To observe NLC the path bettween you and the NLC has to be clear of clouds. This raises the question just how far away are NLC ? This is very useful to know if you are looking at satellite images to determine if NLC should be visible. The observing path goes diagonally through the atmosphere so clouds under the NLC position do not matter in terms of observations. To aid you in intepreting the satellite images I have put a set of visibility distance calculations on the NLC page on my website at link.
Please note this calculation does not include the impact of refraction that will increase the apparent elevation at low angles so allows you to see furthur than calculated. Also for high elevations this is the diagonal path to the NLC rather than the surface distance which will be less.