5The Electromagnetic Spectrum Sun radiates shortwave & longwave energyEarth radiates longwave energy
6Solar and Terrestrial Energy Solar radiation peaks in the visible light spectrumFigure 2.7
7Distribution of Insolation Tropics receive more concentrated insolation due to the Earth’s curvature. (see next slide)Annually, tropics receive 2.5x more ins. than poles*, though total hours of daylight per year are the same.However, in June, more radiation falls in a day on the top of the atmosphere at latitudes like Vancouver, BC than above a similar sized tropical area.*per unit area, top of atmosphere
9Seasonality Caused by changes in intensity & duration of insolation Key related conceptsSolar altitude – angle above horizon (affects intensity at ground level)Daylength (duration)Declination – location of the subsolar point* in degrees of latitude (in tropics)*latitude where sun is directly overhead at solar noon
11Reasons for Seasons: Primary explanation: variations in the duration and intensity of solar radiation received drive the annual march of the seasons. What causes these variations?Tilt of Earth’s axisAxis is tilted 23.5° from plane of eclipticAxial parallelismAxis maintains alignment during orbit around the Sun animation earth sun relationsNorth pole points toward the North Star (Polaris)Thus, the subsolar point (declination) will vary in location between the Tropic lines.
12Reasons for seasons (cont) From March 21 to Sept. 22, declination (subsolar pt.) is in northern Hemisphere. Day length is longer the farther north one is on the Earth. For all places north of Tropic of Cancer, solar altitude is highest June 21.From Sept. 22 to March 21, declination is in southern Hemisphere. Day length is longer the farther south one is on the Earth. For all places south of Tropic of Capricorn, solar altitude is highest Dec. 21.From Sept 22 to March 21, declination is in the ___ Hem. Day length is longer the farther south one goes.
14Annual March of the Seasons (know perfectly!!) Winter solstice ~December 21Subsolar point Tropic of CapricornSpring equinox ~March 21Subsolar point EquatorSummer solstice ~June 21Subsolar point Tropic of CancerFall equinox ~September 22
15Annual March of the Seasons: Know Well! Figure 2.13