Cappotto vs. soprabito
Both the Italian words cappotto and soprabito translate to the English words coat or overcoat, but they differ significantly in their weight, material, and the seasons they are meant for. Understanding the difference is all about the changing weather and the layers of clothing you need.
Cappotto
A2A cappotto is a heavy winter coat, typically made of wool, cashmere, or a thick blend, designed specifically to insulate the body and keep you warm during freezing weather.
Oggi fa estremamente freddo e non posso uscire senza il cappotto.
(Today is extremely cold and I cannot go out without my coat.)
Ho comprato un nuovo cappotto di lana nera per affrontare l'inverno.
(I bought a new black wool coat to face the winter.)
Ricordati di abbottonare il cappotto prima di aprire la porta.
(Remember to button your winter coat before opening the door.)
Lascia pure il tuo cappotto all'ingresso assieme all'ombrello.
(Feel free to leave your overcoat at the entrance along with the umbrella.)
Quel cappotto elegante le arrivava quasi fino alle caviglie.
(That elegant coat reached almost down to her ankles.)
Soprabito
C1A soprabito is a lighter overcoat or topcoat, meant for transitional seasons like spring or autumn, elegantly designed to be worn directly over a suit, a dress, or standard indoor clothing without causing you to overheat.
Per la mezza stagione preferisco indossare un soprabito leggero.
(For the transitional season I prefer to wear a light overcoat.)
Il nonno indossa sempre un elegante soprabito grigio sopra il suo completo aziendale.
(Grandpa always wears an elegant light coat over his business suit.)
Ha piegato il soprabito sullo schienale della sedia perché faceva caldo nel ristorante.
(He folded the topcoat over the back of the chair because it was hot in the restaurant.)
Un soprabito impermeabile è assolutamente perfetto per le imprevedibili giornate di pioggia primaverile.
(A waterproof overcoat is absolutely perfect for unpredictable spring rainy days.)
La sposa ha scelto un bellissimo soprabito di seta per coprirsi al tramonto.
(The bride chose a beautiful silk duster coat to cover herself at sunset.)
Summary
In short, choose the word cappotto when you are talking about a thick, heavy winter outerwear meant for the cold months. On the other hand, use soprabito for a lighter coat, topcoat, or duster worn during the milder temperatures of spring and fall to protect your clothes.