I am writing a literature review for a project and I have a problem. Namely, I cannot figure out the the condition required for surface-based duct to occur. Literature does not help either. In one of the journals I find:
"Surface-based ducts are mostly caused by advection of warm and dry air over the ocean or land, which creates humidity and temperature inversion." 
And in other
"Surface ducts generally appear in sunny weather with relatively stable atmosphere. At this time, the atmosphere often has a relatively stable temperature inversion layer, and the humidity generally decreases with height." 
So one paper says there is humidity inversion while other says that humidity decreases with height. So which is it?
 E. Dinc and O. Akan, “Beyond-line-of-sight communications with ducting layer,”IEEECommunications Magazine, vol. 52, no. 10, p. 37–43, 2014.
 W. Tang, H. Cha, M. Wei, and B. Tian, “The effect of atmospheric ducts on the propagationof ais signals,”Australian Journal of Electrical and Electronics Engineering, vol. 16, no. 2,pp. 111–116, 2019. [Online]. Available: https://doi.org/10.1080/1448837X.2019.16224918