This index takes the average insolation value ( 2 calories per square centimeter per minute) and adjusts it for the angle of the sun and the earth's distance from the sun on a given date as calculated by the the Summit School Almanac for Windows.  In 2001 at Summit School the maximum summer value is 1.889981 on June 14 and the minimum value is  1.047008 on December 20.
    The top red line is the maximum value at the summer solstice.  The second red line (from the top) is the value at an equinox. The third red line (from the top) is the median value.  The bottom red line is the minimum value at the winter solstice.
    Insolation is radiant energy from the sun that strikes the earth
( a contraction for "incoming solar radiation") measured in various SI units.  We use calories per square centimeter per minute.  However, some in the solar applications industry prefer kilowatthours or megajoules per square
meter per day.