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Jon Ahlquist 9/17/2006MET1010 Intro to the Atmosphere 1Chapter 3Seasonal and Daily TemperatureWhy the Earth has seasonsLocal seasonal variationsDaily Temperature variationsControls of temperatureAir temperature dataDaily, month, and yearly temperaturesUse of temperature dataAir temperature and human comfortMeasuring air temperatureChapter 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.SunNightDayEarthEarth’saxis:NorthPoleThe Sun rises in the east, sets in the west.WestEastDirection of motionaround SunEarth’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 yearEarthSunFig 3.1, p. 56Angle 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,less at dawn & dusk.The Earth tilts (Fig. 3.6, p. 58)The Earth tilts23.5 degrees relative to its orbital plane23.5 degEquatorial planeOrbital planecontainingthe SunBecause 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 hemisphereUse the drawing above to visualize these 2 facts.Jon Ahlquist 9/17/2006MET1010 Intro to the Atmosphere 2Solstice 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 Hemtilts toward SunSept-March: N Hemtilts away from SunSept, March: sideways tilt21 Dec: N Hem hasmax tilt away from Sun21 June: N Hem hasmax tilt toward SunSolstice and Equinox (cont.) Solstices: 21 December, 21 June Equinoxes: 20 March, 22 September For simplicity, think of 21st of monthfor all of them. Learn these dates! Winter solstice: minimum solar energy input to winter hemisphere Summer solstice: maximum solar energy input to summer hemisphereWhen do seasons start?Is December 21 (the winter solstice) really the 1st day of winter? (p. 62) Cut the year into four equally long periods Center summer on the warmest 91 days,winter on the coldest 91 days. By that definition, summer starts around 1 June, and winter starts around 1 December for inland areas. Winter and summer start later in the month for coastal areas. Also, inland areas cool off faster than coastal areas in fall and warm up faster in springSolstice and Equinox (cont.)Equinox (equal night and day): 12 hrs of daylight and night at all latitudes Sun overhead at noon on equator Tilt of Earth’s axis not toward/away from Sun but to sideOn all dates other than equinox: 12 hrs of daylight at equator but not elsewhere More than 12 hrs daylight in summer hemisphere,less than 12 hrs daylight in winter hemisphere Sun overhead at noon at some latitude in summer hem. Some tilt toward/away from sun in summer/winter hem.Solstice: 24 hrs daylight/dark poleward of (ant)arctic circle in summer/winter hemisphere Sun overhead at noon at 23.5 deg lat in summer hem. Max tilt toward sun in summer hemisphereSolstice and Equinox: Hrs of daylightFOR NORTHERN HEMISPHERE (opposite for Southern Hem)21 December: Minimum hours of daylight21 December to 21 June: No. of hours of daylight increasing fromday to day21 June: Maximum hours of daylight21 June to 21 December: No. of hours of daylight decreasing from day to day21 March: 12 hours of daylight21 March to 21 September: 12 or more hours of daylight21 September: 12 hours of daylight21 September to 21 March : 12 or less hours of daylightExamples:On 15 January, < 12 hrs of daylight, & days getting longer.On 4 July, > 12 hours of daylight, & days getting shorter.At what latitude is the sun directly overhead at noon? (Graph not in book.)Sep 21 Dec 21 Mar 21 Jun 21 Sep 2123.5º N15º NEquator23.5º S Sun never overhead outside of 23.5º N to 23.5º S Overhead once per year at 23.5º N or 23.5º S Overhead twice per year between 23.5º N and 23.5º SExample: At 15º N, overhead twice during Mar 21 to Sep 21Tropic of CancerTropic of CapricornJon Ahlquist 9/17/2006MET1010 Intro to the Atmosphere 3Where is the Sun at noonat other latitudes? (fig. 3.6, p. 58) Take N Hem latitudes as positive, S Hem as negative. If Sun is overhead at noon at latitude LSun, thenat noon at latitude L, sun is LSun-L degrees down from vertical, toward N if positive, toward S if negative.Examples: At noon on June solstice, the Sun is overhead atLSun= 23.5°N. (Use the figure above to visualize.) Therefore: •At equator, the sun is 23.5 – 0 = +23.5° = 23.5° north of vertical•At 47°N, the sun is 23.5 – 47 = -23.5° = 23.5° south of vertical•At 90°N, the sun is 23.5 –90 = -66.5° = 66.5° south of verticalSun at various latitudes (fig. 3.8, p. 61)Energy received on 21 June (fig. 3.5, p. 58): Almost constant across Northern HemisphereEquator Latitude (deg N) North PoleEnergy received on 21 Dec (from 7thed.): Roughly constant in Southern Hemisphere.Decreasing to 0 in Northern Hem latitudes.Error in graph:This graph shouldgo to 0 at latitude90 – 23.5 = 66.5°N,i.e., at Arctic Circle,not south of 60°N.Heating Difference between Summer and Winter Temperature difference between equator and pole small in summer, large in winter.Fig. 3.6, p.58.Radiation is more “spread out” at northerly latitudes,but more hours of daylight compensate.Approaching S Pole, radiation is more “spread out” andfewer hours of daylight Annual average energy input vs. energy output (7thed.): Visible light input max in tropics, min at poles. Infrared output roughly same at all latsHeat moved by


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FSU MET 1010 - Seasonal and Daily Temperature

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