Solar altitude

Fig. 8-64 Effect of solar altitude angle and cloud cover on albedo tor a horizontal water surface, according lo Ref, 33,...

The relative thickness of the air mass is calculated as the cosecant of the solar altitude a. The turbidity factor is thus a convenient means of specifying atmospheric purity and clarity its value ranges from about 2.0 for very clear air to 4 or 5 for very smoggy. industrial environments. 

An average variation of incident solar radiation for cloudy and cloudless sit uations as a function of solar altitude angle is given in Table 8-5. 

A certain smoggy atmosphere has a turbidity of 4.0. Calculate the direct, cloudless-sky insolation for a solar altitude angle of 75°. How much is this reduced from that of a clear sky ... 

Calculate the heat-generation rate resulting from solar-radiation absorption in a lake with an extinction coefficient of 0.328 m and a solar altitude of 90° on a clear day. Perform the calculation for a depth of 1 ft [0.3048 m]. 

Calculate the absorption rate for solar radiation on bright fine sand for a solar altitude angle of 50° and a turbidity factor of 3.5. 

Calculate the ground insolation fora solar altitude of 30° in a smoggy environment with a turbidity factor of 4.0. How does this compare with the ground insolation for clear air at a solar altitude of 90° ... 

Plot the ground solar insolation as a function of turbidity factor for a solar altitude of 80°. 

A pollution-control enthusiast claims that a certain metropolitan area has such a high concentration of atmospheric contaminants that the solar insolation is attenuated by SO percent. How do you evaluate this claim Assuming a turbidity factor of 4.5, what solar altitude would be necessary to produce the 50 percent attenuation factor ... 

Using the data of Table 8-5, estimate the values of the turbidity factor at 5, 20, 30, 45, 60, and 90° solar altitudes upon which these average computations were based. 

Plot the solar insolation versus solar altitude angle for a smoggy atmosphere having a turbidity factor of (a) 3.5 and (b) 4.5. 

At what solar altitude angle will the solar insolation equal 50 percent of the insolation at the outer limits of the atmosphere when the turbidity factor is 4.0 ... 

Part of the solar radiation entering the earth s atmosphere is scattered and absorbed by air and water vapor molecules, dust particles, and water droplets in the clouds, and thus the solar radiation incident on earth s surface is less than tlie solar couslanl. The extent of the attenuation of solar radiation depends on the length of the path of the rays through the atmosphere as well as the composition of the atmosphere (the cloud.s, dust, humidity, and smog) along the path. Most ultraviolet radiation is absorbed by the ozone in the upper atmosphere. At a solar altitude of 41.8°, the total energy of direct solar radiation incident at sea level on a clear day consists of about 3 percent ultraviolet, 38 percent visible, and 59 percent infrared radiation. 

Solar Altitude Angle - The angle between a line from a point on the earth s surface to the center of the solar disc, and a line extending horizontally from the point. 

Jones, L. A., and Condit, H. R., Sunlight and skylight as determinants of photographic exposure. I. Luminous density as determined by solar altitude and atmospheric conditions. 

Spectral radiation intensity ofgiobai radiation depending on solar altitude [87]... 

Illuminances due to sunlight, skylight and daylight as functions of solar altitude for a clear atmosphere are shown in Figures 10.116, 10.117 and 10.118, respectively. Solar altitude at any location and time may be determined... 

Fig. 10.115. Relationship between the three principal methods of measuring the illuminance due to sunlight, skylight and daylight on the horizontal plane (H), the perpendicular plane (P) and the normal incident plane (N). Part (a) represents a perpendicular plane through a line OS, which is drawn from a point O on the earth s surface to the sun (S), intersecting the horizontal plane along a line CB. Part (b) represents the three-dimensional relationship and defines the terms solar azimuth (A), solar altitude (b) and solar zenith distance [2293].

Fig. 10.116. Illuminance (in footcandles) due to sunlight on horizontal (H), perpendicular (P) and normal (N) incident planes as a function of solar altitude for a clear atmosphere [2293].

The relative spectral energy distributions of sunlight, skylight and daylight as a function of solar altitude are shown in Figures 10.120,10.121 and 10.122, respectively. 

Fig. 10.20. Relative spectral energy distribution of sunlight at the earth s surface for a clear atmosphere for solar altitudes 70, 40, 20 and 10° [2293].

Not all this shortwave solar radiation reaches Earth s surface, however some is absorbed by the atmosphere and clouds, and some is reflected from the atmosphere and clouds back into space. When skies are clear, approximately 80-85% of the solar radiation reaches Earth s sruface when skies are cloudy, approximately 50% reaches Earth s surface. Earth s surface itself has an albedo that averages approximately 0.35. The amount of solar radiation received per unit of surface area also varies with the sine of the solar altitude (the angle between the sun and the horizon). Total shortwave solar radiation arriving on Earth s surface ranges from zero at night, to hundreds of watts per square meter when the solar altitude is moderate, to over a kilowatt per square meter under clear conditions when the sun is directly overhead. 

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