I agree with Ole,
Newton's rings are interference figures created by the reflection of light between film and glass. Film with a very smooth surface are more prone to this kind of problems. Umidity is an issue because the film absorbs the moisture and changes its surfaces (hardness and smoothness). This is what appen to me: Ilford Delta 100 is a very smooth film, but I usually get newtons ring only during the summer or when the eath is on (i.e. when the umidity in my house increases). There are a number of different products to solve the problem (glass, spray, powders), but the principles are just two: 1) avoid to have glass and film surface running parallel; 2) modify the distance between film and glass.
Since the interference is limited to the support side of the film (the emulsion side is irregular, the support side is smooth), I use to put a very thin frame of black cardboard between film and upper glass. Simple, clean and very cheap.
Ciao,

Marcello