What is another word for a dark cloud?

I am writing a novel, and I was wondering what is another word for a dark cloud. Here is the sentence, also the sentence before it: Now the sun is hiding and so is the heat. I look out the porthole on the left side of my bed and see that there are more __________ coming this way.



I have already used the word cloud a lot, so please have your answer not include the word cloud in it. Thanks!What is another word for a dark cloud?
....and see that there is more OF THE THICK HAZE coming this way.



If you want another, use ' the dark,dense haze'.What is another word for a dark cloud?
Black sky.