You've been given a set of solar radiation intensity (flux) observations recorded at Hanford, CA on December 16, 1998 and May 22, 1999. These data came from the National Solar Radiation Database 1998-2005 Update Web site. I have edited the format (but not the data themselves) and added a new column (sun angle), calculated from data in another column (zenith angle).
The data are organized into seven columns in a table. Each column has a header, but not all of the headers are easy to understand. Here is a translation:
- Column Headers
- Year, month, and day (local standard time).
- HH:MM (LST)
- Hour and minute, local standard time, at the end of the hour over which observations are averaged.
- Zenith Angle (deg)
- Angle between the zenith and the sun, in degrees, averaged over a full hour.
- Sun Angle (deg)
- Angle of the sun above the horizon, in degrees, averaged over a full hour.
- ETR (W/m^2)
- One-hour average "extraterrestrial radiation" insolation—that is, the rate at which solar radiative energy strikes a horizontal area of 1 meter2 at the top of the atmosphere, averaged over one hour.
- "W" is short for "Watts", a unit of power. One Watt is a Joule of energy per second.
- "m" is short for "meter", and "m^2" is short for meter × meter, or meters squared, or square meters, a unit of surface area.
- ETRN (W/m^2)
- Same as ETR except on a surface normal to the sun's rays.
- Meas Glo (W/m^2)
- Same as ETR except at the earth's surface.
- Notes on the Data
- On a 24-hour clock, midnight is 00:00, the start of a new day. The hours (time of day) in the table run from 1:00 (1 a.m. local standard time) to 00:00 (midnight), the start of the next day. Each observation represents a one-hour average, and each hour listed in the table represents the end of a 1-hour averaging period. For example, in the first row of data, for which the hour is 1:00 (1 a.m.), the observations are averaged over the 1-hour period from midnight to 1 a.m.
- In their original form (downloaded from the National Solar Radiation Database), the data included zenith angle but not sun angle. Because of the way sun angle and the zenith angle are defined, their sum is always 90°. Hence, it's easy to compute the sun angle from the zenith angle: sun angle = 90° − zenith angle. I did that and added sun angle to the data table.
- At night, the sun is below the horizon. At those times, the sun angle is less than 0° and the zenith angle is greater than 90°. However, in the data table, when the sun is below the horizon for the entire hour over which data are averaged, both angles are listed simply as "99".
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