Meanings
cloud (a visible mass of water droplets suspended in the air)
Der Himmel ist heute von Wolken bedeckt.The sky is covered in clouds today.
cloud, made of steam, ash, smoke or anything that resembles such a mass (may also be Schwaden or Schwade in this sense)
Bei Vulkanausbrüchen entstehen meist riesige Aschewolken.During volcanic eruptions, vast ash clouds do often appear.
nebula, enormous masses of interstellar dust and hydrogen in outerspace (may also be Nebel in this sense)
Interstellare Staubwolken sind häufig die Kinderstuben neuer Sterne.Interstellar nebulae are often the cradles for new stars.
Examples
GermanEnglish
Es war keine Wolke zu sehen.There wasn't a cloud to be seen.
Es ist keine Wolke am Himmel.There's not a cloud in the sky.