"uncle's" is used as a possessive noun in this sentence. One could also argue the case for it to be labeled as a possessive adjective.
"dark billowing clouds" isn't a sentence. Dark and billowing are adjectives describing clouds, a plural noun.
Hazy clouds covered the sun. His memories of the trip to Las Vegas were a bit hazy.
The word "SEE" in the sentence is the base form of the verb "to see". Verbs have both form and tense. In this sentence, "see" is used in the present tense.
When the _______ and ________ are right, the small droplets of water in clouds form larger droplets and preciptation occurs.
The oil slick on the frigid water was iridescent. Iridescent clouds looked like mother-of-pearl.
Venus is covered in acid clouds.
Clouds, sky, mosquitoes 
The planet Venus is completely covered with clouds. The clouds are comprised of sulfuric acid, which traps in the atmosphere and heat.
clouds
the clouds are cumulonimbus clouds
air and water
cirrus clouds are clouds that are in the sky.
"dark billowing clouds" isn't a sentence. Dark and billowing are adjectives describing clouds, a plural noun.
Those funnel clouds are very ominous.
The nouns in the sentence are airplane and clouds.
uranus
Venus