Figure 4: Clear sky absorption and scattering of incident solar energy. Values are typically for one air mass. 20 20 N. Harris, et al., John Wiley & Sons, 1985: Solar Energy Systems Design, New York NY, 774 pp., Out of print.
10 The atmosphere’s effect on the insolation is different in different places and is dependant on weather conditions. Some guidelines of how weather affects the insolation follows below 21 . Cloudy weather: • The clouds reflect approximately 40% (can reflect up to 80%) • The clouds, water vapour, ice crystals etc. absorb approximately 10% (can be up to 20%) • The clouds can give back between 0-50% in diffused insolation to Earth. Clear weather: • Reflection from different particles, water drops and ice crystals is approximately 5%. • Absorption in dust and water drops is approximately 15%. • The Earth’s surface reflects 0-80% depending on surface (see Albedo below). 5.2.6 Albedo Some of the incoming radiation from the Sun is reflected and scattered back to the outer space. Albedo (often with the letter α) can have a value from 0 for no reflection to 1 for complete reflection of light striking the surface. Albedo can be expressed as a percentage (albedo multiplied by 100) that for some is easier to understand. The radiation reflected can be calculated with the following equation: I refl = α∗ I sc Often surfaces that are bright have high albedo which means that they reflect much of the light. With the same reasoning dark surfaces have low albedo. The radiation that is not reflected is absorbed. The absorption of sun radiation is mainly done by the ground, plants and oceans but some is instead absorbed by the atmosphere and the clouds. UV light is mainly absorbed by oxygen and ozone in the stratosphere. Earth has as a mean value of 0.3 in albedo. Some other surface’s albedo are shown in table 2 22 . Clouds Albedo Clouds have a great effect on the albedo of the planets. Clouds reflect much more light back then blue sky does. The albedo of a cloud depends on several factors, including the height, size, and the number and size of droplets inside the cloud. If the cloud contains many big droplets and therefore has a large total surface that reflects, the albedo is high. You can see this when you are under a big cumulonimbus cloud, where not much of the sunlight passes through. A cumulonimbus cloud has an albedo of up to 0.9 and looks black from the ground but light from space. Stratus clouds have an albedo of 0.4 - 0.65 and cirrus clouds have around 0.2 - 0.4. 23 21 Petterson F, 1985: Solenergi Teori, Forskning & Praktisk användbarhet, Stockholm: Ingenjörsförlaget AB 22 University of Leeds: , 2005-03-23 23 ESPERE-ENC; , 2005-03-23 Table 2: Albedo for different terrains. Surface Albedo Fresh snow 0.85-0.95 Dry sand 0.35-0.40 Plains and farmlands 0.20-0.30 Tropical forest ~0.13 Ocean 0.04-0.10