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UNC-Chapel Hill GEOG 111 - Earth-Sun Geometry

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GEOG 111 1st Edition Lecture 5 Outline of Last Lecture I. Two components of solar radiationa. Direct beamb. Diffuse lightII. Factors of Atmospheric TransmissivityIII. Earth-Sun Geometry Outline of Current Lecture I. Earth-Sun Geometrya. Rotationb. Revolutionc. Tilted axisd. Solar declinatione. Solar elevationf. Solar insolation Current LectureI. Earth-Sun Geometrya. Seasonal changes in earth-sun geometry controls the intensity of solar radiation and the length of the dayb. Two independent motions affect earth-sun geometry:i. Rotation- counterclockwise (West to East); goes 360 degrees in 24 hours1. This is why we have time zones2. We go later in time to the east (Europe is ahead of us) and earlier in time to the west. The sun rises and sets later on the western edge of a time zone region and earlier on the eastern edge of a time zone region. ii. Revolution- the earth revolving around the sun on the plane of the ecliptic1. Takes 365.25 days2. Due to the earth’s elliptical path (not a perfect circle), the distance between the earth and sun varies during the year3. Perihelion- around January 3a. Earth is closest to the sunb. 147, 500, 000 km4. Aphelion- around July 4a. Earth is farthest away from sunb. 152, 500, 000 kmiii. The Earth’s axis is tilted 23.5 degrees with respect to the plane of the ecliptic1. This results in seasonsThese notes represent a detailed interpretation of the professor’s lecture. GradeBuddy is best used as a supplement to your own notes, not as a substitute.a. Solstice- December 21-22/ June 21-22i. Summer: longest day in Northern Hemisphere1. SE maxed out in NH2. SD= 23.5 degrees north Tropic of Cancer3. Latitude increases; day length increasesii. Winter: shortest day in NH1. SE min in NH2. SD= 23.5 degrees south Tropic of Capricorn3. Latitude increases; day length decreasesb. Equinox- March 21-22/September 22-23i. At noon the sun will be hitting the equator directly overheadii. Equal day and equal night everywhere (12 hours)iii. SD = 0 degrees Equatoriv. Solar declination (SD)- latitude at which the sun’s direct rays are hitting1. Always between 23.5 degrees north (Tropic of Cancer) to 23.5 degrees south (Tropic of Capricorn)v. Solar elevation (SE)- the sun’s position in the sky1. Highest in June2. Seasonal changes in the length of the day are related to solar elevation3. The rate of change in the day length (i.e. sunrise and sunset time) is positively related to the rate of change of solar elevationa. Greatest changes: late winter-early spring and late summer-early fallb. Smallest changes: late fall-early wintervi. Links between solar radiation and earth-sun geometry1. The intensity of solar radiation is related to solar elevationa. Direct rays give one energy unitb. Oblique rays give half energy units2. The intensity of solar radiation is also affected by atmospheric transmissivity, which is related to the solar elevationa. Solar elevation increases; transmissivity increasesvii. Solar insolation (SI)- the total amount of solar radiation energy received on a given surface area during a given time1. Maximized in the summer and continues decreasing throughout late fall/winter2. Why is SI higher over the north polar region during the solstice? The days are longer so the sun is beaming a longer amount of time, even though it may not be as strong as it is in other seasons or areas. This solar radiation adds up over timea. If this is true, why is it not warmer there? The sun is coming in ata low angle so most of the radiation ends up being reflected by the snow (albedo). The energy is used up in melting the snow and ice and none is left to heat up the


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