Are the bottom of clouds flat or do they just look it?

I was in the train for long trip to Blue Mountains today & was looking at all the clouds over Sydney & it hit me that all the bottoms of the clouds look flat. The lower clouds not upper clouds. Are they flat or do they just look it? I thought seeings as clouds are "puffy" they wouldn't have a flat... show more I was in the train for long trip to Blue Mountains today & was looking at all the clouds over Sydney & it hit me that all the bottoms of the clouds look flat. The lower clouds not upper clouds.
Are they flat or do they just look it? I thought seeings as clouds are "puffy" they wouldn't have a flat bottom really. Maybe it is the perspective? The sunlight shinning through them?
Maybe I had a boring trip & was thinking too much?
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