Callaham's gives this definition for this term, alternatively "terminal or free-fall velocity", which don't sound right here.

[A patent for] terrain avoidance system in an aircraft, the method comprising: generating a altitude vertical error value; generating a vertical safety margin value based on the generated vertical
error value and safety margin limits; determining if an alert condition exists based on the generated vertical safety margin; and
generating a terrain floor height value based on a aircraft groundspeed, a predefined hover speed and approach speed, and a runway distance based terrain floor height value, wherein determining if an alert condition exists is further based on the generated terrain floor height value.

--------------------------------------------------
Note added at 2 hrs (2008-09-25 16:22:32 GMT)
--------------------------------------------------

dust particles hover in the air. The hovering dust has a third velocity, which depends on the ... www.springerlink.com/index/F5252212G4416081.pdf

--------------------------------------------------
Note added at 2 hrs (2008-09-25 16:46:21 GMT)
--------------------------------------------------

или velocity

Вот есть работы русских ученых, опубликованные в изданиях Springer'a - поскольку эти публикации проходят строгий контроль в смысле языка, думаю, термином можно воспользоваться: ничего иного не находится

The dust particle size distribution in the dust suction pipes of ...
constant and equal to the soaring speed v s. The condition for a particle. to fall out of the airflow inside the cover is that it falls a distance H ...
www.springerlink.com/index/JT687T4525432317.pdf - Similar pages
by VA Minko - 1974
Graphite entrainment in reactors with electrothermal fluidized bed
ticles as a result of the decreasing soaring speed of the latter /3-5/-. The relation between entrainment and the proportion of fine particles in the bed ...
www.springerlink.com/index/LUM8165166236U3K.pdf



--------------------------------------------------
Note added at 7 hrs (2008-09-25 21:52:43 GMT)
--------------------------------------------------

Как вариант - soaring velocity
http://tinyurl.com/3vyx6w

--------------------------------------------------
Note added at 8 hrs (2008-09-25 22:32:59 GMT)
--------------------------------------------------

Дим, вот тут есть хорошая книжка:
http://tinyurl.com/3hvgrb

И вообще, залягай меня кошка, но после того, что я там прочитала (стр.75, левая колонка внизу) у меня создалось впечатление, что в английском это просто airborne particles (безо всякого там витания-парения).