The Smithsonian Astrophysical Observatory SOLAR CONSTANT Database The Smithsonian Astrophysical Observatory (APO) gathered solar constant data during at least 49 years of solar monitoring. The solar constant is the total amount of energy received from the sun per unit time per unit area exposed normally to the Sun's rays at the average Sun Earth distance and outside of the Earth's atmosphere. The purpose of this APO project was to determine an accurate value for this energy flux and to determine whether or not the Sun's total energy output is indeed constant in time. Variability in the solar constant would have profound effects on terrestrial climate. Some of the phenomenon which are possibly related to solar flux variations are the advance and retreat of glaciers, occurrence of droughts, water levels in rivers and lakes, and changes in meridional sea level pressure. Whether or not the APO data does in fact show any variation in solar output has been a subject of considerable controversy. Many investigators claimed the measurements revealed real solar variations. Others felt the measurements gave no such proof. One study of many done to correlate solar constant variation and solar activity used the monthly mean "long" method solar constant values measured at all stations and the monthly mean Wolf sunspot number. The linear least squares fit gives the value of the solar constant as S = [(1.94506 +/- .00059) + (.000008 +/- 000008)R], where S is the solar constant and R is the sunspot number. Two basic measurement techniques were used, the "long" method and the "short" method. The "long" method utilized an instrument called a pyrheliometer that measured directly the total irradiance of the Sun. Combining this with a spectrobolometer which photographically recorded the spectrum of the of the Sun, absolute values of the spectral irradiance could be deduced. Taking absolute irradiance at several air mass values (zenith angles) an extrapolation to zero air mass could be made. The extraterrestrial solar spectrum and hence solar constant were thereby determined. The "short" method of monitoring the solar constant required considerably less time to compute and eliminated problems of varying atmospheric transmission. A pyranometer (aureole brightness meter) measures the brightness of the sky in a doughnut shaped ring about the Sun. This method relied on a "function" of precipitable water and pyranometer measurements to give a value for atmospheric transmission. The validity of the data prior to 1923 was questioned by many, including the APO director from 1906 to 1944, Charles Greeley Abbot. Problems ranging from volcanoes to the invention of the improved measuring devices made this part of the data set of limited use. The remaining 31 years of data have been the subject of a controversy of interpretation as mentioned earlier. Abbot and his APO staff claimed that the data showed a 0.2% increase in solar output from 1923 to 1954. Others argued that unconventional data manipulation made such a conclusion erroneous. Notwithstanding all of the short comings and controversy inherent in the data, this program is the longest and most carefully conducted solar radiation program made so far this century. The data are stored in 47 files. Files 1-15 are generally formatted as shown in FORMAT A below. However the information stored in columns 13-16, 21-25, and 71-75 varies from file to file. Depending on which of these first 15 files are accessed, these columns may contain: water vapor pressure divided by 100, apparent atmospheric transmission, atmospheric transmission; apparent solar constant, long method solar constant, short method solar constant, precipitable water, pyrheliometry at 2.0 air mass, solar constant corrected for constant water vapor, solar constant corrected for zero water vapor. The wavelengths at which atmospheric transmission values are given also vary among these first 15 files. FORMAT B shows how the remaining files 16-47 are formatted. The following table lists each file with its corresponding station location and dates. FILE NUMBER STATION LOCATION MONITORING PERIOD ============ ============================= ================= 1 Calama, Chile 1918-1920 2 Mount Harqua Hala, Arizona 1920-1925 3 Hump Mountain, North Carolina 1917-1918 4 Miami, Florida 1948 5 Mount Montezuma, Chile 1920-1930 6 Mount Montezuma, Chile 1940-1948 7 Mount Brukkaros, S.W. Africa 1926-1930 8 Mount St. Katherine, Egypt 1934-1937 9 Mount Wilson, California 1905-1906 10 Mount Wilson, California 1908-1911 11 Mount Wilson, California 1912-1920 12 Table Mountain, California 1925-1930 13 Table Mountain, California 1940-1950 14 Burro Mountain, New Mexico 1940-1945 15 Washington D.C. 1902-1907 16-47 Mount Montezuma, Chile Table Mountain, California Burro Mountain, New Mexico Mount St. Katherine, Egypt 1923-1954 ========================================================================= FORMAT A Column Format Value ====== ====== ==================================================== 1 A1 Site(C) 3- 6 I4 Year 7-10 A4 Month - alphabetic 11-12 I2 Date 13-16 F4.2 Water vapor pressure 17-20 F4.1 Precipitable water 21-25 5X Blank 26-29 F4.3 Atmospheric transmission - 0.335 microns wavelength 30-33 F4.3 Atmospheric transmission - 0.403 microns wavelength 34-37 F4.3 Atmospheric transmission - 0.460 microns wavelength 38-41 F4.3 Atmospheric transmission - 0.511 microns wavelength 42-45 F4.3 Atmospheric transmission - 0.630 microns wavelength 46-49 F4.3 Atmospheric transmission - 0.702 microns wavelength 50-53 F4.3 Atmospheric transmission - 0.830 microns wavelength 54-57 F4.3 Atmospheric transmission - 1.008 microns wavelength 58-61 F4.3 Atmospheric transmission - 1.244 microns wavelength 62-65 F4.3 Atmospheric transmission - 1.610 microns wavelength 66-70 F5.3 Solar constant Eo 77-80 A4 Grade ======================================================================== FORMAT B Column Format Value ====== ====== ======================================================= 1- 4 I4 Year 6- 7 I2 Month 9-10 I2 Day 12-13 I2 Hour 15-16 A2 Station abbreviation - M = Mount Montezuma, Chile T = Table Mountain, California TY = Burro Mountain, New Mexico K = Mount St. Katherine, Egypt 17-19 I3 Air mass - must be divided by 100. 20-23 F4.1 Precipitable water 25 A1 Group designation 27-30 I4 Function 32-34 I3 Pyrheliometry - must be divided by 1000 and added to 1. 36-38 I3 Pyranometry - must be divided by 10000. 40-41 I2 Solar constant - must be divided by 1000 and added to 1. 43 A1 Grade 45-46 I2 Best estimate of solar constant for the day - must be divided by 1000 and added to 1.9. ===========================================================================