Jon Ahlquist 9 17 2006 Chapter 3 Seasonal and Daily Temperature Chapter 3 requires 3 D thinking Why the Earth has seasons Local seasonal variations Daily Temperature variations Controls of temperature Air temperature data Daily Use month and yearly temperatures of temperature data 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 Air temperature and human comfort Measuring air temperature The Earth rotates around its axis in the same direction that it revolves around the Sun Direction of motion around Sun East 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 Sun Earth Day Night West Earth s axis North Pole The Sun rises in the east sets in the west Angle of Illumination Fig 3 2 p 56 Earth Avg distance to Sun is 150 million km 93 million miles varying only 2 during year The Earth tilts Fig 3 6 p 58 The Earth tilts 23 5 degrees relative to its orbital plane 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 MET1010 Intro to the Atmosphere Fig 3 1 p 56 al ori p la ne t ua Eq 23 5 deg 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 1 Jon Ahlquist 9 17 2006 Solstice and Equinox fig 3 3 p 57 Solstice and Equinox cont Earth s axis always points in same direction toward North Star Because of orbit around Sun tilt relative to Sun varies during year Sept March sideways tilt Sept March N Hem tilts away from Sun March Sept N Hem tilts toward Sun 21 Dec N Hem has max tilt away from Sun When do seasons start Is December 21 the winter solstice really the 1st day of winter p 62 21 June N Hem has max tilt toward Sun 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 spring 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 side On 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 hemisphere At what latitude is the sun directly overhead at noon Graph not in book FOR NORTHERN HEMISPHERE opposite for Southern Hem 21 December Minimum hours of daylight 21 December to 21 June No of hours of daylight increasing from day to day 21 June Maximum hours of daylight 21 June to 21 December No of hours of daylight decreasing from day to day 23 5 N 15 N Tropic of Cancer Equator 21 March 12 hours of daylight 21 March to 21 September 12 or more hours of daylight 21 September 12 hours of daylight 21 September to 21 March 12 or less hours of daylight MET1010 Intro to the Atmosphere Winter solstice minimum solar energy input to winter hemisphere Summer solstice maximum solar energy input to summer hemisphere Solstice and Equinox cont Solstice and Equinox Hrs of daylight Examples On 15 January 12 hrs of daylight days getting longer On 4 July 12 hours of daylight days getting shorter Solstices 21 December 21 June Equinoxes 20 March 22 September For simplicity think of 21st of month for all of them Learn these dates 23 5 S Sep 21 Tropic of Capricorn Dec 21 Mar 21 Jun 21 Sep 21 Sun never overhead outside of 23 5 N to 23 5 S Overhead once per year at 23 5 N or 23 5 S Overhead twice per year between 23 5 N and 23 5 S Example At 15 N overhead twice during Mar 21 to Sep 21 2 Jon Ahlquist 9 17 2006 Where is the Sun at noon at other latitudes fig 3 6 p 58 Sun at various latitudes fig 3 8 p 61 Take N Hem latitudes as positive S Hem as negative If Sun is overhead at noon at latitude LSun then at 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 at LSun 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 vertical Energy received on 21 June fig 3 5 p 58 Almost constant across Northern Hemisphere Energy received on 21 Dec from 7th ed Roughly constant in Southern Hemisphere Decreasing to 0 in Northern Hem latitudes Error in graph This graph should go to 0 at latitude 90 23 5 66 5 N i e at Arctic Circle not south of 60 N Equator Latitude deg N North Pole 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 and fewer hours of daylight MET1010 Intro to the Atmosphere Annual average energy input vs energy output 7th ed Visible light input max in tropics min at poles Infrared output roughly same at all lats Visible light input Infrared output Heat moved by advection in atmos and ocean plus transport of latent heat 3 Jon Ahlquist 9 …
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