Though earth's atmosphere exists beyond 50 KM high and up to 100 KM, 99.9% air is only available up to 50 KM height. Water vapor is found only up to 10-12 KM, which factors are not favoring moisture to rise beyond this height?
Weather/Atmospheric/Environmental scientists can answer this question.
Thank you, Shipra, You have correctly stated the fact that after certain height(say ,15 KM or so) in atmosphere all moisture particles try to return back to earth surface. Some force of attraction say gravity or self weight of individual particles forces them to come down in stead of further rise in atmospheric air. If you can further explain this statistics /dynamics from your studies please elaborate.
The Tropical Tropopause is higher than 12 km. It is often nearly 18 km in the deep tropics, and at least 12 km in the sub-tropics. The only moisture it gets is from penetrating deep convection that detrains some moisture into the local Stratosphere. This process occurs even at higher latitude Temperate areas, especially over the Great Plains of the central USA. Most of the moisture is in the form of small ice particles that can be advected long distances. In the Stratosphere, vertical motions are limited by the thermal structure having temperatures rise or stay constant with altitude, so most water stays near the Tropopause. That temperature structure is maintained by ozone absorption that heats the air up there.
While the concentration of water vapor is significantly reduced above the tropopause, it present throughout the atmosphere. Water vapor is routinely measured throughout the Stratosphere and Mesosphere, for example by the Microwave Limb Sounder on the Aura spacecraft which measures water vapor up to 0.002 hPa. https://mls.jpl.nasa.gov/products/h2o_product.php
In the upper atmosphere the water vapor concentrations are reported in parts per million. However, sometimes it will get cold enough that these small amounts of water vapor freeze forming Polar Stratospheric Clouds at altitudes of 15 to 25 km during the polar winters (http://www.atoptics.co.uk/highsky/psc1.htm) and Noctilucent or Polar Mesospheric Clouds at 80 to 85 km during the polar summers (see for example the review articles by G.E.Thomas, 1991 Rev. Geophysics, 29, 553-575, and by DeLand et al., 2006 J. Atmos. Solar-Terr. Phys., 68, 9-29.)
I know there is a very small water vapor concentration in the Mesosphere. The real question is how it gets there, since vertical motions in the stratosphere are normally strongly inhibited. In the winter hemisphere near the poles, some ice might be advected into the stratosphere and get mixed to higher altitudes. Some of the Mesospheric water might be meteoric in origin as well. Flow changes in the polar Stratosphere that occur near seasonal changes might also allow some water to get through.
Yes, and some of the water which reaches the middle & upper stratosphere is broken apart by the solar UV. This is replaced by the oxidation of methane and molecular hydrogen. (See Wrotny et al., 2010, J. Geophys. Res., 115, D04302, doi:10.1029/2009JD012135)
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