it is raining cats and dogs: piove a catinelle.
cielo sereno: blue sky
fenomeni di precipitazione: rain [possibly with hail] or snow
(precipitare: to fall with great velocity)
banchi di nebbia, nebbia a banchi: clouds of fog ('clouds' at ground level)
This expression: /it is raining cats and dogs/ actually describes what happens
when a strong wind accompanies the rain; small animals are moved around and
dropped to the soil at a distance; such string winds are rare in Italy, with the exception of Trieste and the ridge of our many mountains.