Have you seen clouds like the ones in the photo below? It was taken by Astronaut Chris Hatfield (Canada) from the International Space Station.
Notice the mountain range on the right side of the picture.
Wind is blowing from right to left.
As the wind blows over the mountain range, it causes the air to undulate, going up and down as it travels right-to-left in the picture.
This image shows you a similar pattern.
When air rises, it cools and the moisture in the air condenses into visible cloud particles.
When air sinks, the cloud droplets evaporate back into invisible moisture.
So, at the peaks of the waves, clouds form. In between the peaks, where the air is sinking, there is an absence of clouds. That creates rows of clouds coming off the mountains.
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