WILL YOU BELIEVE ME if I say the picture below was taken with a box camera? There is nothing incredible about the thing. And nor is it impossible. To get the clouds into the picture, you require in the first place, clouds in the sky. And to differentiate between clouds and sky, a deep yellow, or orange filter. The picture below was taken with an Agfa Isoly II with a pale orange filter with a filter factor of about 3.
I will be honest here. I don't have a print of this picture with me, only the negative. The picture you see here was made by photographing the negative against a white background using a mobile phone, then using the computer to invert tones. Finally, contrast was added online.
I can assure you, you can get an identical effect by printing the negative in an enlarger, and adding contrast by using a hard grade paper and a high contrast developer.