| "Normal" is a mathematical synonym for perpendicular. As you describe your experiment in moving the sensor during the day, you are keeping its face area "normal" to the incoming solar beam. Were it not for the atmosphere, your readings would be a square wave going instantly to maximum value when the sun breaks the horizon and going to zero at sundown. We do have an atmosphere, and the path that the sun travels through the atmosphere varies from shortest when the sun is directly overhead (referred to as air mass 1 or AM=1) to about 30 or 40 times that when the sun is at the horizon and the beam passes through the slant path. The result is a wavelength-dependent partial extinction of the beam that rounds off the corners of the square wave. The extinction coefficient varies by season and location in a complex way as the molecules making up the atmosphere vary. The sinusoidal curve you are seeing in the weather file is probably the "global horizontal". It is the sum of two components - direct and diffuse. The direct component is the "direct normal" times the cosine of the zenith angle (the angle between the direct beam and a perpendicular to the earth's surface). That angle is 90 degrees at sunrise and sunset and zero degrees at solar noon on a day of the year (if any) when the sun passes directly overhead. That gives a sine curve imposed on the rounded-off square wave, visually an approximate sine wave. The diffuse component is the downward component of light scattered from atmospheric molecules over the solar dome. It's a complicated subject but for practical purposes, it's approximately the "sky blue" light of the sky from short wavelength scattering plus a more variable all-wavelength scattering. For most purposes, it's constant on any given day and makes up roughly 10% of the global horizontal at midday. It gives us most of our useful light at dawn and dusk, but doesn't contribute much during the mid-portion of the day. A great way to grasp the relationships is to see these all plotted out over the course of a clear day. For that, go to http://www.nrel.gov/midc/srrl_bms/display/ where the Solar Radiation Research Lab data are displayed in real time. Ed --- On Sun, 8/23/09, ooikoonbeng2004 <ooi_kb3@xxxxxxxxxxx> wrote:
|