Green flashes and green rays are optical phenomena that occur right after sunset or right before sunrise. The green appearance usually last no longer than a few seconds, the most rarest form of which is a ray shooting upward from the top of the sun. They are usually viewed on an unobstructed horizon such as over the ocean. Because of this, green flashes are most often viewed by pilots. However, they can be viewed at any altitude and, any longitude and latitude. At the equator they rarely last more than a few seconds. Green flashes have also been observed as the sun passes behind clouds or mountains. They occur because refraction of the sun's light enhanced by mirage. As the atmosphere separates out the sun's light hues of blue and red are scattered leaving the green alone for a split second. Due to this, they are most often observed in stable, clean air. Blue Flashes, a rare form of refraction occur sometimes, the cause for which is still much speculation.