uniform size for best results). 
WHERE (WHEN) FOUND: Look toward the sun or the moon, with a thin veil of warm clouds (such as altostratus or stratus) in front of the illuminating source. 

4. Rainbow (Primary): 

PHYSICAL CAUSE(S): Refraction and dispersion of light entering the raindrop, the internal reflection of this light from the back surface of the drop and the refraction of the reflected light. 
THE AGENT(S): Falling liquid raindrops. 
WHERE (WHEN) FOUND: Look away from the low sun (due to geometry considerations, the sun should be no higher than 42 above the horizon) and toward a rain shaft of falling raindrops.   Hence, look to the east in late afternoon or to the west in the early morning.

Part B.   (4 pts. or 1 pt. each item)
1. What is the physical difference between Rayleigh and Mie scattering? Identify the characteristics of the particles responsible for each type of scatter.
Rayleigh scatter occurs when the incoming radiation (typically polychromatic sunlight) is scattered by gas molecules or other particles in the atmosphere that have a diameter typically less than one tenth the size of the wavelength of the incident radiation - or on the order of 0.05 micrometers. This type of scattering is wavelength dependent, with the short or blue and violet wavelengths preferentially scattered more than the longer wavelength red light. 
Mie scatter occurs when the incoming radiation is scattered by large aerosols or cloud droplets that have wavelengths at least as large as the wavelength of the incoming radiation (or greater than 1 micrometer). This type of radiation is independent of wavelength, meaning that the polychromatic white light is scattered and still appears as a milky white light. 
2. Cite an example of each of these two scattering processes.
Blue skies - an example of Rayleigh scatter. 
White clouds (at least when viewed from the side or above) - an example of Mie scatter. 
Note that red sunsets in relatively clean air would represent Rayleigh scatter, but if sufficient aerosol loading of the atmosphere were present, vivid red sunsets would indicate additional Mie scatter. 

Latest revision: 22 July 2004 (1140 UTC)

Produced by Edward J. Hopkins, Ph.D.
Department of Atmospheric and Oceanic Sciences
University of Wisconsin-Madison, Madison, WI 53706

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