# ch03 - Jon Ahlquist Chapter 3 Seasonal and Daily...

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Jon Ahlquist 9/17/2006 MET1010 Intro to the Atmosphere 1 Chapter 3 Seasonal and Daily Temperature ± Why the Earth has seasons ± Local seasonal variations ± Daily Temperature variations ± Controls of temperature ± Air temperature data ² Daily, month, and yearly temperatures ² Use of temperature data ± Air temperature and human comfort ± Measuring air temperature Chapter 3 requires 3-D thinking ± Some students have trouble with chapter 3. You must think three dimensionally to understand relationships between the Sun and Earth. ± If you have trouble with the Earth-Sun relationships, make drawings from different perspectives and/or use a ball to represent the Earth. The Earth rotates around its axis in the same direction that it revolves around the Sun. Sun Night Day Earth Earth’s axis: North Pole The Sun rises in the east, sets in the west. West East Direction of motion around Sun Earth’s orbit around Sun: nearly circular ± Earth’s orbit is almost a circle around the Sun, so orbit is not responsible for seasons. ± Earth is slightly closer to Sun in January, slightly farther from Sun in July. Avg distance to Sun is 150 million km = 93 million miles, varying only 2% during year Earth Sun Fig 3.1, p. 56 Angle of Illumination (Fig. 3.2, p. 56) ± When a light beam is perpendicular to a surface, it illuminates a smaller area, so there is more heating in that area. ± When a light beam is not perpendicular to a surface, it illuminates a larger area, so there is less heating over the larger area. ± Sun: most concentrated heating at noon, The Earth tilts (Fig. 3.6, p. 58) The Earth tilts 23.5 degrees relative to its orbital plane 23.5 deg E q u a t o r i l p n e Orbital plane containing the Sun Because of the Earth’s tilt: ± Hours of daylight vary with latitude. Always 12 hrs at equator. Approaches 24 hrs/day as you go toward the pole in the summer hemisphere. Approaches 0 hrs/day as you go toward the pole in the winter hemisphere. ± More/less intense sunlight in summer/winter hemisphere Use the drawing above to visualize these 2 facts.

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Jon Ahlquist 9/17/2006 MET1010 Intro to the Atmosphere 2 Solstice and Equinox (fig. 3.3, p. 57) Earth’s axis always points in same direction toward North Star. Because of orbit around Sun, tilt relative to Sun varies during year. March- Sept: N Hem tilts toward Sun Sept-March: N Hem tilts away from Sun Sept, March: sideways tilt 21 Dec: N Hem has max tilt away from Sun 21 June: N Hem has max tilt toward Sun Solstice and Equinox (cont.) ± Solstices: 21 December, 21 June ± Equinoxes: 20 March, 22 September ± For simplicity, think of 21st of month for all of them. Learn these dates! ± Winter solstice: minimum solar energy input to
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ch03 - Jon Ahlquist Chapter 3 Seasonal and Daily...

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