Achieving daylighting credits for LEED: Glossary and definitions

Explaining precise meanings for terminology when analyzing daylighting in AGi32.


Clear sky: A sky that has less than 30% cloud cover. 

Equinox: Time of equal day and night. Either of the two annual crossings of the equator by the sun, once in each direction, when the length of day and night is approximately equal everywhere on Earth. The equinoxes occur around March 21 and Sept. 21. 

Fins: The vertical equivalent of an overhang. They are positioned alongside a window and help to block direct sunlight when the sun is low in the sky. On an east or west exposure, fins can be effective in the morning (east) or late in the evening (west) during the summer and winter months.

Overcast sky: A sky that has more than 70% cloud cover. 

Overhangs: The principal method for eliminating direct sunlight from a vertical window. They are particularly effective on the south face of a building.

Partly cloudy sky: A sky that has 30% to 70% cloud cover. 

Solstice: Longest or shortest day. Either of the times when the sun is farthest from the equator, on or about June 21 or Dec. 21. The summer solstice falls in June in the northern hemisphere and in December in the southern hemisphere and vice versa for the winter solstice. 

Sun path diagram: Tells us where the sun will be on any given date and time for a given location. 

Visible transmittance (VT): Daylight transmittance is the percentage of visible light striking the glazing that will pass through. 

Courtesy: AGI website (

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