Effect of cloud

In addition to the effects of the gaseous and particulate components of the atmosphere, the extent and type of cloud cover are of great importance in determining the amount of solar flux that penetrates to the Earth's surface. We follow here the account given by Monteith (1973).

Fig. 2.3 Spectral distribution of solar quantum irradiance at the Earth's surface at three geographical locations (plotted from the data of Tyler and Smith, 1970). (a) Crater Lake, Oregon. USA (42 °56' N, 122 °07' W). Elevation 1882m. 11:00-11:25 h, 5 August 1966. (b) Gulf Stream, Bahamas, Atlantic Ocean (25 °45' N, 79 °30' W). 12:07-12:23 h, 3 July 1967. (c) San Vicente reservoir, San Diego, California. USA (32 °58' N, 116 °55' W). 09:37-09:58 h, 20 January 1967. All the measurements were made under clear skies.

Wavelength (nm)

Fig. 2.3 Spectral distribution of solar quantum irradiance at the Earth's surface at three geographical locations (plotted from the data of Tyler and Smith, 1970). (a) Crater Lake, Oregon. USA (42 °56' N, 122 °07' W). Elevation 1882m. 11:00-11:25 h, 5 August 1966. (b) Gulf Stream, Bahamas, Atlantic Ocean (25 °45' N, 79 °30' W). 12:07-12:23 h, 3 July 1967. (c) San Vicente reservoir, San Diego, California. USA (32 °58' N, 116 °55' W). 09:37-09:58 h, 20 January 1967. All the measurements were made under clear skies.

A few isolated clouds in an otherwise clear sky increase the amount of diffuse flux received at the Earth's surface but, provided they do not obscure the Sun, they have no effect on the direct solar beam. Thus, a small amount of cloud can increase total irradiance by 5 to 10%. A continuous sheet of cloud, however, will always reduce irradiance. Under a thin sheet of cirrus, total irradiance may be about 70% of that under a clear sky. A deep layer of stratus cloud, on the other hand, may transmit only 10% of the solar radiation, about 70% being reflected back to space by its upper surface and 20% being absorbed within it. On a day with broken cloud, the irradiance at a given point on the Earth's surface is intermittently varying from the full Sun's value to perhaps 20 to 50% of this as clouds pass over the Sun.

In desert regions there is rarely enough cloud to affect surface irradi-ance, but in the humid parts of the globe, cloud cover significantly reduces the average solar radiation received during the year. In much of Europe, for example, the average insolation (total radiant energy received m~2day_1) in the summer is 50 to 80% of the insolation that would be obtained on cloudless days.928

In recent years, a large amount of new information on the distribution and optical characteristics of clouds around the Earth has become available from satellite remote sensing. The International Satellite Cloud Climatology Project (ISCCP) has been combining such data from geostationary and polar orbiting meteorological satellites from mid-1983 onwards. Bishop and Rossow (1991) have used the ISCCP data, together with modelling of radiation transfer through the atmosphere, to assess the effects of clouds on the spatial and temporal variability of the solar irradiance around the globe. The results show that the oceans are much cloudier than the continents, and receive a correspondingly lower solar irradiance. For example, in July 1983, approximately 9% of the ocean was perpetually cloud covered, contrasting with only 0.3% over land.

There are marked differences between regions of the ocean: the Northern and Southern ocean waters are almost perpetually cloud covered, but at an equatorial mid-Pacific location (1 ° N, 140 ° W) clear skies consistently prevail. In the northern hemisphere the Atlantic Ocean receives substantially more solar radiation than the Pacific Ocean in the summer, but there is little difference between the two oceans in the southern hemisphere. The irradiance at the ocean surface, as a per cent of the clear sky value is about halved between 30 ° S and 60 ° S due to the persistent band of circumpolar cloudiness centred at 60 ° S. The Argentine Basin and Weddell Sea sectors of the western South Atlantic are, however -presumably due to proximity to land - consistently less cloudy than the rest of this circumpolar region, and are known to have greater levels of primary productivity. Bishop and Rossow suggest that solar irradiance is a major determinant of the rate of carbon fixation by phytoplankton in these nutrient-rich southern ocean waters.

Fig. 2.4 Angular distribution of luminance (approximately proportional to radiance) in a clear sky. (By permission, from Solar Radiation, N. Robinson, Elsevier, Amsterdam, 1966.)

Fig. 2.4 Angular distribution of luminance (approximately proportional to radiance) in a clear sky. (By permission, from Solar Radiation, N. Robinson, Elsevier, Amsterdam, 1966.)

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