dense, heavy, thick
light
fluffy, puffy (esp. AmE), wispy (esp. AmE)
cirrus, cumulonimbus, cumulus, etc.
high, low
broken, scattered
skies of broken cloud

fast-moving, scudding (literary)
black, dark, grey/gray, white
rain, storm, thunder
Rain clouds were looming on the horizon.

ominous, threatening

band, bank, blanket, layer, mass, wisp
a layer of high cloud

seed (= to place a substance in a cloud to make it produce rain)

form, gather, mass
Dark clouds were gathering in the west.

cover sth, envelop sth, obscure sth
break, clear, disperse, lift, part
The clouds broke a little, and the sun came out.

drift, float, hang, hover, loom, move, pass, race, roll, scud (literary), swirl
Thick cloud hung over the moor.
A cloud passed over the sun.
White clouds scudded across the sky.

bank, cover, layer
The cloud cover is quite dense today.

formation

above the clouds, below the clouds
We were flying above the clouds.

through the clouds
She could see the sun through the clouds.

a break in the cloud, a break in the clouds

great, huge
a great cloud of black smoke

dense, thick
billowing, swirling
mushroom
the mushroom cloud from a nuclear bomb

dust, gas, radioactive, smoke

create, form
The wind blew across the beach, forming clouds of sand.

belch, release

cloud of
a thick cloud of steam