![]() ![]() The 23.5° tilt in the Earth's axis of rotation is a more significant factor in determining the amount of sunlight striking the Earth at a particular location. However, the presence of vast oceans moderates the hotter summers and colder winters one would expect to see in the southern hemisphere as a result of this difference. The Earth is nearer the sun when it is summer in the southern hemisphere and winter in the northern hemisphere. ![]() When the sun is nearer the Earth, the Earth's surface receives a little more solar energy. The Earth revolves around the sun in an elliptical orbit and is closer to the sun during part of the year. Because the Earth is round, the frigid polar regions never get a high sun, and because of the tilted axis of rotation, these areas receive no sun at all during part of the year. The more slanted the sun's rays are, the longer they travel through the atmosphere, becoming more scattered and diffuse. When the sun's rays are vertical, the Earth's surface gets all the energy possible. The amount of solar radiation that reaches any one spot on the Earth's surface varies according to:īecause the Earth is round, the sun strikes the surface at different angles, ranging from 0° (just above the horizon) to 90° (directly overhead). Basic PrinciplesĮvery location on Earth receives sunlight at least part of the year. However, the technical feasibility and economical operation of these technologies at a specific location depends on the available solar resource. Solar radiation can be captured and turned into useful forms of energy, such as heat and electricity, using a variety of technologies. On dull days, all or nearly all of the solar radiation reaching the earth’s surface is diffuse, while on sunny days the diffuse may be less than 30% of the total radiation.Solar radiation, often called the solar resource or just sunlight, is a general term for the electromagnetic radiation emitted by the sun. ![]() The diffuse radiation expressed as a percentage of the total is, on average, lowest in April and highest in December. *Kilkenny’s records are from 1979 to 2008. Mean Global Radiation in Joules/cm² 1981-2010 Station The percentage of the energy arriving at the top of the atmosphere, which reaches the surface, has a mean value of 40% to 47% during the months March to September and 30% to 37% during the months October to February. On days with no sun, the surface receives on average 20% to 25% of the energy arriving at the top of the atmosphere. It requires an unusually fine, sunny day with good visibility to get 75% of this incoming daily total to be received at the earth’s surface. At the latitude of Birr, the solar energy received by a horizontal surface at the top of the atmosphere ranges from about 4170 J/cm² a day in late June (4187 J would raise the temperature of 1 litre of water 1☌) to less than 600 J/cm² in late December. ![]() The energy arriving per unit area is expressed in joules per centimetre squared (J/cm²) and may be converted to kilowatt-hours per metre squared (kWh/m²) by using the conversion 1000 J/cm²=2.778 kWh/m². Direct beam radiation can be focused by parabolic mirrors to give the intense beam of solar radiation needed for high-energy applications while the diffuse component is suitable for applications such as water heating. The global radiation at ground level is composed of direct beam radiation which comes straight from the region of the sun and diffuse radiation which has undergone scattering during its passage through the atmosphere. Research Professorship Call 2023 (Closed) Peer-reviewed journal articles by Met Éireann staff members Past Weather Agrometeorological Bulletins ![]()
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