What is the green flash that occurs near sunset?

The “green flash” is an interesting and rare atmospheric phenomenon that can occur at sunset or sunrise.  If conditions are right the flash is seen for less than a second and is not always green.  At times the flash is blue.  Just as the sun sets or rises at the horizon, the upper edge of the sun can be seen. The upper or lower edge of the sun will usually take on the appearance of a green (or blue) rim that is too difficult to be seen by the human eye, unless the rest of the sun is obstructed.  When the sun breaks the horizon, and a majority of the sun is covered, the green or blue flash will be caused by the dispersion of light.