**1. Introduction**

32 Solar Radiation

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The availability of solar radiation is important climate and welfare related environmental factor like temperature or moisture. Living organisms are adapted to the local annual cycles as well as interannual and intraseasonal variations of environmental factors. The importance of appropriate models for global solar radiation has increased in recent decades due to wider use of solar energy applications, including photovoltaic power generation (Šuri et al., 2007, Tiwari and Sodha, 2006). Better understanding of the influence of spectral composition of ground-level solar radiation on terrestrial and aquatic ecosystems has increased the interest in the availability of solar direct irradiance (Lohmann et al., 2006). The annual total does not contain enough information for applications of solar radiation data. Seasonal or even monthly time resolution is often necessary. The annual cycle of availability of solar energy is determined by the annual cycle of noon solar elevation angle, with significant contribution from cloudiness (Tooming, 2002) and atmospheric transparency (Russak, 1990, 2009). Cloud cover at moderate latitudes tends to be thicker and more frequent in late autumn and early winter and less frequent in spring and summer. In the dark half-year not only absolute but also relative availability of solar radiation is smaller.

The attempts of measuring and recording ground-level solar radiation have started about 100 years ago, but usually these activities remained episodic. Most available solar radiation data sets cover significantly shorter time intervals than those of temperature or precipitation. Due to relatively short time series the reasons for variation and regular changes of ground-level solar radiation are still not completely understood.

The present chapter in considering local seasonal and monthly relative availability of solar radiation is based on Estonian solar radiation data. The longest and most complete data set on solar radiation in Estonia has been collected at a typical Estonian rural site at the Tartu-Tõravere Meteorological Station (58o.16'N, 26o.28'E, 70 m a.s.l.). The attempts of recording sunshine duration were made since 1906 (Kallis et. al., 2005). First regular measurements of solar irradiance were performed in late 1930s and continued after 1950 (Ohvril et. al., 2009). Before 1965 the station was based closer to town than its present site. Simultaneous measurements at both sites during one year did not reveal systematic differences. The landscapes at both sites are similar. The Tartu-Tõravere site as well as that before 1965 can be considered typical for Northern Europe. At other geographical regions the contrast between summer and winter as well as the seasonal impacts of cloud cover and aerosols may be significantly different. Here, the variations of solar ground-level integral global and direct irradiance on seasonal and monthly scales are examined. The continuous record of pyranometer-measured daily global radiation extends back to 1953 and that of pyrheliometer-measured direct irradiance back to 1955. The study is based on this long-term data set for years 1955-2010 when both quantities are available. The data set is supported by the conventional meteorological data and visual cloud inspection data.

Much of information in meteorological and climatological studies is obtained from measurement data applying statistical methods. The aim of exploratory data analysis (EDA) is to get an insight into the possible processes behind the variations in the collected data. Often the seasonal or monthly data are analyzed for their trends in time. In EDA, mainly the numerical summary measures of collected data sets, characterizing central tendency, spread and symmetry of data samples during their time evolution are used (Wilks, 2006). Quite often the conventional mean is used as a central tendency measure without checking how adequate it is. To get realistic insights into the processes the chosen characteristics must be robust. Robustness means insensitivity to deviations from the assumptions made. Suitability of different central tendency and spread characteristics of the recorded daily sums is compared in the case of skewed probability density distributions and the appropriate characteristics of seasonal and monthly relative solar radiation are found. Major features of variation and trends in the availability of solar radiation in 1955-2010 are studied.
