TBerwick wrote:
You must have an unobstructed, straight on view of the sun as the top edge starts to dip below the horizon. There can be no haze or any pollution visible. Viewing from a height is preferable. When we watched for it while underway, we used our watch standing binoculars and waited. 90% of the time we were disappointed.
I have seen it from airplane cruising at about 30,000 feet and once from a mountain top. I have seen the same number of good photos of the green flash as I have of aliens.