Solar Radiation

1.1 Introduction

Sunlight, in the broad sense, is the total spectrum of the electromagnetic radiation given off by the Sun. On Earth, sunlight is filtered through the atmosphere, and the solar radiation is obvious as daylight when the Sun is above the horizon. This is usually during the day hours. Near the poles in summer, sunlight also occurs during the night hours and in the winter at the poles sunlight may not occur at any time. When the direct radiation is not blocked by clouds, it is experienced as sunshine, a combination of bright light and heat. Radiant heat directly produced by the radiation of the Sun is different from the increase in atmospheric temperature due to the radiative heating of the atmosphere by the Sun's radiation. Sunlight may be recorded using a sunshine recorder, pyranometer or pyrheliometer. The World Meteorological Organization (WMO) defines sunshine as direct irradiance from the Sun measured on the ground of at least 120 Wm-2. Direct sunlight gives about 93 lux of illumination per watt of electromagnetic power, including infrared, visible and ultraviolet. Bright sunlight provides illumination of approximately 100 000 lux per square metre at the Earth's surface. Sunlight is a key factor in the process of photosynthesis.

1.1.1 The Sun

The Sun is the star at the centre of the solar system. The Earth and other matter (including other planets, asteroids, meteoroids, comets and dust) orbit the Sun, which by itself accounts for about 99.8% of the solar system's mass. Energy from the Sun, in the form of sunlight, supports almost all life on Earth via photosynthesis, and drives the Earth's climate and weather.

The Sun has an effective black-body temperature TS of 5777 K and it is the largest member of the solar system. The Sun is a sphere of intensely hot, gaseous matter with a diameter of 1.39 x 109 m and is, on average, 1.5 x 1011 m away fromthe Earth. The Sun is, effectively, a continuous fusion reactor. It is estimated that 90% of the Sun's energy is generated in the region 0 to 0.23 R (R being the radius of the Sun = 6.95x 108 m); the average density (ρ) and the temperature (T) in this region are 105 kgm-3 and about 8-40 x 106 K respectively. At a distance of about 0.7 R from the centre, the temperature drops to about 1.3 x 105 K and the density to 70 kg m-3. Hence for r > 0.7R convection begins to be important and the region 0.7R<R is known as the convective zone. The outer layer of this zone is called the photosphere. The maximum spectral intensity occurs at about 0.48 mm wavelength λ in the green portion of the visible spectrum. About 8.73% of the total energy is contained in the ultraviolet region (λ< 0.40 µm); another 38.15% in the visible region (0.40 µm<λ<0.70 µm) and the remaining 53.12% in the infrared region (λ>0.70 µm).

1.1.2 The Earth

Earth is the third planet from the Sun. Earth is the largest of the terrestrial planets in the solar system in diameter, mass and density. The Earth, almost round in shape with a diameter of about 13 000 km, came into existence some 4.6 x 109 years ago. The Earth's inner core is a solid made of iron and nickel. The eruption of volcanoes generally occurs at the plate boundary of the Earth. During eruption of volcanoes, various greenhouse gases, namely carbon dioxide (CO2), methane (CH4), nitrous oxide (NOx), ozone (O3) and water vapour (H2O) etc., existing inside the ground, are also discharged through the plate boundary. These discharged gases, at the boundary of the plate, move upwards towards the Sun due to its low density. These gases form a layer between the Sun and Earth (Figure 1.1). This layer is generally referred to as the Earth's atmosphere. The Earth revolves around the Sun once in about a year. Nearly two-thirds of the Earth is covered by water and the remaining one-third is land. Half of the Earth is lit by sunlight at a time. It reflects one-third of the sunlight that falls on it. This is known as Earth's albedo. The Earth is spinning at a constant rate about its axis, inclined at an angle of 23.5°. As a result, the lengths of days and nights are constantly changing. The heat flux at Earth's surface due to heat conduction from the centre is 0.04–0.06 W m-2 with a temperature gradient of 30–40°C km-1.

1.1.3 Earth's Atmosphere

The temperature of the Earth's atmosphere varies with altitude among five different atmospheric layers:

Exosphere: from 500–1000 km up to 10 000 km, free-moving particles that may migrate into and out of the magnetosphere or the solar wind.

Ionosphere: the part of the atmosphere that is ionized by solar radiation. It plays an important part in atmospheric electricity and forms the inner edge of the magnetosphere. It has practical importance because, among other functions, it influences radio propagation to distant places on the Earth. It is located in the thermosphere and is responsible for auroras.

Thermosphere: from 80–85 km to 640 + km, the temperature increasing with height.

Mesosphere: extends from about 50 km to the range of 80–85 km, the temperature decreasing with height. This is also where most meteors burn up when entering the atmosphere.

Stratosphere: extends from the troposphere's 7- to 17-km range to about 50 km. Temperature increases with height. The stratosphere contains the ozone layer, the part of the Earth's atmosphere which contains relatively high concentrations of ozone. 'Relatively high' means a few parts per million (ppm) – much higher than the concentrations in the lower atmosphere but still small compared to the main components of the atmosphere. It is mainly located in the lower portion of the stratosphere from approximately 15 to 35 km above Earth's surface, though the thickness varies seasonally and geographically.

Troposphere: the lowest layer of the atmosphere; it begins at the surface and extends to between 7 km at the poles and 17 km at the equator, with some variation due to weather factors. The troposphere has a great deal of vertical mixing because of solar heating at the surface. This heating warms air masses, which makes them less dense so they rise. When an air mass rises, the pressure upon it decreases so it expands, doing work against the opposing pressure of the surrounding air. To do work is to expend energy, so the temperature of the air mass decreases. As the temperature decreases, water vapour in the air mass may condense or solidify,...