I've been reading on planetary atmospheres and I've encountered this term which I can't find defined anywhere on the web.
What is the definition of Hygropause?
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So there are several "pauses" as one goes up in the atmosphere, a classic one for example is the tropopause that happens between the troposphere and the stratosphere. The "pause" just means that something has stopped changing, usually as a result of coming to a minimum or maximum; in the case of the tropopause this is the temperature: during most of the stratosphere the temperature increases as you go higher and higher; during most of the troposphere the temperature decreases as you go higher and higher; the tropopause is the section between these two when the temperature remains relatively constant because it is at its minimum.
The hygropause is a more obscure version of this which happens mostly in the tropics, and happens just above the tropopause in the lower stratosphere. The minimum is this time of water vapor, more specifically its mixing ratio.
This was somewhat of a surprising find when it was first discovered in 1979 by balloons over Brazil: the tropopause somehow seems to be able to, around the tropics, dry out the stratosphere in some strange way. It may be that strong convective currents move troposphere air into the tropopause and stratosphere where their vapor freezes and then returns back to the troposphere, ultimately drying the region out. Or, atmospheric waves travelling across the tropopause might be able to cool that part of the atmosphere in ways that cause it to dry out. It may also be that somehow the tropopause dries out the stratosphere and then "falls down" but does not take the minimum of water vapor with it. It is not my field of science so I am not really able to comment further than to just tell you some proposals which I found in the literature.
I would go with it’s the point or defined point that water is present or not at some defined amount in the fluid or medium or atmosphere.
See this link