It in Juneau, Alaska. You are right, it is caused by the date, in November the sun is no so high at this location. But for my point of view, they are too dark for this time of year. When I changed the date, the clouds remain dark evens after I redraw them. I had to switch back and forth to an other location for the cloud to appear normal. Maybe some code missing to detect date change in x-plane ?? Thanks for your response.