You should not be surprised, Tony. A tell-tale sign of low contrast (areas within the frame) which in turn exacerbates noise later in the work flow. This happens with *any* digital camera and is inherent to the digital technology.
Sorry Bjorn, I'm not convinced in this case
I have thousands of examples of low contrast images containing skies where I don't see this level of noise, perhaps my work flow methodology is different.
Please don't think I'm being difficult, that's not the case!
I'm wondering if the image shown is a large crop, has had exposure lifted or sharpening applied to the entire image, I say these things because I'm learning still, obviously.
I will add, it doesn't ruin the image for me it's just something I noticed.