@cyder
On "sounding forecast"
https://community.windy.com/topic/6998/windy-offers-sounding-forecast
you can see the temperature/dew point spread at different levels.
e.g. at 21,000 ft the spread is 3 degrees.
0_1541513584684_9178a5a1-a703-41bd-b392-487f877fbbdb-εικόνα.png
P.S. flight visibility is also afected by fine particles (dust, sea salt) in the atmosphere. So, you may have low visibility even in low humidity (dry haze)
especialy if you fly eastwards early in the morning (towards sunrise).