Shade Planning for Schools

5 Shade Planning for America’s Schools Chapter 1 W HAT IS UV R ADIATION ? Ultraviolet (UV) radiation is one component of a broad spectrum of electromagnetic radiation emitted by the sun. The spectrum also includes visible light, which we see, and infrared radiation, which we feel as heat. The ultraviolet area of the spectrum can be further divided into three bands, ultraviolet A (UVA), ultraviolet B (UVB), and ultraviolet C (UVC). All UVC radiation and almost all UVB radiation are absorbed in the ozone layer of the atmosphere. UVA radiation penetrates the atmosphere unimpeded and, until recently, had been considered innocuous. Ultraviolet Visible Infrared UVC UVB UVA 100 280 315 400 700 wavelength (nm) The Sun’s Radiation Spectrum What Are the Factors That Affect UV Radiation Levels? Time of Day At solar noon, the sun is at its highest point of the day. As much as 30% of total UV radiation is received in the hour before and the hour after solar noon, and as much as 75% is received during the 3 hours before and the 3 hours after solar noon. 1 1 Diffey, BL. Solar ultarviolet radiation effects on biological systems. Physiology Medica; Biology 1991; 36(3):299-328. S h a d e P l a n n i n g T o o l b o x Q: What time is solar noon? A: That depends on the day of the year, your latitude and longitude, and your location within your time zone. On July 4, solar noon is at: 12:48 (EDT) in Boston 1:40 (EDT) in Knoxville 12:51 (CDT) in Nashville 12:53 (PDT) in San Diego On February 1, solar noon is at: 11:57 (EST) in Boston 12:49 (EST) in Knoxville 12:00 (CST) in Nashville 12:02 (PST) in San Diego To find out what time solar noon occurs on any day of the year at any location, visit the National Oceanic and Atmospheric Administration (NOAA) Surface Radiation Research Branch web site at highlights/sunrise/sunrise.html. Time of Year Because the angles of the sun change throughout the year, the intensity of UV radiation changes as well. In the northern hemisphere, UV radiation tends to be greater in the summer months. Geographical Latitude UV radiation decreases as the distance from the equator increases. Altitude Because the atmosphere is thinner at higher altitudes and less able to absorb UVB, total UV radiation is greater at higher altitudes. Weather Conditions Although clouds reduce the full spectrum of solar radiation, they do not reduce UV radiation to the same extent that they reduce visible light and