Shade Planning for Schools

38 Shade Planning for America’s Schools Chapter 5 of the equator experience days longer than 12 hours and all latitudes north of the equator experience days shorter than 12 hours. This marks the beginning of summer in the southern hemisphere and the beginning of winter in the northern hemisphere. On winter solstice, the center of the sun lines up with the latitude known as the Tropic of Capricorn, which is at 23.5° south. On the equinoxes, those being around September 21st and March 21st, the Earth is positioned so that the North Pole points neither toward nor away from the sun. On those 2 days, the Earth’s equator lines up with the center of the sun, resulting in days that are exactly 12 hours long, regardless of latitude. Solar azimuth angle is the angle on the horizontal plane between the point on the horizon that is directly beneath the sun and true south. The azimuth angle determines the direction of shadows. Solar elevation angle or solar altitude angle is the angle that describes the height of the sun in relation to the nearest point on the horizon. It varies according to the time of day and the season and determines the length of shadows. K e y T e r m s a n d C o n c e p t s Axis Tilt and Solar Radiation That the Earth’s axis is tilted affects the amount of solar radiation reaching the Earth in three ways. First, the length of the day changes throughout the year. In the northern hemisphere, beginning on the summer solstice, the sun’s daily path is increasingly lower in the sky (making shadows longer) until the winter solstice, after which the days become increasingly longer. As days grow longer, the risk of exposure to excessive ultraviolet (UV) radiation is greater. Second, when the sun is at a lower altitude, its rays are spread over a larger area, reducing the intensity of the sun’s radiation, including UV radiation. Third, levels of solar radiation are also affected by how much of the atmosphere the sun’s rays must pass through. Solar radiation levels, including UV radiation, are greatest when the sun is higher in the sky and solar radiation has a shorter path to travel through the atmosphere. When the sun is lower in the sky, solar radiation has a longer path to travel through the atmosphere, resulting in more of the sun’s radiation, including UV radiation, being absorbed or scattered by the atmosphere. The more perpendicular a beam is to a surface, the brighter the beam will be on the surface. Putting It All Together In order to create shade that falls in the right place, at the right time of day, throughout the year, it is essential that the planning team model the shade that their proposed buildings and plantings will cast. To do that, the team will need to know: