Editing Perlan Project (section)

==Meteorological basis of the missions==
[[Lee wave|Standing mountain waves]] are a source of rising air used in the sport of [[gliding|soaring]]. Riding these waves, similar in some ways to surfing on an ocean wave, has been widely used to reach great altitudes in sailplanes since they were discovered by German glider pilots, including [[Wolf Hirth]], in 1933 in the [[Riesengebirge]].<ref>{{cite web
| url = http://www.nateferguson.com/glider.html
| title = Article about wave lift
| accessdate = 28 September 2006
}}</ref> This method uses the powerfully rising and sinking air in [[Lee Wave|mountain waves]]. Gliders regularly climb in these waves to high altitudes.

Prior to the 4 September 2017 flight, the glider absolute world [[flight altitude record|altitude record]] stood at 15,460 meters (50,727 feet), which is the altitude reached by [[Steve Fossett]] and [[Einar Enevoldson]] during Perlan Mission I. The previous record was 14,938 meters (49,009 feet). It was set in 1986 by Robert R. Harris, flying from [[California City]] and reaching his record height over [[Mount Whitney]], California.<ref>*[http://records.fai.org/gliding/history.asp?id1=DO&id2=1&id3=98 Official FAI Gliding Open Class Absolute Altitude World Record] {{webarchive|url=https://web.archive.org/web/20050219015448/http://records.fai.org/gliding/history.asp?id1=DO&id2=1&id3=98 |date=19 February 2005 }}</ref> This may be near the limit for standing mountain waves in temperate latitudes, although in unusual meteorological conditions much higher altitudes may be achievable.

Standing waves normally do not extend above the tropopause at temperate latitudes. A strong west wind usually decreases above the tropopause, which has been shown to cap or prevent the upward propagation of standing mountain waves. However, at the outer boundary of the [[polar vortex]], in winter, the stratospheric [[polar night jet]] exists. Its wind field can join with the wind field of the polar [[jet stream]]. The result is a wind which increases with altitude through the [[tropopause]] and upward to 100,000 feet or above. When this conjunction of winds occurs over a barrier mountain, standing mountain waves will propagate through that entire altitude range.  [[Einar Enevoldson]], former [[NASA]] test pilot, sought to demonstrate the feasibility of riding these stratospheric standing mountain waves. The weather conditions favorable, although not in every case required to exist simultaneously for a climb into the stratospheric waves, are not exceptional. The following list of requirements for record attempts was given in an article published in 2014:<ref name=":0">{{Cite news |date=2014-11-01 |title=Airbus Perlan Mission II |url=https://issuu.com/mccawmedia/docs/soaringissue39_web_backissue |access-date=2024-09-20 |work=SoaringNZ |pages=27}}</ref>

*Prefrontal conditions
*Ridge top winds ≥ 40 knots
*Winds within 30 degrees of perpendicular to ridgeline
*Strong low-level winds
*A stable atmosphere
*A gradual wind increase in altitude
*A weak tropopause
These conditions often occur during late winter and early spring over the southern Andes and Scandinavia.<ref name=":0" />

These conditions are likely to occur in the southern region of Patagonia three to four times per year between mid-August and mid-October. They probably occur in New Zealand, but less frequently; over the Antarctic Peninsula more frequently; and at several locations in the northern hemisphere, but closer to the North Pole at latitudes above 60° north.{{Citation needed|date=July 2015}}